I recently acquired an interesting 1984 300D which had a factory rebuilt motor installed about 50,000 miles ago along with a tranny rebuild. It truly runs nicely, with the one exception of idle. My first instinct has been that one cylinder appears to have little fuel delivery, as the slightest touch of the pedal, particularly under load, seems to smooth it out entirely, and the engine is very smooth at accelleration. I've reread every post I can find that leads to the 'rack dampner screw' or any variation, as well as injector threads and so forth. I also just did the 2 can diesel purge, which really made the engine quiet down nicely, but no effect on idle.

Now the first thing that confuses me is that all the pictures and such that I have seen of the idle rack adjustment pin show it on the rear facing side of the IP, but I have a solid plate. Now I'm very curious about this, and the missing screw might be related to the fact that the engine put back in was non-turbo. The IP is not the original, either, and may not have originally been paired with the engine. The previous owner claims it was new, but who knows.

Is there an equivalent adjustment on the IP to smooth the idle on the non-turbo pump? If not, should I be chasing injector problems? Seems like at 50,000 miles or less that shouldn't be the first thing to look for, but then I've also read that the new Bosch injectors are made in India and sub-par with the older ones. I am presuming (perhaps incorrectly) that compression is not going to be the answer, since the engine is relatively new post-rebuild and also is smooth with any more fuel delivery.

Thoughts anyone?

Ben

Is your idle set to spec? What does it idle at? When is the last time your valves were adjusted? Have you eliminated all non-engine causes, e.g. mounts, shock, etc.? Be more specific about how it idles, does it lope? vibrate? shake the whole car?

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1982 Mercedes-Benz 300CD
1982 Mercedes-Benz 240D - stick

Thanks for your reply.

The idle speed isn't verified, naturally, but my neighbor has the same car and they sound remarkably similar in pitch (how's that for the scientific method). Like many, my tach is not functional. The roughness definitely follows one cylinder, rather than just being random shaking. I've just done the diesel purge, and after being fully heated, the engine exhibited the same, although slightly lighter, symptoms hooked up to its own external fuel source. I'm usually pleased with the cold idle, you can feel it but there's no shake.

I am fairly certain I need to pay attention to the transmission mount, and driveshaft support areas, as I have some accelleration wobble that is a separate issue at specific speeds, independent of gear or engine speed. It's enough to keep me busy for some time! Need new axle boots too.... However, the thing that stumps me on the engine is the rough idle despite it's relative 'newness'. Assuming they used new mounts at the dealership, they should only be four years old.

It just seems to follow the patterns and descriptions of all the people's posts regarding the idle dampening adjustment, but I don't seem to have one. Could it be located somewhere else on the changed IP? I see another thread has already popped on on rough hot idle since this post.
Ben

Oh yeah, it definitely shakes the whole car. I sit at a stoplight and wonder who around me notices my autonomic motion...

The valves would have been adjusted with the new engine, 30-50,000 miles ago. I haven't ruled that out as a possibility, but wouldn't I notice more engine noise or shake accellerating too?

These cars that have a non turbo engine have an idle adjustment knob on the dash. When they went back with the non turbo, did they install an idle adj knob? It is on the dash, left side of steering wheel, opposite the ignition.

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Jimmy L.
'05 Acura TL 6MT
2001 ML430 My Spare

Gone:
'95 E300 188K "Batmobile" Texas Unfriendly Black
'85 300TD 235K "The Wagon" Texas Friendly White
'80 240D 154K "China" Scar engine installed
'81 300TD 240K "Smash"
'80 240D 230K "The Squash"
'81 240D 293K"Scar" Rear ended harder than Elton John

Well I would start there. They should be adjusted every 15,000 miles. Chances are they used new mounts with the new engine, but you can only be sure by checking them. Valve and idle adjustment are not expensive and may solve your problem.

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1982 Mercedes-Benz 300CD
1982 Mercedes-Benz 240D - stick

No, No adjustment on the dash. There are a coule of adjustments on the IP that don't quite corresond to any of the 300D (old or new) pictures I've seen. I'll try to get a picture and post it. Meantime, I either have to find an MB mechanic or facsimile thereof in my area. No dealer around here. There are a couple of European autoshops, and a large Truck place near the truck stop. I'll have to figure out who would be most reliable.
Ben

Maybe I'll have the good fortune of finding an RPM Amplifier cheap too!