Misfire Problem

[Martin]

Ive got a problem in a Single butterfly twin turbo V6. It misfires at certain RPM`s and it sounds just like a coil that cannot keep up under boost.

The misfire is always at the same RPM and repeatable on run after run on the dyno. The misfire would occur ot 0.2bar boost at 3500Rpm and again at 0.6bar at 4200Rpm.

This engine setup with the same coils have produced 700+ WHp at 1,4bar boost with no misfiring.

This problem showed up after an engine rebuild.

What ive checked.....
Changed ECU
Changed Igniters
Checked wiring from ECU to Igniters
Checked wiring from Igniters to Coils
Logged Coil power supply at 200Hz to see of coil power dipped.
Earthed Coils to the head with separate wiring
Earthed Coils and ECU directly to Cyl Head and Battery negative ( Tappet covers ceramic coated)
Changed all 6 coils
Changed all the spark plugs
Gapped plugs down to 0.4mm
Checked mixtures,
Checked MAP sensor at 50Hz for fluctuating reading
Disable Accel Enrich/Enleanment
Disable Decel Enrich/enleanment
Checked injectors for open circuit errors and short circuit errors.
Injectors Flow tested before engine build
Checked wastegates to make sure they open simultaniously
Changed Crank Sensor (no errors on the ECU whatsoever in any case)

What could this be? Could it be mechanical?


Could the ECU maybe not trigger errors under cerrtain cicumstances?

[AaronM]

Hello Martin,

Does it only misfire when ramped or can you hold it at the particular rpm with constant misfiring?

[Martin]

When you ramp it at full throttle it always occurs at the same RPM.
when you hold it steady it also misfires.

This setup worked at 1.4bar with 3,2ms Dwell at 14volts
Ive upped the dwell to 5mS with no success.


Running Motec M600, Ive changed the Ref sensor even though there were no Diag`s at all.

Ive done a leakdown test but the figures were very good.

Strange thing is that when I run in on the Dyno each It happens at the same RPM time after time. The first one at 3500RPm is at very low boost (0,2bar). The coils held okay with 1.4bar boost and 850Nm at the Hubs before.

What did change were the turbo`s after one failed a while ago. They are 2 GT28`s Not sure of the trim and A/R

have suspected the wastegates to open unevenly, but I opened them fully yesterday and ran the car and it still does it. If anyting its worse

1.Tried NGK R`s, Same thing.
2.Also tried Bosch Plugs, no difference.
3.Disconnected wastegates, problem still there, may even be worse.
4.Changed to Denso "Toyota 1UZ-FE" double ended coils with internal igniters. I wired the coil triggers directly (separate from harness) to ECU and also the power and earth separate from harness (complete new coil harness) and the problem is still the same.
6. I tested the Injectors at RPM and fiddled with the wiring to see if I could get the Injector pulse to break down, no success.

Any ideas?


Could the ECU maybe not see certain problems from the Ref/Sync sensors?

[SprinterTRD]

Could it be a Valve spring problem? At certain RPM, boost, ect your getting valve flutter?

[Dragon]

Did you try richening up the problem rpm/load areas?
Do you have one lambda sensor per cylinder on the dyno and see which cylinder (probably not all) is giving trouble?
I have seen some inlet manifold designs which need over 10% individual fuel trim at places.

[DarrenR]

Hi Martin,

It is extremely unlikely to be a ref sync problem when it is not reported as an error, altho i wouldn't rule that out completely.

It sounds to me you have done everything ignition wise and enough fuel wise to say they are not the problem. I'm with SprinterTRD, look at the mechanical side, a floating or sticking valve most likely, especially since this problem started after a rebuild.

Can you figure out which cylinder is giving the problem or is it multiple cylinders? It looks from the logging it is only happening on one bank??

[Martin]

Darren,
If you look at the logfile of the ramp you will see that it first happens one one bank and then on the other. 3 three files ive loaded are one steady state, and 2 separate ramp`s. Look at both ramps and you will see the repeatability of the misfire.

Dragon,
Ive tried a +5% fuel trim and a +10% fuel trim and it would not go away.
No I only have dual lambda. This whole setup has run correctly for years and made 500+ Wheel Kw`s at 1.4bar.


Sprinter,
Valve springs are new, and are OEM springs. After it misfired the first time then engine was rebuilt (Heads were stripped down bare) again (using same valve springs) but they were shuffled around. Problem still there. We did to a leakage test, all ok, but I do understand that a low seat pressure will not show in a leakage test. This an Alfa V6. Any suggestions on what to check for and how to check it without another engine removal, rebuild?

[Dragon]

Where is ECU earthed?
Did you try running a separate earth cable from engine block to battery direct?

[Scott@FP]

Do you have any solenoids (VVT, boost etc.) on outputs not equipped with a recirc diode? Try disconnecting them or turning them off if possible.

Chased my tail recently with a similar situation, the high voltage spike back thru the output during the solenoid's high DC periods would result in a very bad misfire that was intermittent in nature and over a period of time damaged the box, but the first few hundred hours it was fine, it was a long-term cumulative damage deal.

[Martin]

Scott,
There is only a boost control solenoid, 6 injectors, and 6coils. Fuel pump and fan relay stay energized the whole time. I have run the car with the boost control solenoid complete diconnected (Electrically) and it was the same, but thank you very much for the tip, I didnt even think that that could have been the problem.

Dragon,
ECU earthed on the Cylinder head.No Delta Bat errors. Ive connected the Cyl head directly to batt- aswell to see if it does anyting for the problem but it did nothing. If I cant explain a strange phenomenon involving electrics on a car I usually look at earth problems, and most of the time you`ll find it there.