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Author Topic: Automotive Fuel Pump Speed Controller  (Read 2290 times)
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Russ
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« on: March 25, 2013, 07:53:21 19:53 »

Hi all, I thought I would share with those interested my current project. Due to time constraints itís taking a while to complete. As a hobby I am building a car. The engine is a 7.0l V8 and fuel system uses standard high pressure fuel fed to the engine through injectors for each cylinder. The high pressure fuel pump (Aeromotive A1000) supplies the fuel rails via a pressure regulator. The issue is that normally the fuel pump in this application runs at full power all the time thus the fuel that is not used is recirculated back to the fuel tanks. After a while this fuel absorbs the heat from both the pump and engine steadily increasing the fuel temp. Not a desirable situation. The project Iím working on reduces the pump speed by utilising PWM which maintains a constant (low) return fuel rate.

The electronics utilise a PIC Microcontroller PIC16F1828 monitoring a rotating blade fuel flow meter and drives the pump via a BTN7970. This combination keeps the component count to a minimum whilst providing a degree of circuit protection etc. The software uses fuzzy logic to map the return flow rate to pump speed. There are other inputs such as throttle position so predictive control is also a factor and switching the pump on permanently can be achieved at 90%+ throttle.
If anyone is interested in further detail then please feel free to ask about any aspect. Iíll post more details asap.
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« Reply #1 on: March 26, 2013, 02:27:19 02:27 »

Why would you use a half bridge to drive the pump? I mean, a single MOSFET would not be enough to drive the pump's motor?

I see that you put the circuit protection in the first place, which is wise for an automotive project.

Of course, details are making your post worth reading it  Cheesy
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Interested and hopefully helpful in: DC brushed motor control (mainly R/C - PPM/PWM), analog audio, PIC (mikrobasic PRO). Feel free to ask, and if I can, I will help. But only on forum topics, any started private conversation will continue in a public topic.
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« Reply #2 on: March 26, 2013, 08:33:21 08:33 »

When you punch it, I seriously doubt the pump will spool up fast enough to maintain pressure for the amount of fuel a 7L drinks, even if you can predict what the driver is doing before s/he floors it.

I think it'd be a LOT easier to plumb the fuel return to a fan-cooled heat exchanger (like an aux trans cooler), then back to the tank. The exchanger will be a lot cooler than the gas tank facing blacktop in the summer, so it'll cool the gas in the tank as well.

K.I.S.S.
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FTL
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« Reply #3 on: March 26, 2013, 06:26:47 18:26 »

It may be easiest to solve the flow issues with sudden full throttle with a fuel accumulator.

This is one used on Volkswagons



It consists of a diaphragm with a spring behind it. When the fuel system is pressurized by the pump, the accumulator fills. If the pump cannot keep up for an instant, the accumulator will provide the pressure and volume needed for a bit.

This is pretty much a capacitor for your fuel system.

Edit: fixed typo
« Last Edit: March 26, 2013, 06:28:52 18:28 by FTL » Logged
Russ
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« Reply #4 on: March 27, 2013, 07:47:59 19:47 »

Many thanks for your interest and comments. Unusually I'm not really sticking to the kiss principles, otherwise I would have just bought a car rather building one and opted just to purchase the standard Aeromotive fuel pump speed controller or even a pressure regulator with fuel pump speed controller built in. I'm much more exploring and applying my electronics and software programming skills on something tangible and practical. The engine takes a great deal longer (relatively) to spin up than the motor - there's a lot of inertia in the engine to overcome. Would be considerable different in an F1 engine! The motor never actually stops as well as there is always some fuel returning to the tanks (about 0.5 litres per minute) There are some applications where instead of a mechanical pressure regulator they use an electronic pressure sensor combined with monitoring to the delta of the injection pulses to control fuel pressure - using the same pump as I have. I also have a Bosh 044 that I'm working with.

The BTN7970 is good for a fair few amps ( about 5x what I need) and if the PWM frequency isn't too high (max 25KHz - I'm using 15KHz to 20KHz in the tests) it stays relatively cool with minimal heat-sinking (it will have way more than enough heat-sink in the final application) - I can always double up too for heat sharing and resilience.

