Heavy Misfire after WOT pull plus general hesitation while cruising

[BADD281]

Not sure if this is the right place for this question but it has to do a bit with a recent data log so I figured why not give it a shot...

I finally got around to datalogging the car in hopes to capture what it is that is going on lately. Basic run down of the car is built 2v motor, vortech, with Siemens Deka 60 lb injectors, twin GT pumps, dual fpdms, etc. Ever since doing long tubes it seems like the car just doesn't drive smooth while cruising. The car was retuned following long tubes and has continued to idle and run at WOT very well. While cruising it hesitates similar to an EGR issue where it feels like a very slight buck, BUT the EGR was deleted a while back and removed from the tune altogether.

Now moving onto the big misfire, took the car out today and did a WOT run up to about 6k rpm which was then followed by severe misfiring while trying to simply cruise at a constant speed. I managed to hold a constant rpm in 1st gear as the car misfired for a good 20 seconds. Taking a look at the datalog itself everything seems in order EXCEPT the Short Term Fuel Trim Bank 2, if you look at the pics below you can see while the car is misfiring its stuck at 1.25 and then when I press the gas pedal a bit it immediately returns to 0.991 and continues as normal.

Thus far I have done new plugs, COP's, fuel filter, o2 sensors, cleaned maf, and cleaned the air filter. No CEL has come up and I am absolutely stumped at this point. I really want to figure out the cause of the issue before throwing more money into a bottomless pit. If you guys can help me out a bit I'd greatly appreciate it.

 

[BADD281]

Clogged/dirty injector? It's very odd that everything is fine at WOT air/fuel wise but when cruising it has issues. Running out of ideas, the fuel pressure and duty cycle are normal so I know it's not pump related.

 

[gmcconn1]

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[1QUICK10TH]

Do a compression test, when you are in boost the blower is giving the motor " artificial' compression (best eat I can describe it). You maybe be looking at a cracked piston

 

[BADD281]

I doubt it's anything damaged on the motor itself, the motor has only 5k miles with very conservative tuning plus it has rarely been taken into full boost. The injectors were used when I bought them btw.

 

[1QUICK10TH]

Did you have the injectors flow tested before you used them

 

[BADD281]

They were not flow tested. My tuner including others pointed out that the 1.25 is actually a rich condition yet my wideband on that bank shows 17-19 when it's misfiring. Brand new o2's so it has to be something in the harness? I'd like to get the injectors cleaned anyway but I'm not really sure where to go from here regarding this issue. I'm going to try switching o2's to the other sides and data log.

 

[1QUICK10TH]

Sounds like an injectors issue to me

 

[BADD281]

I'm not sure if its injectors anymore since the upstream o2 is reading a rich condition yet the wide band right behind it is reading lean. Seems like a clear sign of 02 sensor issues or wiring. I'm going to datalog it again tomorrow with injector pulse width and o2 sensor voltages. Hopefully I can get it to misfire again following a WOT pull to see if the voltages are showing issues like the STFT did.

 

[iggster]

I'm not sure if its injectors anymore since the upstream o2 is reading a rich condition yet the wide band right behind it is reading lean. Seems like a clear sign of 02 sensor issues or wiring. I'm going to datalog it again tomorrow with injector pulse width and o2 sensor voltages. Hopefully I can get it to misfire again following a WOT pull to see if the voltages are showing issues like the STFT did.

Dunno if you are talking about stft at 1.250 but that means the car is running lean and the ecu is adding fuel to "try" and compensate for it.

Before wasting any more money, like suggested earlier do a compression test, doesn't matter how many miles or pulls the motor has it can go bad.

I was ignorant a year back when my car started to run rough saying to myself no way can it be the engine it only has 1,000 miles on it and not to many pulls on it. I put a new mass air on mine, new spark plugs and tried different tunes with NO luck. Till one day I got my head out of my ass and checked the compression and sure enough 2 dead holes. (bent exhaust valves)

Could be compression, could be injector/wiring/o2 sensor etc etc...

