ForceFedC5

TT C5 tuned via SD developed a noticeable miss. Engine shakes slightly more than usual at idle and continues to 2k where afterwards its a bit harder to detect. Lost 7mph in the 1/4.

Tested the car while running by removing each plug wire one at a time. Each time the miss got worse. Did the same things with the injectors with the same results. Pulled all plugs which look ok and performed a compression test. All is well.

My mechanic friend thinks its either a sensor (no codes) or a vacuum leak which we looked for and couldn't find. Turning on the misfire option in the ecm also didn't show any noticeable misfires.

Any ideas would be greatly appreciated!

And thanks in advance!

stevieturbo

Pull injector plug and keep an eye on engine vac to see how it changes and runs each cylinder.

Compression test....what are the results ?

Leakdown test.

Ensure all valvetrain are sound.

If it has this problem at idle, what are lambda readings at idle on each cylinder bank ( open loop ), or better still a 4gas analyser results for each bank.

ForceFedC5

Thank you very much for the prompt reply!

Compression test was inline with previous ones:

1 147
2 139
3 143
4 140
5 149
6 145
7 150
8 148

I'll try your other suggestions and respond in this thread. Thank again!

realcanuk

Would check for broken valve spring.

BLOWNBLUEZ06

This was my first inclination. Is it auto or manual trans? Either way, get a infrared temp gun and shoot each primary but do it with the engine loaded. Set the e-brake and power brake it or load it with the clutch lightly. Check the temp of each primary. The low temp cylinder will be your culprit.

Pekka_Perkeles

If it is tuned by HPTuners, I think 2/3 bar SD CoS won't support any misfire codes (P0300...P0308) no matter if they are active or not in the DTC list.

ForceFedC5

They actually will, I've used it before when I previously had a head gasket issue.

New plugs and wires, same issue. Going to either get fresh e85 or through some 93 in it and see if that helps. Maybe something with the e85? Ethanol content is about right at 82%...

Thanks guys!

ForceFedC5

Update:

I used a combustion leak tester to test for a blow head gasket:

https://smile.amazon.com/gp/product/...?ie=UTF8&psc=1

Performed the test twice when she was warm and the color didn't change.

Next I used my friends infrared tester as suggested. The car is an auto, but unfortunately I was alone so I couldnt load the engine. At idle and after reving it up for a bit, #2 cylinder was not as hot as the others, by about 30 degrees. I'm not sure however if this was because its getting air flow from the fan. I blocked that off best I could and it was still running colder....

Next I switched the coil packs between 2 and 4, no difference, switched injectors from 2 and 4, no difference.

Guess I'll spray around with carburetor cleaner since it's the next easiest test before I have a leak down test down.

Stupid question, would a cylinder that does great on the compression test, and misses once in a while, be caused by excessive leak down?

Suggestions and comments are greatly appreciated!

stevieturbo

Problem with hydraulic lifters...is it can easily mask some problems to make a compression test look ok, and a leakdown test has limitations too.

Again, a 4 gas on each bank would highlight what sort of misfire if any, and you've swapped the injector and coil pack about cylinders....but try just swapping injector wiring.

Even firing at the wring time, it will still allow fuel into the cylinder. Or have you verified good signal to the injectors ?

realcanuk

Did you check the valve train yet?

ForceFedC5

Hopefully without sounding too stupid, I believe my LG 3" exhaust mixes the exhaust gases in the mufflers so I wouldnt be able to test there? I've also noticed that the ECM compensates up to 30% for a rich or lean condition at idle, wouldnt both of these effect the results?

How do I check the signal at the injector?

No yet. I plan on pulling the pass side valve cover soon, it's the easiest. Should I be visually able to see a broken spring?

realcanuk

You should see if one is broken if you look carefully. The inners can be hard to spot but look with a light and you can see.