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Fluctuating Fuel Pressure

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Hey guys,

i've been running into some pretty annoying misfires on a customers car today.

It runs perfectly fine all the way up to 7500rpm with about 1bar of boost, but as soon as I cranked up the boost, I couldn't rev it cleanly above 6200rpm.

It started misfiring pretty bad... it revs up, but with all those popping and bangings and didn't dare to rev past 7000rpm.

The dynoplot shows that the power and torque curve above 6200rpm are bouncing up and down all over the place on those runs.

I changed the spark plugs, increased the dwell time from 2ms to 3.5ms, ran a straight wire from the battery to the fuel pump.

Also, I upgraded the firmware on the Haltech PS1000 from 1.09 to 1.13 which seemed to have cured the problem, but only for 2 ramp runs. Then I changed the rev limiter from soft cut to hard cut ... also worked for about 2 ramp runs.

Started playing with boost, to see exactly at which boost level it starts.

Well, to be honest, the result wasn't really consistent ... first, it was OK at 1.2 bar, sometimes OK at 1.3 ... then it started misfiring occasionally at 1.2 bar, and after some further testing it was OK again.

The only thing that really got me thinking was the fluctuating fuel pressure. It seems like it's raising with boost, but veeery jumpy - I attached a screenshot from one of the ramp runs, this time it already started missing at around 5800rpm.

The weird thing ... the lambda readings track within 1-2% of my tuning ... although looking at the logfiles, I could have been tuning the VE table around the FP problem the whole time and wouldn't notice then?!

Anyways, car is a 2.3l 4 cylinder with a custom tubular exhaust manifold and a custom 6.8l plenum volume intake manifold.

GT3076R Turbo, Aeromotive A1000 FPR (looks like a knock off, to be honest), 6AN fuel files, Bosch 044 fuel pump mounted inline after a fuel surge tank.

Actually I would say it's a pretty serious car, apart from that annoying problem I've been tracking all day.

Please let me know, what you guys think ... other than testing my own personal Haltech ECU and switching out the AFPR, I'm kinda running out of ideas.

thanks :)


Attached Files


Hope this helps.. i had a similar issue for a few months.. i realize my fuel pressure was fluctuating on boost especially when i smash the pedal. I changed the pump and regulator same thing, one day i had a clutch slave leak and i realized i had a crushed fuel line before the rail ,so as soon as i went up in boost i was unable to supply the engine with fuel so it behaved like a limiter and at times just misfire. It appears your rail is running out of fuel, try running sequential injection and see if your fuel pressure is more stable




thanks for the hint, but the car is alreay running fully sequential fuel injection and ignition.

U have logged ur trigger miss counter ? Fp dose look very up and down but ur rpm looks abit unstable to me

With the sort of fluctuations you're seeing, the fuel pressure reg is a good place to start although I haven't actually seen too many regulators cause issue over the years. Ultimately they are a pretty simple device and even the Chinese copy cat manufacturers seem to get them right more often than not - This doesn't mean I'm an advocate for Chinese FPRs by the way :)

One thing to check is where the regulator is referenced to. If you're taking it off a single runner then you may see pulsations in the fuel pressure. It's important to have the reg referenced to the plenum so it has amore stable vacuum/pressure source.

As a test you could try disconnecting the vac reference entirely and raising the pressure to whatever you're seeing on boost for a single run on the dyno just to prove the point. This will have the engine run rich at idle and low boost if you don't make adjustments to the map to compensate though so keep that in mind.


For some unknown reason I've had issue's running 80 psi(550KPA) fuel line pressure as well, maybe this could be an issue.I guess some fuel injectors just don't like that much pressure, could upset there spray pattern.Is your base pressure 58psi (400 kpa) usually i start with around 42psi

I've also had the same issue with soft valve springs,Ask the customer about that too.Inlets bang in the inlet manifold,exhausts seam to make it miss under higher boost levels

Maybe the FPR? I've had an A1000 FPR produce similar results. Replaced the FPR with a Weldon and all is good. Does it hold pressure after the fuel pump is off?