 

[BADD281]

Dunno if you are talking about stft at 1.250 but that means the car is running lean and the ecu is adding fuel to "try" and compensate for it.

Before wasting any more money, like suggested earlier do a compression test, doesn't matter how many miles or pulls the motor has it can go bad.

I was ignorant a year back when my car started to run rough saying to myself no way can it be the engine it only has 1,000 miles on it and not to many pulls on it. I put a new mass air on mine, new spark plugs and tried different tunes with NO luck. Till one day I got my head out of my ass and checked the compression and sure enough 2 dead holes. (bent exhaust valves)

Could be compression, could be injector/wiring/o2 sensor etc etc...

STFT is the opposite of LTFT. STFT above 1.00 is a rich condition whereas above 1.00 for LTFT represents a lean condition. The upstream bank 2 is giving a constant rich signal which is causing the ECU to lean out the cylinders, thus why my wideband on that side reads nearly 19.0 while the car is misfiring. I'll do a compression test but I highly doubt its anything mechanical at this point.

I'm going to datalog tomorrow and if it misfires again I'll keep an eye on o2 voltage. If bank 2 gets stuck on both STFT and voltage I'll know something is up with the wiring.

 

[iggster]

STFT is the opposite of LTFT. STFT above 1.00 is a rich condition whereas above 1.00 for LTFT represents a lean condition.
.

Who told you this? this info is wrong man.


Here I got this off the don lasota book.

 

[NoviGT2000]

Check your tune. Seems he went the changed the wrong variables in your tune and leaned it out more instead of fatting up the tune a little to compensate for your long tubes. Common mistake I have seen in some tuners.

 

[iggster]

Check your tune. Seems he went the changed the wrong variables in your tune and leaned it out more instead of fatting up the tune a little to compensate for your long tubes. Common mistake I have seen in some tuners.

Sounds like this could be it, Also when changing to long tubes you have to change some settings in the o2 heaters circuit.

Even then if the maf table is pretty dead on it should still run good without the adaptive working (if the o2 isnt getting hot enough to go into closed loop)

 

[BADD281]

It was the injectors, switched banks and the stuck STFT followed them to bank 1.

 

[Abilor]

Problem solved?

 

[BADD281]

Well the problem is back. I went ahead and had the injectors cleaned which did not fix the issue. I then bought a new set during black friday and it seemed to fix the issue until this afternoon. I was cruising on the freeway and reached a point where I was going up a grade when it misfired once ahead. This time it did not cause my wideband to go nuts proving the stuck stft moved back to the passenger bank. The original misfire was on the passenger bank, after switching the original injectors side to side it moved to the driver bank, now with the new injectors it is back to the passenger bank. I am really not sure what the hell is going on at this point. I have replaced pretty much everything at this point. Might yank the engine harness and inspect it for anything obvious, other than that I am stumped.

 

[BADD281]

I don't think a mechanical (engine) problem is causing it, the chances of it shifting banks like that leads me to believe it was injectors but we know that cannot be it. Another thing is if it is wiring related how did it switch banks like that when the 2 upstream o2's are on 2 different harnesses? (Engine/injector harness holds one and the tranny harness has the other). A buddy of mine who works at a local speed shop thinks it may be maf related, I am going to datalog again with maf voltage to see if all is well in that case.

Any more ideas?

 

[BADD281]

Compression test checked out just fine. Changed the plugs again and confirmed a 0.028" gap on all 8 before installing. Datalogged once again and this time it misfired on the passenger bank. Tried to hold the misfire long enough for an engine light to come on but nothing. Here is a snap shot of the misfire since I cannot attach the datalog itself. Watching the lap top, the misfire values never went above 2 and there very few even showing up during the misfire. When the misfire began the actual misfire value showing up on the snap shot was on the opposite bank so I assume they mean nothing.