I also have a surge tank that is fed by two low pressure pumps that take their input from the two separate fuel tanks I have. Sandwiched between the base of this 1 litre aluminium canister and the floor of the engine bay are a couple of large peltier devices. When decelerating they switch on and they cools the container and over time the combat some of the induced heat in the fuel. There are some timing parameters around the frequency of switching etc so there is a little bit of intelligence built in - makes for a little bit of energy recovery ;-) Not serious but again fun to try and evaluate in the real world.

I like the idea of the accumulators, i'll have a look into those.

I did look at discrete MOSFETS but then they need protection diodes, gate drivers, bootstrap drivers (as I want to ground the pumps -ve terminal and switch the +ve. together with gate diodes to aid transition times and possibly gate zenners to protect against spikes etc. Plus additional circuitry for current monitoring and control.  The component could was becoming significant for me.

To power the digital side I've used a simple DC-DC converter with wide input operating voltage and protected by suitable TVS diodes - All the components are automotive spec (or above) for environmental concerns.

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solutions
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« Reply #5 on: March 27, 2013, 11:39:19 23:39 »

I think you realize that it is not the inertia of the engine that governs the almost-step-function of fuel required when you floor it - it's the speed of opening the throttle butterfly to a 7 liter air pump (3.5 liters of air per rev at current RPM when she goes wide open), and maintaining stochiometric ratio. Your pump cannot spool up fast enough to keep up with it, IMO....the accumulators may be a solution, but if you use those, I think you can get away with bang-bang control of the pump. As I said before, you'll get much bigger gains and reliability with a fuel cooler, especially one that cools incoming and return fuel.

Cracking the throttle open on big displacement engines is why the Holley "double pumper" continues to be used by hot rodders to this day.

As far as K.I.S.S. goes, never make it so complicated it will strand you, or worse - your wife, girlfriend, sister, or daughter - and always have a limp home more...that means keeping the return lines, IMO. Then again, you might be building this car for your mother-in-law....
 Tongue
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Russ
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« Reply #6 on: March 28, 2013, 02:00:51 02:00 »

I think you may have missed my objectives for going this route - they aren't about gains or reliability - I know that it may be an alien concept but its more about the journey, the practical application of knowledge and understanding together with the realisation of turning decades of theory into practical reality. I'm under no illusions over reliability that's why I have a big switch on the dash that says "manual override"! The system is also configured to fail safe, as far as possible.

I'd be interested in how you formed your assertion that the pump won't respond fast enough - do you have any timings in mind or practical data you could share. You'll appreciate there are many factors at play when it comes down to it and theory is only useful to get you close to a solution. I'm a little skeptical as I mentioned before commercial systems are available and return-less fuel systems are commonplace. It isn't a drag car its actually a road legal car - just happens to have a custom built GM LS7 in the back.

As for the mother-in-law there is another rotary switch on the dash that says "traction control 0..100%"..but that's the next project that I have only done on paper so far. More on that in a separate thread going forward.

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solutions
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« Reply #7 on: March 29, 2013, 04:00:54 04:00 »

I think the guys at the big 4 or 5 or 10 did their homework on all this and concluded that the return was the cheapest and most reliable way to do it. But, it's your marshmallow roasting party. I'm curious as to what kind of car this is, 'cause you said "motor in the back".

OK - numbers,

I had to guess operating points, but I think I am darned close to real numbers, and the relative numbers are dead nuts right. I'm assuming 100 deg F air going into the cylinders which drops the HP a bit. If I drop the temps to 55F, I get 485HP, GM claims 505. 7%...close enough.

You're loping along at 1000 rpm and pulling 20 inches of pressure making 48 HP and flow 24 lb/hr or 0.4 lb/min of fuel

Now you floor it to get 30 inches of air pressure into the cylinders. You still want to maintain the 14.7:1 ideal fuel mixture

You need to flow 44 lb/hr or 0.73 lb/min which then calls up 88HP...the car accelerates

I'll keep it floored until 6000 RPM on your LS7

You'll be flowing 741 CFM, 216 lb/hr, and making 432HP

So, assuming your car (you said "engine in the back" for an LS7...what car is this?) launches from 1000 RPM and accelerates in 1st gear for, say 2.25 seconds, and assuming your pump makes pressure and flow at 5000 RPM, your pump has to accelerate from 500 RPM to 5000 RPM in 2.25 seconds, all the while loaded to keep the pressure in your fuel rails, or a rate of 33 revs/s/s.That's the easy part.

