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  #1  
Old 06-23-2017, 06:24 PM
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'82 300CD rough idle at coasting 800 rpm only

Ok, I've scoured the web looking for something similar as to what's wrong and I need some advice. 1982 300CD, daily driver with 210k on the clock but a replaced engine by the previous owner at around 100k. (her husband ran it out of oil, is what I was told).

Problem: over the past 3 weeks the car has had a rough idle while slowing down/coasting below 30mph to almost a stop. Replaced a tie rod that was bad but it was still doing it. It's been getting worse, so I replaced the fuel filters the other day and threw in some diesel additive. The white bottle, they were out of Redline. The car has definitely picked up horsepower, but the shaking has gotten worse.

I've experienced rocking/vibrating engine in my 81 years ago that a valve adjustment cured, this is much worse. Only the harsh vibration at 800 rpms. Below at a stopped idle of 650 rpm, nothing. Anything over 800 or 850, it's smooth as silk. Very weird!

My conclusions are Damper Bolt adjustment or replacement, or it needs a valve adjustment. Although I'm sure it could use a valve adjustment I just don't feel like that would surface so quickly and now violently. I'm guessing the increase if fuel is exacerbating the issue, whatever it is.

Has anyone else experienced similar rocking? It started in what I thought was a suspension problem since the front of the car was swaying side to side from 30mph down to 5mph. Applying brakes or a little accelerator would immediately stop it.

Well, those are my 2 conclusions. Anyone have anything else or similar experience? Thanks in advance!

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1982 300CD (daily driver) 186K clicks

Last edited by JellyDonuts; 06-23-2017 at 06:25 PM. Reason: Moving to a different forum. This was the wrong one. Sorry.
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  #2  
Old 06-23-2017, 11:29 PM
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Newish poster and welcome aboard. Sounds like a good car.

My guess and only a guess is you might be taking air into the fuel system. When you slow down the presence of the air is making itself known.

The other item is one should always install new fuel filters. All too often they can produce problems. You never count on them being the issue. Yet it is the only way to make sure they are not. People on occasion have landed up like a dog chasing their tail over problematic fuel filters.

If they are not the problem. You take the removed ones and put them into secure fuel proof containers in the trunk and carry the tools to change them. They then are emergency spares.


This can save you money sometime in the future. Also carry some fuel in a small container to fill the secondary fuel filter before installing it. This can all be contained in a small box. If on a trip and you have a filter problem develop without them the costs before you are running again can be substantial today.

Last edited by barry12345; 06-23-2017 at 11:44 PM.
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  #3  
Old 06-23-2017, 11:44 PM
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Thanks Barry! The new fuel filters I just put in this week were Mahle, same as the ones that were already on it. I've always used them on this and my 81.
I haven't read anything about air leaking into the system. Any idea where to possibly look for a leak? Over the past 2 weeks I've crawled all over the engine bay and underneath. There were any leaks ... it's definitely running quicker after the filter change and additive in the tank. Blessing and a curse now that the rough idle is more pronounced. After driving 30 mins the roughness will also occur at a standing idle.
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Old 06-23-2017, 11:53 PM
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If you noticed a running improvement with the new filters. I might suspect the lift pump. The overflow or return line from the injection pump should be putting out fuel at idle. If it is not the fuel pressure supply at idle is too low.

When you used the primer pump to prime the system. Did the effort to pump steadily increase to fairly stiff and you heard the relief valve make a noise? If this occurred chances are reasonable the lift pump is still okay. If not post it.

You do not have to actually adjust the valves. You do have to make sure that they all have clearance though. As the valve stems heat up they expand. If any are too tight the cylinder will stop firing. This may not be the case at elevated rpms. Yet occur at low rpms as the valve leakage then is more critical. If you have no ideal of when they were last really checked especially. You pretty much have to get this possibility out of the way. It is a regular maintenance item anyways. to check then every 15K anyways. I do not blame a lot of past owners as they probably only have known of hydraulic lifters in their lifetimes and totally ignored this requirement.

Last edited by barry12345; 06-24-2017 at 12:05 AM.
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  #5  
Old 06-24-2017, 12:02 AM
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It was only a couple pumps to prime, just enough to fill the small filter. I always fill the larger filter with fresh fuel before installing. I sounded like normal when I did it. Any way to test the pump?
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  #6  
Old 06-24-2017, 12:23 AM
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Quote:
Originally Posted by JellyDonuts View Post
It was only a couple pumps to prime, just enough to fill the small filter. I always fill the larger filter with fresh fuel before installing. I sounded like normal when I did it. Any way to test the pump?
The finger pump has to develop real increasing resistance until you hear the relief valve on the injection pump open. This noise occured? It makes a detectable noise. This has to occur before accepting things are okay In the area. In itself it is a test. Sounds from your description it did but not certain. When the pressure does not increase and the squeal is not heard. Either the relief valve is open or the lift pump check valves are not sealing well enough.

