MAJOR misfire problem back

1999 Z-24 (2.4L) 5 spd

This problem has foiled several shops now.

Last winter I had a serious misfire problem.
It seems to occur when there is major temp change in weather.
i.e from summer to fall, fall to winter (I don't know why).

All spring/summer car worked perfectly.

Fall is here........& problem is back.

Car starts fine, idles fine.

Once on highway car will start misfiring after a few minutes.
Tach needle goes nuts, from 0 to 3000 RPM. Several shudders (multiple cyl misfire). I let off the throttle to minimize effect, engine catches itself. Once car is warm (about 8 mins highway driving) it works perfectly again. If cruising around town, front wheel will actually squeal due to misfire. The car visibly dives and shakes during misfire. Tonight I let a buddy try driving it and he couldn't believe it (he was laughing the whole time). I cannot believe the engine has not destroyed itself yet.

From what I've read, the car is in "open loop" mode when cold. Since the problem happens when the car is cold, I expect the PCM is in open loop when the misfire is occuring. Does anybody know what the PCM is doing in open loop? What sensors is it using (if any) that could cause this effect. What specifically can it be narrowed down to?

I would really appreciate some insight into this problem.

Thanks in advance.

One other point to add.

Car starts and idles fine until the temp gauge his 1/4 (40 C?). Between 40 to just before 90 all hell breaks loose. Once warm, it runs perfectly.

:banghead:

Well, first thing I'd check is coils and plugs. I'd also check wires, but that wouldn't be a problem on a 2.4, now would it? I would seriously be considering something computer related or electrical related as opposed to mechanical failure. Cold weather makes plastic as well as metal contract, so this could translate to something going wrong when it gets colder.

If all else fails, try swapping computers and giving that a shot.

Well, first thing I'd check is coils and plugs. I'd also check wires, but that wouldn't be a problem on a 2.4, now would it? I would seriously be considering something computer related or electrical related as opposed to mechanical failure. Cold weather makes plastic as well as metal contract, so this could translate to something going wrong when it gets colder.

If all else fails, try swapping computers and giving that a shot.

Thanks for your reply OverAllComa.

I know it's not the plugs. I changed them AND the dealer changed them. I made sure to use the AC delco plugs as well.

I checked the coils per the haynes manual but I can see how there could still be a problem there.

I agree with your logic. I think my next plan is to find someone with a 2.4L willing to let me borrow ($) their coil / icm /cover assy for a few minutes. I would rule out the ICM, the spring / boot assy, the harness connector and the coils.

My worst fear is that it IS electrical. If plan A doesn't work & nobody else is having a similar problem on this forum, I'm going to have to have it checked over by an auto electrical shop. I checked everything I could think of checking with a meter, but it's intermittant and herin lies the biggest challenge.

I now refer to it as the entity. :evillol: