P0302 - Misfire DTC

[TransAm-98]

It was just the computer pulling timing due to the o2 readings. The tune actually took away my misfire at idle. It used to sound like a boat engine idling in water. That's the best way I can describe what my idle used to sound like.

[bartonmd]

Say what? The computer pulls timing on the lean swings on this engine, or something?

Mike

[Trail X]

Merged over the last few posts at the request of bgwolfpack.

So does this indicate that O2 sensors could be the root cause of the misfires?

[bgwolfpack]

The O2 sensors could play a large part. I changed both of mine when the problem first happened (thanks Mike) and of course with the battery disconnect everything was fine long enough for me to post about it. Since then, I've spent money on all sorts of parts being thrown at the engine, with the thought that changing now is just as smart as later when they fail. The problem continues, especially with the cooler weather or the change in gasoline due to season.

TransAm-98s post about the reading of the sensors has brought me back to an original doubt I've had with the ECM.

IIRC the only engines to have this trouble has been the 4.2L older series (2002-2005) Could it be there is a calculation problem within ECM, where the ECM cannot understand the newer gasoline being pumped with higher levels of "Seasonal" additives?

[chevycrew]

Say what? The computer pulls timing on the lean swings on this engine, or something?

Mike

The way our computers are setup, if the knock sensors indicate a knock or ping, the PCM will pull timing or retard untill the knock or ping goes away.

If you have dirty injectors, it could be pulling timing under normal conditions.



My computer has been tuned, but before and after the tune I still ran better with 91 octane. (even at 4500 feet where 85 octane is the low grade)

[TransAm-98]

Running 91 will help with mileage too if you keep running it and give the computer time to learn it. Higher octane= slower burn=equal less fuel. The random misfire can be caused by a few things. I never threw a code for a misfire except for when my coil went out. But the hiccup i had in my idle was easily audible when listening to the tailpipe. We ended up putting a little more timing into the earlier tables and taking a little bit of fuel out. The lack of timing mixed with the excessive fueling that GM put in the car was causing my irregular idle tone.

**Note that correcting this was just us being bored and improving on GM's tune. My car cad a slight hiccup at idle, but it was no more than you would here on a corvette or a tahoe, and nothing that would actually register as a misfire in the PCM. A lot of the factory cars do it. So I'm not saying there was anything wrong with my car, just that there was some room for improvement.

O2 sensors are also overlooked far too often. Just because you're car isn't throwing a code, doesn't mean that the O2 is working properly. We see this A LOT at the shop since when need them to be in working order to read fuel trims. And a lot of cars come in with weak or dead O2's without throwing an O2 code. Then we end up having to swap it out so we can finish the tune. A lot of things can cause an O2 to go out and they are much more sensitive to contamination or getting banged around than given credit for. O2's are pretty cheap through RockAuto, so if you notice your car is running poorly or sipping gas faster than usual, do yourself a favor and swap them out.

[TransAm-98]

The O2 sensors could play a large part. I changed both of mine when the problem first happened (thanks Mike) and of course with the battery disconnect everything was fine long enough for me to post about it. Since then, I've spent money on all sorts of parts being thrown at the engine, with the thought that changing now is just as smart as later when they fail. The problem continues, especially with the cooler weather or the change in gasoline due to season.

TransAm-98s post about the reading of the sensors has brought me back to an original doubt I've had with the ECM.

IIRC the only engines to have this trouble has been the 4.2L older series (2002-2005) Could it be there is a calculation problem within ECM, where the ECM cannot understand the newer gasoline being pumped with higher levels of "Seasonal" additives?

A lot of different places have a lot of different qualities of gasoline. Out here I almost always try to fill up with 76 or Chevron cause they have been the most consistent for me. I found out the hard way a long time ago that not all fuels are created equal. With my old motor setup in my Trans Am I was running a little over 11:1 compression with an open loop speed density tune. Meaning my tune was based off of my MAP sensor and Intake temp sensor only. This made my car much more sensitive to changes in fuel, temperature, and altitude. At the time I was working at a Mercedes dealer and had access to free gas from time to time. I used our gas card to fill up my Trans Am a local CFN station. Big mistake, my car started pinging very bad as soon as I tried to go full throttle. It took me 2 tanks of good gas to get it to run normal again. The motor in it now has closer to 12:1 compression and needs all the help it can get to run on our crappy California gas. So I'm much more selective now on where I fill up.