Misfire when let off throttle?

[chad99]

So all this recent posts of misfires have caused me to notice some strange things with a 1.8T 6speed b6 sedan I have owned for a month or so.

When I check blocks 014 015 016, I find zero misses when engine just starts from cold, zero misses while idling, and zero misses while pressing and holding down the gas pedal (steady state or WOT) but as soon as I let off the pedal I get one or sometimes two misses registered, this is repeatable almost 95% of the time (this is all while stationary).
So far I have felt only 2 of those misses but that is in about 1000 miles of driving (since I have owned it).
No codes stored in ECU as read from vagcom, no CEL.

This seems strange to have a misfire off throttle, does this classify it as a afterfire/backfire? but I never hear any pops that sound like a backfire.
At idle car pulls a solid 21inhg
Stock no tune
Boost hits 10ish and holds fine
I have replaced coil packs with a red OEM 2.0T used 10k mile set I had laying around.
Replaced coil pack harness to the back of the motor
New iridium plugs the -11 ones .044 something like that
New front 02 sensor Bosch
New OEM fuel filter
Used N249 connected (old one nipple was broken off from PO doing a N249 delete)
No drop in coolant levels in the 1k miles I have driven it or any of the typical headgasket issues, no smells, no hard starts

Since reconnecting the N249, LTFT most of the time runs at +3 or 4, sometimes +5 but have not seen it go any higher, STFT is anywhere from -0.38 to 0.00, and I do actively monitor it on every drive as I do my AFR.

I also notice the intake noise at idle is louder than my 1.8T B6 avant, both cars have factory intakes with factory insulation in the boxes.
Maybe PO punched out the cat?, cat looks factory from the outside as do the welds, car really sounds like it has an intake even though intake is all factory, exhaust back boxes are factory not sure about the rest of it.

Other than that the car drives nice, great MPG, easily hits 23 24 MPG even with 60% city driving with SF bay area stop and go driving.

[CardFan]

Could be a vacuum leak. Try to do a boost leak test. I'm having the same problem, misfires on decel, but I haven't got around to testing it yet. I have a Motoza tune, and it's been tweaked enough to eliminate everything but a vacuum leak. Car runs fine otherwise, which isn't helping with my procrastination.

[chad99]

How do you know it's misfires?did you get a cel, could you feel it or were you just checking measuring blocks for fun like me.

Above I mentioned I'm pulling 21inhg and my STFT is - or 0 so no leaks.

If I can't come up with any other ideas I might boost test it, I did test it when I got the car a month ago and only found a leaky SJP, which I replaced.

[5ktq]

Bk6reix-11, you mean? I think you should use the plain variant, with 0.8mm gap. The -11 is 1.1mm, maybe too big. though i kinda doubt this is your problem, either.

[Detroiter]

Bk6reix-11, you mean? I think you should use the plain variant, with 0.8mm gap. The -11 is 1.1mm, maybe too big. though i kinda doubt this is your problem, either.

With his 2.0T coil packs he can run the larger gap without issue.

Wouldn't hurt to try some cheap bkr6e's with a smaller gap just to rule it out but I'm sure you're fine since I have stage 1+ tune and am running a .05 gap with no issues.

[CardFan]

How do you know it's misfires?did you get a cel, could you feel it or were you just checking measuring blocks for fun like me.

Above I mentioned I'm pulling 21inhg and my STFT is - or 0 so no leaks.

If I can't come up with any other ideas I might boost test it, I did test it when I got the car a month ago and only found a leaky SJP, which I replaced.

Measuring blocks. It's showing up in the logging of data I send back to Motoza. I've changed plugs and verified the gap, changed coils (upgraded), fixed some vacuum leaks, and Dave at Motoza has tweaked the tune, but the problem remains. I'm getting the same vacuum reading at idle on my boost gauge also, and none of the other data in the logs suggests a vacuum leak, hence all the other changes above. I just haven't gotten around to leak checking it again.

[chad99]

Measuring blocks. It's showing up in the logging of data I send back to Motoza. I've changed plugs and verified the gap, changed coils (upgraded), fixed some vacuum leaks, and Dave at Motoza has tweaked the tune, but the problem remains. I'm getting the same vacuum reading at idle on my boost gauge also, and none of the other data in the logs suggests a vacuum leak, hence all the other changes above. I just haven't gotten around to leak checking it again.

Ahh got it, really sounds like we're in the same boat.

I guess I'll boost test it this weekend

[BARRY]

sounds like a vacuum leak. i'd confirm this as well as all your plastic check valves to confirm they are not faulty.

additionally your spark plug gap is too wide. with all coils besides Okada Plasma Direct i'd blow out spark at full throttle with a wider gap as you have.

[chad99]

With his 2.0T coil packs he can run the larger gap without issue.

Wouldn't hurt to try some cheap bkr6e's with a smaller gap just to rule it out but I'm sure you're fine since I have stage 1+ tune and am running a .05 gap with no issues.

