3 Cylinder Misfire/No Service Engine Code

2001 Pontiac Grand Prix 3.1L 136,000 miles.

Initially noticed an engine miss after car sat in rainy weather for about 3 hours that only lasted for a few miles.

About one month later the car began missing consistently when driving. Appears to occur at about any speed but mostly between 40-70 mph. Doesn't stand out when accelerating, and does not miss when idling.

Took it to my service guy who plugged in and said it missed on cylinders 2, 4, and 3 during the miss. It is not a 100% consistently miss but does happen frequently.

I changed spark plugs from the AC Delco 41-940s that were in the car to the AC Delco 41-979 that Auto Zone said was the correct plug for this car. The problem persisted after changing the plugs.

I then changed the spark plug wires from the originals to the AC Delco Wireset 9726UU.

The problem still persists.

Figuring the ingnition coil is about the only thing left, however according to the numbers my mechanic gave me, it's missing on 2,4,3 those are each on different ignition coil packs and all the reading I've had says it is pretty well impossible for all three coil packs to fail simultaneously.

Any thoughts on where I need to head next?

The car still runs and drives well and the wife is still driving it but this is driving me crazy.

The PCV valve was replaced last year as well. The car has an oil problem as well although I think that's a whole different story.

Buy a spark tester and test all six terminals......something your mechanic should have done while he had the car....

If you take the vehicle out on the highway, get at a constant speed of about 55-60, and then find a slight upgrade......then lightly accelerate(this keeps the TCC engaged) up the upgrade from 55-60.....if the car starts to jump/misfire, that is usually secondary ignition.....plugs and wires were replaced, that leaves ICM or coils.....ICM's tend to misfire at all times, rather than under a load......

Other possibility is mice eating wire insulation to injectors under the upper plenum....

Well I replaced two out of the three ignition coils and there was no improvement.

I was discussing with my father and thought I'd mention here, when I was driving with the mechanic and he told me the misfires were occurring they occurred all together and not at individual times.

Meaning when the car misfired he read off the cylinders missing as 2/4/3 quickly.

Not sure if that helps at all. Guessing the ICM is about all that's left with the ignition system and it's going to leave something with the fuel injectors not sure if the throttle position sensor can do anything of this sorts.

Someone mentioned a potential vacuum leak, but I hear no leak when the car is running.

You might as well keep changing parts, because you are doing that instead of diagnosing the problem....if your mechanic is suggesting these replacements, time for a new mechanic...

Nope the mechanic isn't recommending it at this point. I haven't taken it back since he told me it was missing on 3 cylinders and to change the spark plug wires.

Now I've just talked to some guys at O'Reilly who suggested a leaking upper plenum gasket is a common problem for this misfire. Seems like everyone has an opinion.

I attempted to spray brake cleaner around where the plenum attaches the the engine and got no real results there.

I attempted to spray brake cleaner around where the plenum attaches the the engine and got no real results there.
...no real results could be expected either cause the intake manifold is at negative pressure...it will not bubble out....but if you expected the cleaner to be sucked in then you were right...
however intake manifold leakage is a known problem with series II. I also like the mouse issue....:evillol:

I have NEVER seen an upper plenum leak on a 3.1......IT IS NOT A KNOWN PROBLEM!

The idea behind the brake cleaner was if you had a vacuum leak it would pull the cleaner into the engine through the leak and cause the engine to rev slightly due to the combustion of the cleaner.

So is a vacuum leak significantly less likely in this situation? It appears the engine is starting to miss significantly more.

So this has changed immensely and figured I'd keep you guys up to date on this story as it develops.

So I finally gave up and took it to my mechanic to let him do the full bore check and just tell me what's wrong so I can get it fixed.

Well I drive it there, park it, leave it. Then he calls me and informs me the transmission is shot. It's whining and won't even engage the drive shaft at this point (these are the same guys who repaired it three years ago).

Anyway, the transmission is bad now and I'm no longer convinced my jerking was truly even an engine miss, although he swears his machine read off a three cylinder miss.

It may be time for a new mechanic afterall.

Quick question.

Does it make sense that a leak of a gasket on the intake manifold could cause the transmission to fail?

The repair place is telling me that having a leaky intake manifold gasket causes a coolant leak (I do believe this) which causes things to run hotter which can cause the transmission to run hot and fail.

The car itself has never over heated is it possible that even without the engine over heating for the transmission to be over heating in this situation?

I'm not trying to get anyone in any trouble here I just have to conflicting answers and I'd like some more opinions on which is the case.

I have NEVER seen an upper plenum leak on a 3.1......IT IS NOT A KNOWN PROBLEM!
Obviously......I was thinking 3.8 series II...my mistake.