Flow

I bought a SS a few weeks ago, SC, and it drove great for a day, now it's having issues.

Issues:
Misfire at idle, noteable, seems almost in series but it's not, random. Poof - poof poof - poof poof poof - smooth for a little then - poof poof - poof.

No detectable misfire under very light load.

Sometimes but not all the time misfires hard past (25% give or take throttle) misfires light some of the time - runs like a bat out of hell 5% of the time. When it's running good it will run good for a few pulls then back to poop.


Things I've checked.
- New plugs - NGKs for the Stage 2 package, 4644s?
- Replaced a coil, went down the line, coil didn't change anything.
- Intercooler pump is operational.
- Checked all vacuum hoses, replaced worn hoses, checked gaskets. No leaks detected.
- Intake manifold is free of coolant from leaking laminova cores.
- Replaced MAP, MAF, SCIP.
- New throttle body 11-14-15
- Compression is 190 +/- 2lbs across the board
- Short term fuel trim is 0 to -7 long term fuel trim is 0 to -20.7

Codes I have gotten once and have not returned.
SCIP circuit low
MAP malfunctioning
TPS voltage high
TPS voltage low

Codes that I get that are returning
P0420 (exhaust, I have no cat)
P300 (always stored through)


Areas of concern for me is o2 sensor? It seems dumb but I'm watching it on the scan gauge and it reads .0XX to .1XX two or three times and .9XX once. The car is definetly rich at idle and when it misfires under load with throttle it blows black smoke.

Other than that I'm pretty lost on what it could be and I'm all ears.

Flow

By the way, 3.1" pulley and getting 17 psi all the way till 7k sounds a little off, not sure if that helps at all.

Sharkey

iTrader: (1)

when i was trying to help in your last thread we were talking about stage 2 and thought you had stage 2 injectors. i see in your mod list you have 60lb injectors. my theory was that you had the stage 2 injectors (42lb/hr) with the stock tune (designed for 33lb injectors) but you confirmed it had the stage 2 tune with the 7000rpm redline, so that tossed that idea out the window.

im going back to that theory. im thinking you have 60lb injectors on a tune meant for 42lb injectors. it makes complete sense, the black smoke, misfiring on all cylinders. i dont think the o2 sensor is a problem, although with the age of the car it wouldnt hurt to put one in.

i dealt with a similar issue with a supercharged corvette. customer installed and flashed it with a supplied tune, was missing, blowing black, and fuel trims at full rich. turned out the kit was shipped with larger injectors than the tune was designed for.

Flow

That did cross my mind as well. I made a new thread for the location and extra info that isn't all jumbled.

Like I said the car will pull really well on occasion. I'm 99.99% sure I found the original owner on here (same issue) and he said it started missing right when he installed the header and cat pipe. The reason I think it's the original owner is the over spray matches in the pictures from the car being repainted and the injector harness matches the one that was on my car (notably messy). Here is his thread, let me know what you think.

https://www.cobaltss.net/forums/prob...p-mode-320764/

60lb injectors on a tune made for 42lb injectors, along with the exhaust changes might be throwing the trims off just enough??

Thank you for all your help, again!

Flow

Replaced the o2 yesterday, still misfiring, but here was the old sensor.

Flow

Put new 60lb injectors borrowed from a friend and it was no fix to the misfire getting into it. However the misfire at idle is gone.

Flow

42lb injectors went in today, still missing bad when I get into it. On the scan tool car looks rich until I get on it then it leans out.

RonSS

Too bad you don't have access to HP Tuners and could get a real log of what's happening.Scan tool will show trims, but trims aren't active at WOT.
For that you need a wideband.

What does #1 plug look like lately?

That O2 sensor looks like it was put in with a hammer........

Flow

It just rapid misfires. It mostly does it going into WOT but sometimes in between gear shifts when driving normally however they're much lighter. I can build boost, it will get to 10-15psi before misfiring but never makes it far into power. Obviously boost builds very quickly being roots blown and by the time to boost comes up the car misses. The spot where it misses almost seems to be the same spot on the pedal, like the pedal has a dead spot from 25% down and on. I'm running out of ideas. Possibly fuel related since the misses feel worse on smaller injectors?

No codes besides cat since switching to the 42s 500 miles ago. No p0300 but the car still doesn't run, no nothing other than p0420.

Plugs aren't telling me much, they all look nearly identical, and are not showing a rich or lean condition.

Right now im just pulling sticks and thinking possibly it's tuned for e85 and I'm running 93 in it, was thinking of putting a gallon or two of race gas in to see if the issue goes away. I'm lost boys. I'm fairly decent around a car and I'm just puzzled, and I'm sure it's something stupid.

Maybe a tune? I personally believe the car has the GM stage 2 tune on it seeing as the car has the pulley and other things. The previous owner stated the issue started with the header when he put the header on the car. Would a GMS2 tune be able to handle an intake and full exhaust without needing to be retuned?

RonSS

I tune V8 cars, I haven't messed with my Cobalt yet.
Intake on MAF equipped cars usually requires some tuning.
Exh not quite as much depending on the header type used.
Just typical things we see.

Race gas won't change a thing.

If it was tuned for E85, and you have 93 in it, I'd expect fuel trims to be way off.

Shotgunning injectors is a crap shoot at best.

I'd crawl under the car and see what wiring the guy could have messed up when he installed the header. Judging by the bent O2 sensor, anything is possible.

A tune would be a good idea as the tuner is going to have to troubleshoot the issue it has now before he attempts a tune.

mrbelvedere

iTrader: (3)

I would get the tune looked at and retuned and post pics of the plugs

Flow

Well, I figured it out or atleast 99% sure I did, haven't had a miss in over 24 hours of ******* on the car. Boost levels are normal and not stupid high now. I did order a PCM from ZZP so that will be here on Friday, not sure what the tune is on the car or who did it, so I'll just start with a canned tune from them.

Anyways it ended up being the purge valve. The connection from the fuel tank to the back of the purge valve was leaking. The plastic clip to hold it in is worn, so I guess under boost it was sliding out and causing the car to miss.

I cant tell you how dumb I feel. I knew it would be something simple, just wish it didn't take me a month to find it.

So im finally able to give a full evulation to my cobalt. So far even broken I liked it quite a bit more than my focus. The focus makes a little more power though so I'll have to put them against each other and see what's up!

Thanks for all the helps guys!

steelmesh

I chased LTFT more than once in the first year I got my Solstice LE5 which had an early gen werks turbo kit on it that needed maintenance because aftermarket turbo kit. First time was broken exhaust manifold studs and second time was cracked leaking exhaust manifold. Things expand and contract where certain conditions cause leaks to spring up or change throwing actual real fueling off making economy and driveability issues. Okay a bonus example is my transition in the MAF Low and High airflow tables (for MAF based fueling) at the same MAF sensor frequency had different airflow values, causing a "bad spot" hesitation.