1983 300SD intermittent rough idle

[83w126]

I have tried adjusting the valves several times and changing the fuel filters, but it doesn't make much difference. But, sometimes (especially after getting off the freeway, but other times too) it will idle so rough the the a/c lines are hammering against the frame, and the hood star (along with the rest of the front of the car) shakes side to side an inch or two, then at the next stoplight it will idle almost as smooth as it probably did when it was new. Is there any way to fix this?

[interzonearts]

Originally Posted by 83w126

I have tried adjusting the valves several times and changing the fuel filters, but it doesn't make much difference. But, sometimes (especially after getting off the freeway, but other times too) it will idle so rough the the a/c lines are hammering against the frame, and the hood star (along with the rest of the front of the car) shakes side to side an inch or two, then at the next stoplight it will idle almost as smooth as it probably did when it was new. Is there any way to fix this?

Did you look at the engine mounts?

[oldsinner111]

Rack damper bolt,tank strainer,air leaks,weak lift pump, can all be your problem.

[83w126]

Engine mounts have been changed twice, first with cheap ones then with decent ones, which did cut down on the ability to for the a/c lines to bang against the car, but it can still idle rough enough for it to happen sometimes. As for the other suggestions, how about I go about checking those things? Most of the time I find this car pretty easy to work on once I know what I am doing, but finding information to fill in the gap between what I know and the minimum knowledge expected to understand the answers to questions here can be difficult sometimes.

[Zacharias]

Shaking at hot idle is a common issue with the five cylinder motor.

What you describe can have several causes but I would suggest you start with the rack damper bolt.

If you do a search there should be lots of threads on this. The OE one was silver, with age all these cars need the updated version, which is gold. Shop around, prices vary widely, lowest I have heard in last year was $27 I believe.

This has to be adjusted with the car very hot, preferably after a highway run.

Given how bad your issue is, however, and assuming the mounts have not let go because of it, I would look at the injectors as well. A bad injector can act up when the motor is very hot. Forum member Greazzer provides reliable and reasonable injector cleaning and rebuilding.

The last possible issue is that the internal governor in the pump is worn. This also contributes to shaking after a highway run, but I have never heard of that alone creating the serious problem you seem to have.

What brand motor mounts did you install?

[83w126]

The motor mounts are fine, I looked at them just recently. The shock absorber things are probably dead though. The current motor mounts are phoenix brand. Bad injectors are certainly a possibility too, because it smoke at idle a lot more than when I got it too. Is there any written procedure for adjusting the idle and rack damper bolt? I don't even know where the rack damper bolt is, and I never felt very comfortable making adjustments to the injection pump.

[Zacharias]

If there is that sort of evidence for one or more injectors being bad, my advice would be that you should address that before trying to adjust the rack damper bolt.

The rack damper is there to smooth out the idle, all other things being equal - and they aren't from the sounds of things.

Once you have addressed the injectors, you can try to adjust the rack damper bolt that is now in there, but you chances of success will be limited based on my experience. The bolt has an internal spring and the upgraded one is stronger, to compensate for wear in the pump.

Replacing the bolt is difficult because of where you have to work but it isn't an internal component in the sense I think you have of it. It's an external adjustment tool, it has no contact with fuel.

The rack damper (aka damper pin, I believe) is located at the very back end of the pump, facing the oil filter canister. It has a locknut.

[otto huber]

I'm dealing with the same issue on my 300SD. The shaking isn't as excessive as yours but the idle can be bad after an hour of driving or it can be fine. I switched filling stations which has helped a little. To test your lift pump, run the cigar hose(fuel return line) into a large bottle. If the fuel flows fast and easily your lift pump is good. I have 5 good injectors from an old engine that idled well, so installing those will be my next attempt to remedy this situation.

[SunDance]

Adjusting the Rack Damper Bolt

You only want to do this with an engine as near to perfect as you can get: valves adjusted, all filters and fluids renewed, no chain stretch. Get all that right first.

Adjusting the Rack Damper Bolt

[barry12345]

Make sure you are not processing a lot of air at times. A clear hose on the return output of the pump if yours is too dark to observe is a good ideal. An air leak can be intermittent in nature as well.

Usually it is also good practice to change out the fuel filters on suspicion especially if the existing ones have been on a long time. Make sure the injection pump is flowing return fuel into the return line is important in my opinion.

A hot injection pump from lack of the cooling effect of fuel exiting the relief valve might cause a real reaction from the injection pump. That's if its operating temperature has risen substantially above normal. Say on a highway run is a good example.

All the time when those internal pistons are compressing fuel they are building heat. High enough retained heat and even the fuel might boil in there. If that where to occur the engine might run as you describe.

Someday hopefully I will temperature scan some pumps with and without the overflow functioning. After a highway run. At this time what I am thinking is not scientific enough in nature. Still there is some possibility.