This link was relevant to our situation... http://www.evolutionm.net/forums/evo-engine-turbo-drivetrain/403415-warning-all-those-who-use-aeromotive-fp-regulator.html

A little update - it wasn't the pressure regulator, but its vacuum source. I put a cheap plastic fuel filter in the vacuum line, and it was much more stable after that. still not ideal, but better.

That didn't cure the misfires though ... I'm still not sure, but I'm leaning towards the crank sensor or some sort of interference of the charging circuit.

I increased the gap between trigger wheel and the sensor (customer set it to 0.2mm instead of my requested 0.5mm), and just for testing purposes only I did two runs without the alternator belt on and it worked.

Next day, belt installed, no other changes -> misfired again!

And now for the worst part, the car died occasionally after a bad misfire and I wasn't too concered about it, but now it wouldn't start again.

ECU shows rpm during cranking, it counts trigger and sync signals, smells like it's getting fuel but all coils wont fire.

I was so upset and frustrated at once, that I took the car off the dyno and left it in a corner of the shop for the weekend ....

Sounds like you have your hands full with this particular car! Unfortunately it happens from time to time.

I'd say that your next step would be to get a scope onto the trigger inputs and see exactly what the ECU is receiving. Personally I like to have the gap to the sensor set around 1.00 mm. This should give sufficient signal level during low speed cranking to allow the engine to start well but provides enough clearance to prevent the sensor being hit by the trigger wheel. You'd be surprised how much the sensor and/or trigger wheel will move at high rpm and it only takes a gentle brush of the sensor to damage it permanently.

I should get a new crank sensor tomorrow, so I can test if it's the sensor. I also increased the gap to 1.1mm today ...

The customer put in another alternator over the weekend, as he wanted to do some body work anyways.

I'm now able to do ramp runs in 3rd gear with 1.4bar of boost to the set rev limiter, as soon as I put it in 4th gear though, it starts missing above 6200rpm.

Well, at least I found the culprit of the non-start condition - the ignition fuse was blown (Haltech flying lead wiring harness with their relay box). ... replaced that with a 20A one instead of 15A, now it doesn't die on me :)

Anyways, I did one test run in 4th gear with about 30%-40% throttle ... it built boost (about 1.3bar) and I could rev it out to the rev limiter.

I then did the same to about 5500rpm and then mashed the throttle open ... misfired like crazy.

Is the spark getting blown out? Seems like at a certain air speed and mass the coils can't keep up firing the plugs?

Plugs are Denso IK27 (gapped 0.4mm from the factory) and the coils are Bosch Motorsport PS-T (http://www.bosch-motorsport.de/media/catalog_resources/Single_Fire_Coil_PS-T_Datasheet_51_en_2777995659pdf.pdf).

I'm still scratching my head around this.

so, to make a long story short....

After a remote support session with the guys at Haltech, we found out that the injectors had too much resistance and also they were set up as low impedance injectors within the ECU.

To be perfectly honest, I've never touched that setting, nor did I check it when we got the car .... I assumed that the previous tuner did that part of his job correctly and the customer knew what injectors he had bought.

Turns out - don't trust anyone than yourself :)

We were able to get a better and cleaner result after some changes and let the customer off to the race weekend with 450whp at 1.5bar of boost.

ID1000s are on the way and we'll probably also switch back to the factory VR sensor on the crank as Dave from Haltech told me, that the GT101 Hall Sensor is not a good choice on a Motronic trigger wheel - which I thought should work well.

thanks to all of you guys for your suggestions and Support.

Good to see u got it sorted it's very easy to over look somthing like that

I've just had a hell of a time with a customers Evo which had been tuned before by a specialist, he came to me for bigger injectors and more boost.

The car had difficulty starting and a stutter on initial throttle tip in, the owner told me that's how both his and his friends cars came from the tuner, they were told it was something to do with using a later model power FC in an earlier model car.

It drove me insane whenever I had it in trying to move it around until he dropped it off while he was away for a couple of weeks. Turns out the throttle tip in, cranking and cold start tables were hugely over fuelling, quick trim and it behaves like a normal Power FC controlled car.

Take nothing for granted and trust no-one else's work lol

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