How long does it take you to floor it? Well, lets assume you can stomp your foot at 15MPH, which is 22 FPS, or 264 IPS. Let's assume your accelerator pedal strokes 2.64 inches. SO, it takes 10mS for you to floor the pedal. In that time, your pump had to go from 500 RPM to 917 RPM, or pick up 417 RPM in 10 mS. The required acceleration, under full load because the pressure is constant with your controller, has to be 700 revs/s/s, over 20x that of very aggressive, fully floored, 0-45 or so in 2.25 sec.

I'm estimating my 3HP router accelerates to 22,000 RPM in 3 sec. That's 122 revs/s/s. Even if it ramped up in a second, it's still be half of the rotor acceleration you need...and the router is UNLOADED, is a big diameter machine compared to a fuel pump (aka torque -> HP), only has to overcome rotor inertia, and has 3 HP available to it.

You can try, but the physics seems to be stacked up against you. Then again, you could use another LS7 to run the fuel pump...

Is that detailed enough for you?
 Grin
« Last Edit: March 29, 2013, 04:13:06 04:13 by solutions » Logged
Russ
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« Reply #8 on: March 29, 2013, 10:22:32 10:22 »

That's great, many thanks for spending the time to reply. It may take me a little while to reply as I'll have to convert all those funny units to proper ones :-) seriously though i'll post what I've seen so far from bench tests etc. the car is an Ultima Gtr, the engine is a custom built LS7 that produces 730hp @ 7200 rpm. That'll make your figures a little more conservative and concerns greater I would imagine. More soon. Just one quick observation, the fuel pump goes from 0 to full flow at something like 56 psi in under a second. It has loads of torque ( loads of current and a big kick when turned on) but little speed. The opposite of your 22k motor I would suggest. Its the torque that gives it the abbility to accelerate quickly. The current curve is almost a step responce. I think I have it logged somewhere. I'm also thinking it would be useful for the discussion. ;-)
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Sideshow Bob
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« Reply #9 on: March 29, 2013, 03:00:59 15:00 »

I know that BMW has used the aircon unit to cool down the fuel for a while. And you can get kit for that http://www.vaeth.com/en/fuel-cooling-system.html I share the scepticism given by other. By using PWM your system will not be able acommodate fuel demand say during acceleration
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Russ
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« Reply #10 on: April 03, 2013, 01:08:36 01:08 »

OK - managed to grab some time...

Letís make some realistic assumptions
1) BSFC for a NA engine is 0.5
2) The dyno chart for my engine starts at 260HP @ 3000 rpm and goes pretty much in a straight line to 730HP @ 7200 RPM
3) lets assume we go back to 0 rpm = 0 HP plotting a straight line through the HP dyno results gives us 0.1 HP\RPM and a correlation coefficient of 0.952 - pretty close to a straight line.
4) Working out fuel demand per RPM at 100% load using a BSFC of 0.5 gives us 0lbs\hr @ 0 RPM and 365lbs/hr (730*0.5) @ 7200 RPM
5) So Fuel consumption\demand is a function of (amongst other things) RPM.
6) Now letís consider how long it takes for the engine to rev to 7200 RPM under full load!?! As I referred to in my original post itís not an F1 engine so itís going to take seconds. Just blipping the throttle isn't going to generate 730HP and thus doesnít require 365lbs\hr of fuel. Letís take the 2.25 seconds above as a first step.
7) 2.25 seconds to go from 0 lbs\hr to 365lbs\hr. Thatís 162lbs\hr\sec demand. Hold that thought.
/8) Looking at the spec of the fuel pump we see that it can flow 500lbs\hr@50psi@12v and 600lbs\hr@50psi@13.5v
9) The transient response of the primed pump is shown below - This is to reach and maintain 50psi.
10) From rest the pump takes about 65mS to reach full flow i.e. 500lbs\hr and is pulling about 14Amps@12V
11) Thatís about 7692 lbs\hr\sec or about 47 times quicker than required.
12) If the pump is already running (as it will be) returning about 50 lbs\hr the pump reaches full flow somewhat quicker than from rest.
13) The blue trace is the voltage dropped across the +ve feed wire to the pump and thus essentially shows current. The red trace is the voltage across the pump terminals. (Ignore the -ve sign thatís just where I had the ground leads)
14) The previous assumptions about the fuel pump were way off reality.
15) Hopefully that makes some sense.