If it did squeal valves checked are pretty much next. Or you could choose to insert the return line from the injection pump into a container with some fluid in the bottom and watch for any air coming out as well as fuel flowing in at idle. No overflowing fuel means there is a problem. If there is it may or may not be your issue but just an age thing that also needs addressed as well. Air on the other hand showing up as bubbles if present. The source of it has to be found.


Now if the hot idle is so rough it could be a cylinder down. You might locate it by loosening each injector nut in turn.

Last edited by barry12345; 06-24-2017 at 12:44 AM.
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  #7  
Old 06-24-2017, 08:55 AM
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OK, I get what you are saying now about the squeal. Yes, it did that.

I guess the thing puzzling me most is the fact that the vast majority of the time the roughness is at 800 rpm and stops going higher ... and lower? I never noticed the 2 different "idle" speeds before, one in motion and one at a complete stop. That's where I haven't found anything on the web to associate to on this.
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  #8  
Old 06-24-2017, 10:15 AM
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Lubro moly

If the fuel additive helped you might want to get some lubro moly
(liqui moly) and purge your entire fuel system.
It has made a lot of difference in some of my cars.
I think there are instructions on this forum.
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  #9  
Old 06-24-2017, 11:14 AM
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If the idling issues are not present until the engine warms up. You have to at least check the valve train for some clearance present. Does not mean you have to get right into a valve adjustment. Just that reasonable clearances already exist.
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Old 06-24-2017, 02:35 PM
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Rough idle only rears it's head after driving at least 30 mins, and that's sometimes. It's intermittent at best. Driving today, I went about 32 miles and the idle was VERY slightly rough, but not concerning. Yesterday it was knocking my kidneys around like Mike Tyson in the 8th round after the same 32 mile route. lol
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  #11  
Old 06-24-2017, 04:52 PM
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Just out of curiosity what sort of shape are your engine mounts in? Those often only effect idle
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  #12  
Old 06-24-2017, 06:19 PM
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Mounts are good and relatively new. I got the car 3 yrs ago? Replaced air cleaner bracket, noise was bothering me. Also tried adjusting the damper bolt. Easier than I thought it would be, fyi cut your 12mm crap wrench in half to make it much easier.
Result, no effect on the idle turning the bolt. Hmmm, i think we may have a winner. New one was ordered last night just in case.
On the "you've got to be kidding me" side, after a start up and test ... the car didn't shut off with the key. Great. First time that's happened with this one. My 81 had that and a valve adjustment made that go away.
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  #13  
Old 06-24-2017, 06:33 PM
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I asked if the car had to warm up before the problem. The valves stems lengthen with the heat. If the valve clearances are not enough combustion issues can then occur. At higher revolutions the piston speed can make this less of an issue. Again no certainty it is the issue. It just could be.


There are other possibilities of course. This should be checked and gotten out of the way.
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  #14  
Old 06-24-2017, 07:57 PM
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I can tell you she's running like a bat out of hell. Wrenches, feeler gauges and damper bolt on the way. Hoping with those two, we'll be back to good standing.
Now to search for what I did today to prevent it from shutting off with the ignition. hahaha
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  #15  
Old 06-24-2017, 10:00 PM
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You may have done nothing to add the shut off issue to the mix. Actually in becoming defective it might also have caused some of your idle issue.

Do not jump on it though with that thought in mind. Just have a look around to see if you did disturb a vacuum line.

The test for the shut off itself is either with a hand held vacuum tester. Or take a piece of hose. Attach it to the device on the injection pump. With the engine running and sucking on it should shut the engine down if it is good.

If that test works get the car warmed up leaving the original vaccum hose off and plugged with a small bolt or golf tee. See if your hot idle is the same. This is a really long shot but unless you locate something you have disturbed you have to pay some attention to possible clues. Once again not really signifigant yet if you do not find anything you did. I would test it.

There is a very, very slight chance the vacuum control valve at the back of the ignition switch was going out and in the process was creating some form of effect at hotter idle. It finally let go totally. Just keep an open mind.


Last edited by barry12345; 06-24-2017 at 10:12 PM.
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