1981 300D runs rough at low rpm
 

I've recently acquired a 1981 300SD w/230,000 miles. This is my 3rd early 80's 300D. The car starts right up but runs rough at low rpm and has somewhat excessive shaking when idling as if only 4 cylinders are firing. Some unburnt fuel is blown out of the exhaust as a darkish cloud.

The problem has intermittently gone away, ie after driving up the road and back then idling in the driveway the engine runs perfectly smooth and blows no smoke. But taking the car back out it could be running rough again at the first stoplight.

The problem doesn't appear to be related to engine temperature.

I have checked the glow plugs which are all reading 0.5 Ohms. New oil+filter, secondary fuel filter, air filter. I've run two tanks of fuel with injector cleaner. I don't hear any unusual noises when the engine is running at low or high rpm.

I'm thinking this could be a bad injector or sticking exhaust valve, but have not seen this before. Any other suggestions?

RE: 1981 300D runs rough at low rpm
 

To find which injector it is crack the line loose at each injector.Watch fuel can/will get on the ground.Listen for the rpms to drop if they don't that injector is giving you the problem. Also the best way to run injector cleaner through is pull the fuel lines to and return and with some hoses added use a jug to run the cleaner through it as a concentrate. this works better. not as good but quicker is empty or use new filter and fill it with the cleaner. Were do you get your fuel from and what kind?

RE: 1981 300D runs rough at low rpm
 

I have an 83240D with exactly the same problem.

About atwo yearsago (after running beautifully the previous year and a half) my alternator went. It happened to be summer, and I was into riding bicycles, so I let my car sit for a couple months. Eventually I fixed the alternator and everything started up wonderfully and all was well.

Maybe a couple weeks later on city roads, I started idling REALLY heavy - first time ever felt anything like that. After that moment, it started running low speeds and DID NOT like any form of hill whatsoever.

I took it in to a shop, just to be sure, they replaced my fuel filter at my request in addition to priming it and getting it back on the road. Was glorious for about 3 days - and back to the same thing. I found that I couldn't even go near the highway - at about 45-50 the car just wanted to shut down. And at stop lights on city roads, I would stall out unless I put it in park. If it stalled out, it took a long time of pumping the gas pedal to get it back started again. The discharge out the tail pipe was always gruesome at this moment.

Anyhow, its been sitting again this summer, so I fear I've only made things worse, but I'm going to replace the fuel lines and check the injectors.

The guys at the shop - who were not Benz experts, thought it was my fuel pump...

Any thoughts?

RE: 1981 300D runs rough at low rpm
 

A bad/old rack damper bolt can cause rough idle. Also check all filters and at that time you might try taking off the fuel filler cap and then blowing air through the fuel line from the first filter to the tank and if thatclears it up for a whilethen your tank strainer needs to come out and get cleaned. You might try running some diesel purge to clear the injectors.

RE: 1981 300D runs rough at low rpm
 

The fuel is straight diesel (from kwik fill). When I changed the secondary fuel filter I filled it up with injector cleaner before putting it back on instead of pulling off the fuel line in front of the primary, figuring that would be just about as good. The primary looks clear/new.

I was expecting to hear a noise related to having a bad injector, but this engine sounds just about the same as my buddies 79' 300D and 84' 300D. Based on reading a few other posts here it sounds like maybe a valve adjustment would do the trick?

I will try pulling off a single injector and see if I don't observe any rpm drops at idle first though. Thanks for the input!.

RE: 1981 300D runs rough at low rpm
 

Sounds like how mine used to be. I changed injectors and adjusted valves, now its smooth as silk.

RE: 1981 300D runs rough at low rpm
 

Update: I did the value adjustment last week and found only 1 set of intake/exhaust values to be tight, the others were just a little loose or just about right and left them alone. The valve adjustment didn't make much difference as I had expected after only adjusting one valve.

Next I cleaned out the banjo bolt and line on the back of the intake as part of the ALDA service, this was completely clogged. I also found that the switchover valve had been removed leaving a direct linkage between the intake banjo bolt and the ALDA. After cleaning out this line there is no more smoke when I start the engine - this I did not expect. If I dont touch the throttle at all when starting the car the engine runs great for about 5-7 seconds then spudders and shuts off. Then when I restart the car it will blow a large cloud of smoke and continue to run rough. If I give it some throttle for about 20 seconds after starting it will continue to run rough but does not spudder and shut off.

The only time I see smoke is when restarting the engine after it stalls out but its a _big_ cloud of smoke.

This car also has alot of pressure through the breather hose - in fact today it split in half. There is also a some oil leakage around the seal on the oil cap. When I checked the valves I replaced the valve cover with one from a different engine - and both have this leakage around the cap.

For now I am waiting on receiving new injector nozzles and a rack damper bolt..

RE: 1981 300D runs rough at low rpm
 

Update #2: I rebuilt injectors with new nozzles, two were unbelievably dirty inside. After reassembling the engine runs much quieter, quite noticeable difference in power but the rough idle remains.

This evening I changed the damper bolt. I removed the original silver bolt, tuned in the replacement and the engine idle smoothed right out. I also replaced the breather hose and the shock mounts for the air cleaner. I haven't noticed any oil around the fill cap so far.

Thanks for all of the input.