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Discussion Starter · #1 ·
06 Pontiac Grand Prix
I have been dealing with this problem for about 3 years now. I have done every single test and checked every single ignition part on the car, including the MAF. I have a very random misfire, all the ignition parts have been changed out. But I have a very random misfire, doesn't happen all the time only once in a while. Sometimes during a trip to work, sometimes not, and sometimes when it happens it kicks off cruise control. The problem is no codes, 3 shops have looked at it. A GM master tech of 40 years has looked at it. He couldn't find anything. The fuel pressure never dropped, but the car will jerk for a split second. It feels like transmission slip. But its not because it will happen at idle in park too. The other day when we had a -17 out, car drove fine in the morning. No misfire no problem. But when I got out of work, she started just fine waited for about 1-2 min they headed for home. It died on me twice, first it started up fine, second time coasted in the mall parking lot. She would start, then die. Only with a little gas, she would stay alive for 30 seconds running very rough then die. The shop picked it up, it started just fine for them, ran and drove just fine, and get this no codes again. Temperature doesn't change the problem, it happens when its warm and up to temp all the way to -17 cold start.
 

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A scantool should've been plugged in at some point (not just a code reader, something that can log multiple sensor data and evaluate afterwards). It's still amazing that people who modify cars equip ourselves better for troubleshooting than most shops. A Snap-On scanner comes close, but it has a worse interface than a Casio watch.

You'll want to find out what the MAP & MAF sensors read for vacuum/airflow, what the O2 sensor is saying, and how the fuel trim %'s are being applied for adding/reducing fuel based on these readings. You can also monitor the cam/crank sensors, but the computer is pretty sensitive to errors there, compared to the less direct readings of air/fuel.

Information is the way to solve this, so you can start with a $15 ELM Bluetooth OBDII scanner to monitor from your phone, and move up to something more robust as you get a sense of what is/isn't right.
 

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Discussion Starter · #3 ·
I have look at data with someone else driving for like an hr. Fuel trim and everything else was perfectly fine. 3 people other than drove the car with a GM Tech 2 Bluetooth module and they only saw nothing wrong. It literally just blips than falls back to normal. I am seriously just about to buy a pico, but they are expensive.
 

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May want to inspect wiring and grounds to ensure it's not intermittently having anything disconnected/frayed. Costs nothing and helps you think of things as you look it over.
 
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