I might give this a try too, the thing is though my misfires are off throttle it never misses while accelerating.

One thing I noticed is my rear CTS has the temps up to 13 deg F different from each sensor, by that I mean the ECU temp and the cluster temp are up to 13 deg F different. I know the sensor had two different sensors inside but I just remember my other car would read something closer along the lines of 2 or 3 deg F difference max. And it seems my oil temp is closer to my cluster temp and not the ECU temp, so maybe the ECU is getting the wrong temp?

Even then a 13 deg F max incorrect value, should that cause that much of an issue?

Clearly not enough for the ECU to think out of line yet since no codes.

[chad99]

sounds like a vacuum leak. i'd confirm this as well as all your plastic check valves to confirm they are not faulty.

additionally your spark plug gap is too wide. with all coils besides Okada Plasma Direct i'd blow out spark at full throttle with a wider gap as you have.

I have so far replaced 4 check valves with the US plastics, none of them were bad or leaky they just didn't look 100%

And my misfires are only right after you pull your foot off the throttle, never while on throttle.

I will do the boost leak test this weekend and I also have an used 50k mile MAF i pulled off a b6 awhile ago, I might clean up and swap in if I'm still out of ideas and since boost testing is only two screws away from replacing the MAF

[BARRY]

suction jet pump is intact? my current b6 had its SJP separated at the seam...was very hard to locate, but did cause misfire upon steady throttle or at lift throttle. was random. the updated one isn't glued, but screwed on.

ya let us know how the boost leak test turns out!

[chad99]

suction jet pump is intact? my current b6 had its SJP separated at the seam...was very hard to locate, but did cause misfire upon steady throttle or at lift throttle. was random. the updated one isn't glued, but screwed on.

ya let us know how the boost leak test turns out!

New OEM SJP about 1000 miles ago

I will keep this thread updated

[chad99]

Just a little update.

I had some free time so I swapped in that low mile cleaned MAF and an extra silicone tip I had laying around (old rubber tip had a ton of what looked like oil blow by on the turbo inlet).

Drove her for a 100 something miles and noticed the STFT went to 0 and is now at +0.19 and LTFT dropped to +2.XX and now will only reach a max of +4.65.

Measuring block still says misfires 90% of the time as soon as I take my foot off the throttle, of course still no misfires when on the throttle and no CEL or stored codes.

I pulled my car on ramps to do a leak test today, then realized my trusty old little 5G harbor freight compressor had some how broken itself in storage, i think I can fix it, if not I'll just pick up another one during the Labor Day 25% off, then I will run a leak test again and update further.

Prior to replacing the MAF and tip, I took her on a 250 mile road trip for work and she averaged 30.4MPG (measured from gas receipt not DIS, I think DIS said 34MPG something) at 75-80MPH

[customa4]

Just bring it back to Harbor Freight. I'm sure they'll work something out with you.

[old guy]

Misfires are calculated by measuring the engine speed fluctuations. The torques pulse between firing events is evaluated by metering the time between two ignition events. The assumption is that a change in engine torque will change the engine speed. So essentially anything that causes a change in the engine speed between two firing events (outside of the expected parameters) is going to register as a single misfire event. Single events are registered but for the most part ignored. Once you begin to get multiple events you will get a CEL.

I'm going to do a little speculation here. Since you only get a single misfire count and it only occurs when going off throttle you may have a mechanical cause for the change in engine speed between two firing events. Maybe a clunk in the dual mass flywheel when going from loaded to unloaded?

Whatever it is I really don't think it should be cause for concern if it is only a single event, you aren't getting backfires and it only occurs when going off throttle.

[chad99]

Just bring it back to Harbor Freight. I'm sure they'll work something out with you.

Haha I have seen people do this but I would feel too guilty, the compressor paint is all faded and more than a few years old. I was able to fix it anyways so no big deal, but right now I'm too lazy to pull the car back up on the ramps to check.

[chad99]

Misfires are calculated by measuring the engine speed fluctuations. The torques pulse between firing events is evaluated by metering the time between two ignition events. The assumption is that a change in engine torque will change the engine speed. So essentially anything that causes a change in the engine speed between two firing events (outside of the expected parameters) is going to register as a single misfire event. Single events are registered but for the most part ignored. Once you begin to get multiple events you will get a CEL.

I'm going to do a little speculation here. Since you only get a single misfire count and it only occurs when going off throttle you may have a mechanical cause for the change in engine speed between two firing events. Maybe a clunk in the dual mass flywheel when going from loaded to unloaded?

Whatever it is I really don't think it should be cause for concern if it is only a single event, you aren't getting backfires and it only occurs when going off throttle.

Well that makes me feel better coming from you old guy, I keep telling myself "it can't be that bad if the car never complains".

I might still leak test it latter. I will keep an eye on my LTFT to make sure they don't creep up.