Hello,

91 2.5T W124 runs rough at idle when warm and intensity varies widely.
Could this possibly be due to air in the fuel and if so where are the possible fault points that air most commonly would enter the system ?
* Any history known or similar symptom that anyone has experienced

thankyou all

between the Tank and the I.P.

This engine has a "Fuel Thermostat" sitting on the support of the Driver's side
Motor Mount that has a O-Ring seal Notorious for going bad. it can Leak Fuel and Suck Air

Motor AND Tranny Mount(s). ???

Do you have a Clear PreFilter (The Little one) are there ENORMOUS amounts of
bubbles in there?

Any Oil in the Surge Tank? (Radiator Fluid)
(whitish smoke?)

How Many Miles on this Particular OM602 ?

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'84 300SD sold
124.128

Also check injector return lines, these leaking can cause a rough warm idle.

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1993 320TE M104
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past:

1983 230E W123 M102
1994 E300D S124 OM606 (x2)
1967 250SE W108 M129
1972 280se 3.5 W108 M116
1980 280SE W116 M110
1980 350SE W116 M116
1992 300E W124 M103
1994 E280 W124 M104
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"music and women I cannot but give way to, whatever my business" -Pepys

Will they really!?!?!
This is the one thing I havent tried on my 240d, which has a rough warm idle. I've literally tried almost everything else, aside from replacing the IP and lift pump, at the advice of this forum and local mechanics.

But I've never tried the return lines, because they don't appear to be leaking. Will a slight dampness cause a noticeable idle issue?

What I had to do to get rid of the my when Engine hot vibration/shaking:

Raise the Idle speed within the normal range (this helped some; it was around 700rpm). I raised it to about 750 rpms.

(Your 240D does not have one of these)I bought a new Rack Damper Bolt but it did not help much. I have a turbocharged 300D Tubo and it has a Rack Damper Bolt.

Rebuilt my Injectors with new Nozzles (This took away most of the shaking)

Doing a Valve Adjustment (took away some more shaking). If you can adjust the Valves yourself do that first except for the new Valve Cover gasket is free.

Changed Motor and Transmission mounts (took away some more vibration)

Found that I had a small air leak that showed up when the Engine was hot due to hard fuel
inlet lines that did not seal even with the clamps tight.

(You do not have one)Finally was able to use the Rack Damper Bolt to tune out a little more shake. (that bolt was made to reduce the shake on a new engine; not to compensate for a bunch of worn and out of spec parts

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84 300D, 82 Volvo 244Gl Diesel