Random Misfire on 4.3 Vortec Engine, GMC

[Matt Brandsema]

Random misfire on my 1997 GMC Sonoma, 4.3L Vortec engine. When engine is cold, misfires do not happen much, as it warms up it gets very bad. Misfires happen at any rpm. Here are the things I have tried / points of information.

1. Checked for vacuum leaks with carb cleaner: none found
2. Observed that the fuel trims, short and long, are both very close to 0%, going no higher than 7% in either direction.
3. Replaced the spark plug wires as they were questionable (the good fuel trim values lead me to believe it is ignition related)
4. Measured fuel pressure. With the key on, engine off, the pressure primed was about 55-58 psi. spec says it should be 60-66, but I don’t know if this small amount of discrepancy would cause an issue. It uses one of those spider injector systems. Fuel pressure also remained constant for 10 or so minutes, indicating there are no internal leaks. It did drop off (very slowly!) eventually but I don’t believe this is of major cause for concern.
5. I just bought this truck from a guy in town, and he said he just replaced the fuel pump. Fuel filter was done about 5000 miles ago if I remember correctly.
6. Observed spark plugs in cylinder numbers 1,2,3, and 4 (5 and 6 were tough to get to) and they had a little bit of a white soot like appearance, but I believe this is not bad. They all looked the same.
7. Did a compression test on cylinder number 2, showed 150 PSI.
8. MAF live data looks good, (g/s reading matches spec).
9. Coolant temp sensor giving good data to computer, It isn’t jumping around or anything. Stays relatively constant around 90 C, indicating that the thermostat is working.
10. MAP sensor data appears to be ok. Startsoff at 95 or so kPA and drops to 33 kPA on start up and at idle. I still need to put my vacuum gauge on it though.
11. Cap and rotor were done less than 10,000 miles ago and look good. I cleaned the contacts just in case and did a resistance check to check for carbon traces. All good.

I still want to check the distributor gear, as well as maybe do compression tests on more than 1 cylinder, but I figured since it is a random misfire, it must be affecting all cylinders, such as a skipped timing chain or something. I also want to check the EGR gasket and valve to see if I am getting leaks there.

Any thoughts????

Topic

[Rob]

I would replace the wire and at least clean the contacts on the ignition coil.. maybe use some sand paper and some alcohol to clean it and use some dielectric grease on it to make better contact

[Matt Brandsema]

Ok! I replaced the coil and I was still getting arcing. I thought to myself that somehow the path through the cables to ground is an easier one than the path through the spark plug gap. So SOMETHING in that circuit has excessive impedance. I removed the distributor cap and cleaned the contacts with sand paper (I had looked at the cap before but I don’t work too much on cars with distributors so I didn’t know what to look for). After cleaning, it put it all back together and no more misfires! I took it for a very long drive and looked at the codes in the computer, there is a PENDING code for cylinder #1 misfire, but It isn’t a hard code yet (it could be I didn’t clean the contact as good). I don’t FEEL any misfires anymore so for the most part, that seemed to have fixed the problem. I am going to replace the cap and rotor with quality parts and hopefully call this one done!

I had disregarded the cap and rotor because the previous owner that I bought the truck from (I just bought this truck a few weeks ago) said he replaced the cap and rotor about a year ago.

Thanks so much for all the help with everything!

EDIT:

I forgot to mention that if I put my fingers on the ignition cable, I get a little bit of arcing to my fingers (cant feel it so don’t worry). I have heard that once breakdown occurs through the wires, they are shot. Is this true?

[Rob]

I think more the boot would be bad more the the wires in it… but I would just replace the cables and seee if that helps… you can buy cblaes one by one so it might only cost a few dollors.. but I would still put some di grease on it… will halp to provent carbin build up

[Matt Brandsema]

Update. The morning after I cleaned the contacts, it was back to misfiring. When I went to install a new cap and rotor, there was a TSB inside the cap box that said the 4.3 Vortec engines have two vents located inside the distributor that clog up/have insufficient flow, and allow moisture to build up inside the cap and this corrodes the contacts and causes misfiring. When I replaced the cap and rotor I sprayed brake clean down the 2 vent holes and then blasted them out with compressed air and the truck has been running like a champ ever since!

[Author]

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