Comments\observations\the spotting of mistakes (Iím only human) welcome of course.


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solutions
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« Reply #11 on: April 03, 2013, 02:15:29 14:15 »

I think your head is stuck in the wrong place.

I am saying your pump cannot maintain 14.7:1 the instant you floor it, not WHILE you are accelerating. My analysis showed you might be able to keep up with acceleration.

Remember, your engine is a 3.5L/rev AIR PUMP. The throttle restricts the air going into the engine, which determines both its speed and power. At equilibrium, say 1000 RPM, the volume (and therefore mass) of air that is being moved into the engine is not 3.5L, and the attendant fuel mix is also not full flow. You floor it, and you have a 10mS STEP function (pun intended) to open throttle....to 3.5L/rev. You must supply fuel for that new air mass, or it will go extremely lean and hesitate, if not stall out.

If you drive like a granny, where you slowly open the accelerator over your 2.25sec, yeah, your pump will keep up. Stomp it, however, and I think you're screwed without an accumulator or dump/return line. I've built cars and if the accelerator pump was inadequate, it would hesitate/stall....increase the pumper volume and it fixes it. Too much and it'll puff black smoke out the back.
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« Reply #12 on: April 08, 2013, 01:26:11 01:26 »

I might be a bit slow, but can't see what benefit you are trying to achieve by going this way, apart from it being an academic exercise without practical benefit.

There is no performance or reliability increase to be had here for the added complexity.

Also, significant fuel enrichment is required during heavy acceleration to prevent detonation and hesitation in most vehicles. In carburetor engines, the accelerator pump(s) provided this.

The moment the butterfly valves open, the air demand increases dramatically, long before the engine begins to spool up in rpm. Any lack of fuel at this point can cause significant hesitation and the potential for serious engine damage.

Cheers.
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« Reply #13 on: April 09, 2013, 05:08:12 17:08 »

As said before.
What benefit you are trying to achieve?
A engine that put out 730hp don't sound cheap, I think you are only buying yourself some serious trouble.
The engine will cough when you floor it!

How much current does a Aeromotive 1000 on full fuel pressure draw?

/Zed
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Russ
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« Reply #14 on: April 10, 2013, 11:52:29 11:52 »

Thanks for the continued comments.

"Solutions" I think I'm beginning to understand where your head is.

Let me try and bottom this out.

As for the benefits, in no particular order:-
1. Longer pump life
2. Reduced heat absorption due to fuel mass recirculation
3. Reduced chances of vapour lock
4. Reduce chances of low pressure side inlet cavitation
5. Reduced noise

How often could\would I drive a road legal car at WOT at 7200rmp when it accelerates from 0 to 60mph in 2.8ish seconds. Hmmm now let me think.
So why have the fuel pump circulating 500 lbs\fuel an hour through aluminium fuel rails bolted to aluminium heads bolted to an aluminium block. Heat transfer from engine to fuel is pretty significant - not even contemplating the heat generated and transferred by the pump going full tilt.

Complexity has been mentioned a couple of times - to me it's a relative term - it means different things to different people and is usually dependent upon knowledge - or lack of it. What is not understood can seem complex. Is EFI more complex than Carbs!?! Letís not get into a debate about what constitutes complex or simple!

