Another 3.4 Missing and running rough

[Danspeed1]

Hello,

I need help diagnosing a strange problem that just appeared out of nowhere. I have a 1995 Chevy Monte Carlo 3.4L DOHC V6. Car has 235,000 miles on it. I am having a problem at idle and when accelerating from a 900RPM to around 2500RPM. The engine is running rough, sounds like it is missing, and it is surging between 600-900RPM’s. What’s interesting is that there is NO loss of power. If I jump on it, it will peel out and take off like nothing is wrong. If I just drive it normal, it will buck, and miss till about 2500RPM and then the issues clear up. Of course when the converter locks at 65MPH on the highway this problem is amplified and the car shudders. It sounded like a MAF initially so I unplugged it, drove it, and there was no change. Then I though IAC… same result. O2 Sensor was unplugged next… no difference. This problem occurs whether the car is cold (open loop) or warm (closed loop) makes no difference. I checked all the vacuum lines, and there are no leaks. I changed the spark plugs… no difference. There is no MIL light, and when accelerating the on the highway the problem appears to clear up. From time to time it will idle like normal, but as soon as you pull away from a light it begins shuddering, and missing. It’s not a fuel issue because it would happen more at WOT… The coil packs are 2 years old, and about 40K miles?

I have no idea how to fix this problem…..

Any thoughts?

DG

[robertisaar]

the fact that you can unplug the MAF, IAC and O2 sensor, yet no change occurs is disturbing.

what kind of coils? aftermarket ones are generally much less reliable than even used Delco ones, especially with where they are mounted on the early LQ1/LH0s.

[RedZMonte]

have you done a T-belt recently? Sounds like Timing might be off a little. My 1st lumina Z slipped a tooth and was acting kinda what you described... run/idle rough but ran fine in upper RPMS. i would atleast pull the cam gear cover and see what she looks like under there.

S

[Danspeed1]

have you done a T-belt recently? Sounds like Timing might be off a little. My 1st lumina Z slipped a tooth and was acting kinda what you described... run/idle rough but ran fine in upper RPMS. i would atleast pull the cam gear cover and see what she looks like under there.

S

Timing belt has no more than 20K miles on it with all new components

DG

[JDz34]

Is the feeling sharp and fast (miss)? If so I would look real close at the spark plug wires. Where they are so long and extend into the head I've seen them crack or even get a carbon track down the insulator of the spark plug and cause an intermittent misfire. Especially under light load and cruise speeds. If you still have the old plugs look down the sides of the porcelain for black marks like a pencil would leave. It will usually leave the same tracks down the boot and continue to misfire. I have seen very little problems with the coil packs. Usually when they die they kill both cylinders completely.
Just my 2 cents worth I hope it helps.