Its a common misconception that a rough idle will trigger a misfire code. The computer is monitoring the spark, not the motion of the engine. The old sun diagnostics would monitor the smallest variation in the engine idle and compare it to the spark to detect a cylinder that was not contributing it fair share to the power curve. Some people think the PCM does the same thing, but a Sun diagnostics computer cost more than my Saturn cost when new.

The PCM is only seeing the amplitude and duration of the spark. The amplitude and duration are affected by fuel/air mix and compression. There has to be an allowable variation between the sparks due to spark plug wear, spark plug wire resistance and other factors, but when one spark is significanty different, it sets a misfire code.

A leaking intake manifold gasket will have a small affect on the A/F ratio, but not a whole lot and no affect on the compression so it can cause a small roughness in the idle, but not set a misfire code. But with one cylinder getting more air, the O2 sensor reads the whole engine as running slightly lean, so it adds a tiny bit of extra fuel until the O2 sensor averages out, this causes the idle to run a little on the high side.

 

Thank you, Keith. I've never believed that a rough idle will necessarily trigger a misfire code, as there are numerous reasons/conditions that will lead to a rough idle, and they certainly are not all misfire related.

I always just had it in my head that if the intake manifold gasket was sufficiently distorted/sucked in, that also meant a large enough volume of unmetered air was being added to the mix in Cyl 1 that the conditions would be altered so significantly that there would truly be no firing of the charge in Cyl 1, hence a true misfire as defined.

That, coupled with the # of reported P0301s over the years since you've joined the forum that did indeed turn out to be intake manifold gasket issues, kind of lulled me into mentally associating a P0301 with certain SOHCs as a prerequisite for taking the intake manifold gasket into consideration as a cause.

However, as you've already pointed out, it's not.

 

If the A/F ratio is off enough to make a significant change in the spark, then it would set a misfire code. Gasoline vapors ionize at a different potential that air so if the A/F is off far enough, the ionization potential will change and so will the spark voltage and duration.

Mine never got that bad so I never got a misfire code, the engine idle wasn't that rough either, but I could feel it and my gas mileage fell off from 38 to 35.

 

This is why I've always been glad that you're still with us on the forum. You teach us alot of the finer points and you do so using simple language.

Thanks for sticking around.