Yes, another post on rough idle. :)

My 240 doesn't like to start when the engine is cold. It wants to stall, and sometimes does if I don't open the throttle a bit to keep the RPMs around 1500. If left to idle on it's own, RPMs drop and the computer tries to compensate by revving the engine. Then RPMs drop, then self-revving, etc.

When warm, the idle is at 900 RPMs but it isn't a smooth idle. The tach shows the RPMs going up and down a bit, maybe 1000 RPMS or so. I know that four-banger engines don't idle smoothly to begin with, but the entire car quivers because the idle is so rough. Normally it isn't this bad.

I have a fuel economy vacuum gauge. Normally during idle it reads full vacuum - the needle is pegged to the left. Now the needle is no longer pegged but not that far from it. When coasting in gear, the gauge is pegged left and the engine vacuum appears normal.

Sometimes I coast to a stop by putting the manual tranny into neutral (trying to save a little fuel). I've noticed a reduced amount of braking power when the engine is at idle as opposed to braking at higher RPMs when in gear. I'm assuming that the reduced engine vacuum is lessening the braking power (?).

Everything seems fine while driving. No loss of power when taking off or crusing. Engine seems happy.

I've checked most of the vacuum hoses and found no leaks and heard no obvious hissing sounds, but I did replace a few hoses that were stiff. I have a newer engine wiring harness. My first thought is the air mass meter is getting funky. It's not the original, but it's at least 10 years old. I haven't yet tested the AMM as described in the Bentley book. I'm a bit queasy about messing around with the engine wiring.

Any thoughts?
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Mike F - 1984 244 DL - 249,000 miles - Undergoing reconstructive surgery with POR-15