Here I go into an impossible to find solution. I have an '89 L98 and there is an occasional stumbling issue at around 3000-4000 rpm. It started yesterday and has happened a few times. Maybe 1 in 5 times it might stumble near that rpm while driving. It idles flawlessly and revs nicely in neutral. The timing is at 6btdc and advances smoothly as it should. I recently installed a new fuel pressure regulator as the old one was torn and blowing fuel through the vacuum line into the plenum. I have replaced all of the spark plugs recently and they are all gapped and snug. The wires are the nice MSD ones (don't know how old as they came on the car, but they seem okay by looking at them). The plug wires are a bit disorganized without the retainers. I may need to route these properly if they are a problem. I don't know if these nice big insulated wires are known for crossfiring if they touch. I checked the exhaust manifolds after driving and they are NOT glowing red or anything that would indicate a clogged cat. I know that the car ran really rich when I bought it and that could have destoyed the cat, but it seems to flow fine. Performance is great except for the occasional misfire. Maybe I need to look into replacing some distributor parts such as the module, module harness, coil, cap , and rotor. I also have a new fuel filter. Maybe I have a bad tank of gas? I just hate to start throwing parts at this car as that gets expensive. Has anyone else had this isuue? Any ideas on what I should do to correct this issue? Oh yes, the car has 100,000 miles on it. Thanks for any replies.

 

Define stumbling. Do you mean stumbles like the motor cuts off for a split second at part throttle and comes back to life or is it a like a momentary and random lack of throttle response when throttle is applied?

 

My 1990 six-speed coupe did what yours is doing for a couple of months. Sometimes it would cut off, but I could re-start it. Eventually, it wouldn't start until I let it cool for 10 minutes. Finally, it would run for only 10 minutes and wouldn't re-start for the rest of the day. Meanwhile, I replaced the distributor cap, rotor, rotor button, ignition coil, ignitiion module, fuel filter and fuel pump. It still wouldn't run for more than 10 minutes.

I gave up and had a wrecker take it to an independent mechanic. He replaced the ECM, and that did the trick. But he charged me $500. Hopefully, you won't replace a box full of parts like I did. I learned that the mechanic had put a junkyard ECM in my car, and that ECM lasted only a year. The original problem was slowly starting again. I bought a remanufactured ECM for $75 and solved the problem.

Something may be causing the ECMs to go bad. I hope to replace this car with a C5 before I have to buy another ECM.

 

The stumbling is like a momentary and random lack of acceleration. It doesn't shut off or anything like that. I drove it some more tonight and it is running fine. I'm sure that the issue will re appear. Also, there has always been some sense of minor slugging. It would never be felt in an auto car, but it is a little irritating with the manual transmission. It feels alot like when an air conditioning compressor kicks on in a 4cyl car or something. But I have 8 cyl and the air is off. Maybe the AIR system or EGR kicks in this feeling? I need to eliminate that stuff if it is a problem.

 

The AIR system is used to warm up the cats on a start up. If air is still flowing to the AIRports on the exhaust manifolds after the computer goes into closed loop mode, it will think the A/F ratio is lean and add more fuel needlessly. This could cause some drivability problems. The EGR is used on very light acceleration or part throttle cruise. If too much EGR is delivered at once when in WOT, this could cause some momentary lack of response and can even cause a stall. You should also try checking the connection to the MAF sensor. A bad connection there would either cause a stumble or if bad enough, a stall out. Any codes?

 

I had a similar problem with my 90. Replaced the plug wires and the problem went away. The wires were old, looked ok, but in reality had seen better days.

 

First I'd ohm the plug wires, and you need to kinda twist them while you ohm them. I found a couple bad ones that never showed up in the dark.Hate intermittents cause they can be hard to find without a code.
You also might want to try some SeaFoam, its worked for me and alot of others.And you should check the TPS, from closed throttle to WOT, watch the voltage on it, it should increase smoothly.

 

There are no trouble codes. I will check the connection to the MAF and ohm the plug wires. Thanks for the replies an keep them coming