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Discussion Starter #21 (Edited)
Still looking for an answer. Misfiring is pretty bad at idle (intermittently) and sometimes at low RPM when driving. Fuel mileage hasn't improved. The pending P1401 is gone.

I collected more data while it was misfiring right after I started it. This time I let it sit there and misfire for about minute while I collected data and the CEL finally came on with a P0301 code - Misfire Cylinder 1. After idling for a couple minutes, it started running fine, which is how it typically goes. At that point I turned it off for a few minutes, then turned it back on to collect data for the second file I attached, while it was misfiring again (about once per second on average).

Notice the LTFT on both sides is above 20% consistently. Also in the second file of data you'll notice the bank 2 STFT go above 20% for a few seconds as well, something I didn't notice before. I also noticed that the O2 sensor voltage for Bank 1 Sensor 1 and Bank 2 Sensor 1 was switching properly but was staying constant for Bank 1 Sensor 2 and Bank 2 Sensor 2. Could that be an issue?
 

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Have you ever replaced that O2 sensor? And you're sure you've got no vacuum leaks? Or a crack in the intake manifold?
 

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Discussion Starter #23
Have you ever replaced that O2 sensor? And you're sure you've got no vacuum leaks? Or a crack in the intake manifold?
Don't know about the O2 sensors. It would be weird for them to fail in pairs in different banks though. Plus I was under the impression there was only 1 sensor per bank so I'm not sure if the sensor 2 numbers even mean anything.

We did multiple smoke tests and never found any leaks. We even sealed off the line to the HVAC system in case it was leaking. I haven't checked the intake manifold in depth but there's nothing obviously wrong with it. Someone suggested replacing the intake gaskets - is this enough of a possibility to just go ahead and do it or is there anything else I should consider first?
 

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Intake manifold gaskets would be a good way to start, both upper and lower. Especially if they've got 250,000 miles on them. They're not too expensive either.
 

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On your software can you scroll all the way to the right and post the "Report" data. Like the Mode $06 data?

I have the same software and am having some minor misfires at idle only - does not sound as bad as yours. But I would like to see your Mode6 data and compare to mine.

I am suspecting a small air leak - but my LT fuel trims are not as high as yours.

I fail the $A1 $83 - Manufactured Defined Test. (I think I will start a new thread on that for some help) But pass all the other tests. I only see my misfire count go up on random cylinders at IDLE ONLY.
 

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I'll be getting the cat replaced next week but I don't think that's what's causing all the symptoms. Poor fuel mileage and no power after lots of driving probably, but the misfiring under low RPM load and idle problems are probably something else.

I had a smoke test done today and they couldn't find any leaks anywhere, even with the HVAC system.

Any other ideas?
Sounds like Mrs. Viktor's '99 Duratec Sable was doing when the vacuum lines for the EGR valve and fuel pressure regulator was doing.

I don't know what they look like on the Vulcan, but on the Duratec, they're hard plastic lines, and they just broke with all the sub-zero temperatures we had this winter. It wasn't fun trying to fix them, either, because they kept breaking as I was attempting to patch the gaps with rubber vacuum hoses.
 

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viktor, if that happens again, may try to snag a good complete vac. harness at a self service salvage yard. Those small hard red (fpr) and green (egr) lines do get brittle due to age/temp fluctuations.
 

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viktor, if that happens again, may try to snag a good complete vac. harness at a self service salvage yard. Those small hard red (fpr) and green (egr) lines do get brittle due to age/temp fluctuations.
That's the problem, though...any salvage yard replacements stand a good chance at being just as brittle as the ones currently on the car.
 
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