Back to the analysis - I'm still not seeing any evidence that it isn't viable to control the pump speed while maintaining capacity to meet demand. I understand that on WOT there is an requirement to  "go rich". This is taken care of by the ECU by the Power Enrichment mode. I've looked at quite a few data logged WOT runs from cars with similar engines to mine (EFILive and HPTuners logs). What I can see is that on DBW throttle bodies the butterfly takes between 200 and 400ms to open to 100% (from idle) and is also non linear. (not the 10ms assumption made above).  When the throttle position usually hits around 75% and the MAP has exceeded a threshold then the demanded afr drops from about 14.6 to around 11 or 12. The resulting action is to crack open the injectors - on the logs, Iíve seen this go from about 29% duty cycle to about 37% in about 95ms. With my 50lbs\hr injectors thatís an increase of 8% (32lbs\hr) in 94ms. Before PE mode was triggered the injectors were at 29% (116 lbs\hr) The pump has already responded to the increase in throttle position and is accelerating at 7692lbs\hr\sec and has been accelerating for 100+ms. By my calculations the pump would be flowing 400lbs\hr well before PE mode kicks in and we see a jump in fuelling demand of 32lbs\hr. That all leads me to the conclusion that there is way way more capacity and responsiveness in the pump to respond to demand...even worse case WOT from idle. A few things to remember - Air has mass - it cannot change velocity instantaneously. When the engine is at idle (1000 rpm) a cycle takes 120ms to complete i.e. the time it takes 8 pistons to complete 1 inlet event each. The pump can reach full flow 500lbs\hr @ 56psi in 65ms Ė half the time it takes for a complete cycle of the engine @ 1000rpm.

Its also worth mentioning that many modern manufacturers are now implementing return-less fuel systems (primarily for evap emissions control) where the pump is controlled via PWM to vary the output pressure (and hence fuel mass delivered via injectors (for the same opening duration)) is varied in response to load and demand. These use pressure transducers rather than the mechanical regulators as well as fuel rail temp sensors. I may even have a go at one of these ;-)

In answer to the last comment about pump current draw please see point 10 above - the pump draws around 14A & 12V when supplying 500lbs\hr of standard fuel at 56psi (4bar ish).

Regards
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solutions
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« Reply #15 on: April 11, 2013, 05:10:34 05:10 »

You'll have much bigger horsepower gains and reliability simply by running the supply fuel through a cooler - cool the incoming air as well. One aspect we did not touch upon is that the recirculating fuel actually cools the fuel rails....if there's any vapor lock to be had, it'll be in a hot fuel rail, where you've allowed fuel to stagnate.

Anyway, it's you that'll get stuck by the road (been there, done that, pushed that), which is deserved. Not a good thing if a wife or daughter is behind the wheel after dark. That aside, you stubbornly refuse to accept a 15MPH foot going to the floor - are you arthritic to be calling up the power under the good with a 400mS pedal press, or even 200? Stomp it, and it'll bog or die, as WE have told you and the pink slip will go to someone else. Jokes on him, though, with that fuel system which is impossible to resell, repair, and find replacement parts, which will be non-existent...after ten years, your flash will have died, and microchip will have stopped making the micro. And if you die before the car does, she'll sell it before your body hits room temperature.

But, you're wasting YOUR time, so go for it. You were warned, and chose not to listen. Let us know what your times are at the 'strip with this contraption of yours when you get it built. Stay away from train crossings and stopsigns...you WILL get hit.
« Last Edit: April 11, 2013, 05:13:43 05:13 by solutions » Logged
Russ
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« Reply #16 on: April 11, 2013, 11:59:35 11:59 »

Thanks "solutions" for your comments - I think.

I'm not after bigger horsepower - I have enough, actually, more than enough, with 730 in a car that weights just over a ton. I'm also not worried about incoming air temp - I live in the UK ;-) and besides the air filter is directly below the rear clip air intake scoop (please donít start a debate about if this is in a low pressure zone or not etc.)

The stale fuel in the rail is flushed out pre hot re-start by running the pump at 100% for a few seconds. Which the ECU does by default anyway.

If you think the wife or daughter are going to be driving this then you are more than mistaken!

You are also mistaken when you suggest that I am stubbornly refusing to accept a 15mph throttle pedal press - I'm not and I do believe you. What you seem to fail to recognise is that there isnít a mechanical cable linking the pedal to the butterfly valve. The ECU reads the pedal position through a potentiometer attached to the pedal - The ECU then drives the stepper (or servo) that is attached to the butterfly. There is also a potentiometer attached to the butterfly valve that provides feedback to the ECU on the actual position. You will know doubt understand that there are pedal position to throttle position configurable maps in the ECU - primarily to inject a non linear response to the pedal travel - much like cams on carbs I think. This is to control the drivability at lower speeds. From the logs I've seen and posted (which I donít believe you actually bothered to look at before posting your response) there is a definite correlation of throttle position response curves across the board suggesting to me that the ECU is controlling the butterfly opening and not simply snapping it open causing a step response in everything else - perhaps this is deliberate! Perhaps the 400ms ones I've seen have been programmed that way - there were more down the lower end of 200ms than at the upper end. My personal observation on my own car is that with a stick attached to the throttle pedal and poking out the window with the engine not running if I blip the pedal full throw and back again as fast as I can the throttle blade doesn't get to open fully Ė Moving the pedal a little slower so the blade tracks the pedal position there is definitely a perceivable lag. Once I get the engine connected back up I'll actually measure it and report back so you can provide more informed advice to the next person that you choose to impart your wisdom on ;-)

Your points relating to saleability, maintainability and longevity are well made and I would suspect well understood by anyone developing systems. To suggest the same system will be in place in 10 yearsí time though is simply misguided - This is a hobby and it'll probably change to something else in a year or so. As I said earlier I'd like to try the return less fuel system next. In 5 years I may even look to convert it to LPG - who knows. I may even replace the engine with a turbo diesel! When it comes to predicting what will be around in 10 yearsí time I suspect your crystal ball is just as cloudy as mine is.

As for re-sale, if I ever do sell it (certainly not my current plan) then I'd simply take out the system and hard-wire it back to permanent 100% - its just one wire and a couple of screws after all! You'll also understand that itís not the everyday person that would buy a car like this - itís the enthusiast that will be walking in with their eyes wide open and will actually expect some level of customisation, as thatís what itís all about.

If I die before the car my wife will probably bury me in it!

Interesting you mention breakdowns- My wife came home only last week saying the her car had just stopped when turning a corner in a car park. I took it for run and sure enough 10 minutes into the drive it cut out pulling away from a stop sign at a busy junction. Wasn't able to start it either for 5 minutes. Not pleasant and had to be pushed out of the way. Turned out to be a failed crank angle sensor that was failing when it got hot. So..does shit happen yes... do things break down, hell yes (probably in part because they are designed by humans). Do I expect my car to break down - hell yes. Is it a car I will rely on every day, hell no, I have other, more "standard" and sensible (well kind of) cars.

I guess in summary you havenít convinced me with any credible evidence that my endeavours are folly. Simply stating I've failed before so you will too doesnít cut it for me, sorry. If you were to suggest you've attempted the same or similar and failed or officered perhaps even a modicum of insight and experience along the same lines I'm progressing down then I'd take your comments as credible. As they stand I can't say I do. I heavily suspect your experience is with mechanical carbs and not ECU, EFI and FBW throttle configurations - if it were (is it!?!) I don't see how you could be posting what you do.

A little tip from me though to conclude with - If you ever break down on a train crossing - GET OUT OF THE CAR ;-) Don't wait for the next train to come along. Feel free to ignore that advise if you wish.

I'll also fully understand if you don't want to engage further as I'm sure you would much prefer to spend your time on something more constructive. Your choice my friend. If you decide to continue then thatís fine too and it would be welcomed.
« Last Edit: April 11, 2013, 12:46:28 12:46 by Russ » Logged
solutions
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« Reply #17 on: April 11, 2013, 06:12:03 18:12 »

I'm willing to bet there's a "performance" mode on the ECU and you are not in that mode with a 400mS, or non wide open, butterfly opening. You are likely running it in a silly EPA mode with what you're showing, and your engine is outputting what my lawn mower does as a result ;-)
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« Reply #18 on: April 11, 2013, 07:05:41 19:05 »

LOL. So what are you willing to bet? Make it something worth while.

ps you might want to read my posts in a bit more detail first. Anything you don't understand just let me know ;-)
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