Thread: SS M90 misfiring due to tune?

I have been running an Intense canned tune for my SS M90 2001 Grand Prix GT. It's been okay, but there is a few problems with it (rough idle, fuel gauge won't read etc.). So I decided to get a ZZP PCM designed for the SS M90 kit. I put it in, and was happy at first, as my fuel gauge was reading correctly. I then started the car and got a random misfire code (P0300). When I scanned each cylinder, I found cylinder 2 was misfiring bad (especially at idle). I know it's not the spark plugs, plug wires, coils or ICM because it works fine with the Intense PCM. ZZP keeps telling me to check the plugs etc. How could the tune be causing it to misfire? I don't know anything about tuning, so I'd like to try explain possible scenarios to them. Thanks.

Originally Posted by Bio248

how low is your idle set?

Idles between 650 and 750.

650 is low, im idling at 850.... and i have stock cam.

Originally Posted by matt5112

650 is low, im idling at 850.... and i have stock cam.

It's usually around 750. But it drops down to 650 at the lowest.

If the cylinder is missing in Open loop fueling(before the car warms up) then its pretty normal in ss m90 kits

Uh, sorry, there is no way that a "ss m90" kit is going to change anything to do with idle.

Misfires at idle are just common in 3800s, but otherwise, make sure your plug wires are on right, etc.

It probably should be said that random misifres are common at idle, but random multiple misfires are not, so there is another problem that causes a P0300 code to flash.

I've never messed with a SSM90 kit personally, but I have heard that there are inconsistencies in the machining for the injector seats. That being said, if there were a vacuum leak that was bad enough, you'd be looking at lots of problems caused by lean issues, or even a bad fuel spray pattern. All that together can cause misfires, idle issues, even loss of performance from lots of reasons.

Originally Posted by Reptile

It probably should be said that random misifres are common at idle, but random multiple misfires are not, so there is another problem that causes a P0300 code to flash.

I've never messed with a SSM90 kit personally, but I have heard that there are inconsistencies in the machining for the injector seats. That being said, if there were a vacuum leak that was bad enough, you'd be looking at lots of problems caused by lean issues, or even a bad fuel spray pattern. All that together can cause misfires, idle issues, even loss of performance from lots of reasons.

I don't have these specific problems with the Intense PCM though, only the ZZP one. So it couldn't be a vacuum leak, because otherwise it would be happening on every PCM I use.

True, I was just thinking about other issues I've heard of. I remember that there are more than 2-3 idle tables. I can't remember where it was on here, but Overkill was having issues with a startup idle surge, which he said was because of another idle table for startup/cold conditions. Not saying this right here is your problem, but just throwing that out there.....A random misfire is common, but a random multiple misfire isn't. If it actually flashes the SES light, then that's not right, there is something causing the problem. You probably won't find out though because whatever tuner you hook up might not show you what was modified depending on what ZZP uses to tune their pcm's now.

Late model cars in the stock PCM seem to ignore the idle timing tables on cold start ups.

They go straight to the regular tables which causes the timing to change which causes surging because of the high cold idle.

flashing SES means there really is something wrong there. Electrically or mechanically

I don't get a flashing SES, but a P0300 code popped up on my scan gauge. I checked the misfires, and all the misfires were occurring on cylinder 2. I'm not sure what's going on. =(

Check your harmonic balancer for separation.

Check your ICM.

Check your wires.

Check your plugs.

Check your coils.

If all of those check out then I have no idea whats going on.

Oh and try moving your injectors around to see if the misfire follows.

ZZP asked me to record the the STFT and LTFT at difference intervals. Maybe someone here can understand what is going on with these numbers:

The numbers fluctuated A LOT so I tried to record them on flat road with cruise control on to keep them steady.


STFT at idle: Fluctuates from 5.4-9.3
LTFT at idle: 16.4
STFT at 60 km/hr: -1.5
LTFT at 60 km/hr: 11.7
STFT at 80 km/hr: -0.7
LTFT at 80 km/hr: 11.7


The transmission is also shifting very slowly and I'm still getting the misfire on cylinder 2 with a P0300 code.

vac leak?

How much vacuum do you have at idle?

Originally Posted by matt5112

vac leak?

How much vacuum do you have at idle?

19-20 inHg.

Exhaust leak near the O2?

Your fuel trims suggest you're running lean.

LTFT's that is.

Originally Posted by matt5112

Exhaust leak near the O2?

Your fuel trims suggest you're running lean.

LTFT's that is.

I don't think I have any exhaust leaks, and I'm pretty sure my Intense PCM doesn't run this lean. The numbers are closer to 0 I believe. I'll have to double check that though.