I have a '91 420 with 150K miles that is giving me fits. Timing chain and throttle position switch were changed 8 months ago, as well as small leak near fuel pump - but this problem has developed in past 2 months.

Engine wants to "miss" or stumble when in gear at idle, and slightly upon acceleration from complete stop; there is also a slightly noticeable uneven acceleration in lower rpm ranges. There are no noticeable stumbles when idling in neutral or park, just when the car is in gear and stopped (most noticeable). The stumble is very quick, not constant rough running, but more of a quick hit, and then the engine is right back up.

It seems to do this when cold and hot. Also, does not seem to get worse with hard acceleration.

Recently, the "stumble" is occurring more frequently; when stopped with the car in gear it does it every 10 seconds or so. It is somewhat intermittent in that it seems worse at times (not hot or cold related though).

It got really bad and I tried some fuel injection cleaner (thought it might be water in gas) and this seemed to help slightly, but its still there.

I've done a quick visual on all vacuum hoses, and can't see anything loose. Spark plug wires all seem in good shape. I am using the appropriate Bosch copper plugs - these were replaced 8 months ago, so I assume plugs are okay...

Any suggestions or ideas???

ljones

UPDATE: Have had a couple suggestions to try replacing ignition coil, distributor, and plug wires. I suspect the problem is ignition related rather than fuel related, however still have yet to pinpoint the problem.

Am planning on taking it to a new independent mechanic on Monday. I figure it will be cheaper to pay him to diagnose than for me to just start replacing parts... Hopefully I'll have this resolved soon.

I repaired one today with the exact problem. Tried everthing but turned out to being the fuel distributor.

Yesterday, problem got worse and car ran really rough all the time. Noticeable loss of power and rough running was almost constant - seemed not as bas when I was only giving about 1/4 throttle, but still very rough. It was like the car didn't want to run at all, but it wouldn't die, just ran real rough

This morning, car started and ran great. There was a little hint of a stumble maybe twice, while sitting stopped with the car in gear, but not the rough running I was experiencing yesterday.

I am beginning to think it is more ignition related, most likely spark plug wires or other ignition problems.

Hopefully I will have it properly diagnosed by Monday...

Would the fuel distributor be this intermittent???

I would put my money on something ignition related. Although fuel distributors would come in second. If you have access to a portable oscilloscope you can watch the watch the ignition coil primary voltage from the diagnostic socket while driving the car. That will tell you if its ignition related or not.

__________________
I got too many cars!! Insurance eats me alive. Dave

78 Corvette Stingray - 3k
82 242 Turbo Volvo - Manual - 270k
86 300e 5 speed manual - 210k
87 420sel - 240k
89 560sl - 78k
91 420sel - 205k
91 560sel - 85k
94 GMC Suburban - 90k
97 Harley Davidson Heritage Softail - 25k
00 GMC Silverado 1 ton 30k