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Thanks. I’ll check the vacuum system one more time. I have verified using the method above that the coils and plugs are not the problem. Also, the fuel injectors are good as well. What are your thoughts about a little oil being on the spark plug threads? I can’t remember what ones they were, but they weren’t just from the misfiring cylinders. The compression is excellent….and I also did a leakdown test and they were perfect as well. There’s no oil leaking from the valve cover either.
Would my scanner pick up a small leak like you’re describing? The MAP sensor says it’s at 15psi with the key on and when the motor is running it’s down to 5psi. From my knowledge that’s within speck. Thanks for all your feedback! Much appreciated!
I did run smoke through the vacuum system with a homemade smoke machine a few months ago. Perhaps I should check again and also check the brake booster.
Thanks for the reply. I will run some more tests and see what I come up with! Also, I forgot to mention the misfiring I found only occurs at idle.
Thanks for letting me know about the results from your AutoEnginuity scanner! So, focusing on the random misfire I’ve ruled out it’s not the spark plugs, spark plug wires, coil packs, fuel injectors, fuel pressure, cylinder pressure or cylinder leakage (strangely, I do find a little oil on the threads of the spark plugs. And not just on the misfiring cylinders). Misfires occur mostly on cylinder 2, and a little bit on cylinder 8.
A bit of history: before the random misfire code occured there was an evap leak code on the Suburban. I left it unfixed for several months and sometime during those months a random misfire code turned on to. But because the check engine light was already on I have no idea when misfires started to occur. I’ve since replaced the canister vent valve and the evap leak code has disappeared. And, there is a small rear main seal leak as well.
Yes, this is the same thing that is going on with my scanner. However, I’ve checked it on another vehicle and this did not occur. What model/year vehicles was it hooked up to? Somebody else told me this happened to them on some older GM models. Aslo, can you let me know what the manufacturer says about your device? Is it the autoenginuity scanner? Thanks!
