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100% reproduceable stumble @ given RPM / load

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looking for a little bit advice here.

The setup:

VW 2l 16V, GTX3076, full sequential injection, wasted spark, ID1000, MaxxECU Race (previously KMS MD35, same issue), E-throttle, NKG 7's plugs.

The issue:

at pretty much exactly 2350 rpm and 100 hpa (at 20% ish throttle) the car stumbles. At the same time the lambda reading goes lean to 1.15 and it looks like it's lean. I can dump as much fuel into it as I want, VE map around 90% where it should be maybe 70%. Leaning it out to 70% VE makes the issue worse to the point where the map sensor picks up pressure spikes to 135 kpa in the manifold.

There is no way I can tune around that. In my opinion it looks like a misfire which leads to the lean reading by the LSU4.9 but actually is pig rich, just not combusted properly.

The intake manifold pressure also feels a bit high to me at 20% throttle already above 100 kpa. The owner says the cams are timed correctly (whatever that means really).

2350 rpm and 100 kpa is always reproducible. Annoying as you drive at this rpm a lot around town. Under full throttle etc. no issue at all. Car runs really nicely.

I attached a screenshot of the (rough) fuel map while we are trying to sort out the issue.

Also attached a log file.

What does not make a difference:

- turning lambda control on/off

- changing injectors, plugs

- changing dwell time of ignition module (VW Polo 6N wasted spark)

- changing fuel pressure

- switching from batch fueling to sequential

- using different intake manifold

- changing ECU. we ran KMS MD35 before and replaced it by MaxxECU race. no significant effect

Maybe we are blind as to what the issue is. So far, nothing we tried alleviated the problem.

Thanks for your suggestions.


Attached Files

I bet you have some resonanse issues in the fuelrail. Do you by any chance a fuel pressure pulse damper on the car? If not, try one. Radium got a few aftermarket ones. You can also try to change the lenght of your vacuumline to your fpr

Good point. No, the car hasn't one, and I had suggested it to the owner as well.

Will bring it up again.

Another guy mentioned that it could maybe be related to valve springs. The engine uses Schrick cams and the according springs, however the valve seats are also modified to the guy thinks that the springs may not have enough pretension.

Sounds like a long shot but we will keep it in mind.

It's probably cheaper and easier to put a pulsation dampener in the fuel system than it is to test to see if it is necessary. For example, high speed pressure transducers can cost many thousands of dollars/Euro and so do such acquisition systems.

Might be an electrical misfire - this can be a bit tricky to trace as it will show up as being 'lean' at that point and adding fuel will not help and may even make the 'lean' indication worse if it goes into a rich misfire - remember, the lambda is responding to the OXYGEN in the exhaust gases and if the fuel isn't being burned and using up the oxygen, it will appear to be lean.

Could be some sort of weird resonance condition that causes it to go over rich on one, or more cylinders - next time you're testing, fit some new plugs and run it at that misfire point for a bit then cut it clean and check the plugs - you may even see them shiny with fuel on them.

Check all the connections and, if it uses leads or even a dizzy cap, theck they are in good condition with acceptable resistances. You could also try adjusting the spark plug gap.

Thanks for your reply.

We changed plugs and the condition was exactly the same. In fact, the log is from a drive with new plugs.

They, as the original ones, looked rather white-ish, at least on the light side of brown.

The wiring hardness is completely newly made and made sure the quality is fine.

Ignition coil and leads will be tested, yet we think it is unlikely.

We are on the resonance train ;).

Also, what is the consensus regarding having 80-100 kpa at 2500 rpm and 20% throttle opening? Feels a bit high to me, to be honest.

Don't need much throttle at low rpm. If it is port injection and timing is late due to high load low rpm the wasted spark could actually be causing partial ignition on overlap. Can you advance the timing without it knocking?

The owner of the car got the recommendation by the guy who he has the head from to replace the hydraulic lifters by solid ones for a test. Not sure if this will lead to anything.

Timing can be advanced further. When I get back and the engine is still running I will give it a go. The explanation with the wasted spark causing partial ignition sounds also quite plausible.

Fuel damper still no1 in terms of probably though. I would be doing ignition as it is cheapest and easieat then damper if it didn't change before messing about with valve gear.

Let us know how it works out.

I'd also be trying a fuel pressure damper. We've seen some weird issues with our 350Z in particular as a result of fuel pressure pulsations.

I agree with you both, yet the owner tends to not listen and rather make a big mess instead of following the low hanging fruits.

I found long ago that if the customer isn't prepared to listen and follow sound advice then sometimes it is better for a shop to fire that customer. Otherwise you get into a situation where you waste a lot of time that inevitably doesn't get charged out and the customer will still blame you for any issues.

I totally agree about firing a customer who doesn't listen.

In this case, it's a friend who I help out.

And guess what. I spoke to him once more to try a fuel dampener and he promised to do so.

They rigged something up using mk4 Supra parts and: car runs fine!!!

Maybe he has learnt now who to listen to.

Thanks to everyone who chimed in on this!

Glad to hear you finaly got him turned. To me it has become almost a monthly routine to try to explain that the oems are not stupid and the the dampers are not there by mistake but are infact put there for a reason.

We usually reply within 12hrs (often sooner)

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