servohead

2002 Z06, 94000 miles, stock exhaust, mild cam, previous owner had it tuned, engine is a fresh rebuild due to spinning a bearing while racing and has a new crank but stock except mild cam. New NGK spark plugs, MSD wires all measure 24-35 ohms.
- was told the air injection system had been disabled when tuned. So we left it blocked off.
- We bought the car after it sat for 2 years with the engine out.
- We got the car running and on the first drive trying to accelerate basically at ALL it would backfire badly. However idle was stable and can be revved fine not under a load.
- checked the fuel and it looked like urine or apple juice. So I purged all the old gas out jumping the fuel pump relay into a jerry can. We added 5 gallons of premium gas.
- the car began running much better but still it would miss at partial throttle accelerating.
- engine codes? I’ve gotten a code for the air injection oddly even though it was “deleted” I had a code for the left bank HO2S heater, it’s been replaced with a new Bosch sensor. Oddly I’ve not seen any codes yet for fuel trim or misfiring through the instrument cluster.
- I drove the car on a trip gingerly with the tank having been filled with 93 and a Bottle of Techron. Still after that entire tank, anything more than 1/2 throttle the engine misfires. In 6th gear for example at 1500 RPM And just easing in the throttle, it’s begins to break up.
- (on that trip P0153 HC set for right bank front O2 slow response now which is interesting) not sure if that’s related here or just a faulty sensor.
- I’m beginning to think it feels more like ignition but then I had found such dramatic improvement with new fuel but this condition remains.
- I haven’t checked yet for intake leaks but felt that all went back together properly, but I know that’s something I need to consider.
Anyways… this whole situation is driving me kinda nuts… any help you guys may have with situations like this would be great. I know this was long but I wanted to front all the info I could think of.

-Steve

C5 Diag

Without a scan tool and a scope you'll only be guessing my friend...your situation is not unique so the best thing I could say is take it to a diagnostic shop and pay for an hour or two of diagnostic time and let them make an quick and accurate diagnosis...since the car sat for a long time I would remove the injectors and have them cleaned and reinstalled...you will not see any DTC's for "fuel trim" but a P0171/P0174 for a lean condition...and yes that P0153 should be addressed.

servohead

Just to update - I was able to confirm 58psi fuel pressure constantly at idle and driving and when it breaks up partially loaded.

C5 Diag

This does NOT mean you may have some clogged injectors !!

purrvert

Just a guess, had same backfire problem minus the bad gas after changing plugs, turned out to be a cracked spark-plug insulator.

servohead

I absolutely agree man… just looking at and compiling all of my data at this point.

servohead

also who do you recommend to send the injectors to to clean?

Smoken1

Try FIC injectors service
https://fuelinjectorclinic.com/Servi...-Cleaning-Port

KOZ

So many variables that you cannot answer without a proper scan of the engine function. And that is not just 'pulling codes'.

servohead

this week I’ll be able to use the Tech 2 and figure this beast out.

servohead

So, I found the problem… almost assuredly. #8 plug is absolutely cracked in half. It was either damaged when I received it and I didn’t notice it or the plug wire was fitting loosely on there and overheated it. This whole time I’ve been telling myself the car was misfiring… I should have believed myself before way over analyzing things.
What is really STRANGE is that the tech 2 showed NOTHING, not one misfire at all, and never set ANY misfiring codes at all.

I still need to sort out the P0153 but I’m thinking at this point it’s just a bad HO2S.

I did check the intake for vacuum leaks and found nothing at all.

thanks everyone for your advice!