'91 420 occasional miss, uneven acceleration
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???