Intermittent Rough Idle When Engine Hot
 

Hello Forum.

My 1985 300D shakes a bit to much at traffic lights sometimes. This is with engine up to temperature. One can feel the whole car moving from side to side when it is at its worst, or so it seems. Passengers notice it and comment.

At other times, it might sit at a light and shake none, or hardly any at all, like I would expect my gas 300SE not to shake.

When it is shaking at its worst, if I give a bit of fuel to increase the rpm a couple hundred turns per minute, then it smooths out. If I let up, it is much more likely than not to resume the shaking.

Have you had and solved this problem before? What do you think the solution is?

Thanks!

It's most likely the rack damper bolt. Either needs adjusting or replacing. It's on the back of the injection pump. Look down between the oil filter housing and the IP. You'll see a nut and a adjusting screw sticking out.

Could be a number of things. More than likely getting air in a fuel line. Fuel filters... When is the last time you changed them?

Do you have the old or new style primer pump? What condition are your fuel lines in to the pre filter and lift pump? What kind of hose clamps (factory or other)? Do you have the "cigar hose" or just a piece of hose?

Worn engine mounts and shocks can cause it too. But there are other simpler things to check first.

Check the rack pin after you have ruled out the leaks. Look at the color of the pin. If it is silver, it is the older one. Gold color is the newer one.

How's the cold idle? Typically rough hot idle is caused by the rack damper. Clogged filters, and air leaks will cause fuel starvation(rough idle) at all temps.

Not in my case. Fixing a small leak at the hose barb at the lift pump fixed my "hot shake". The cold idle was smooth as silk with "the old filters from the PO".

$70 later, burnt fingers and a new rack pin that I didn't really need... It was a simple fuel/air leak.

If I had only checked the fuel lines... Sigh....:rolleyes:

That new rack pin sure is pretty though.:D

Check your engine mounts.

Good to know. Always a good idea to check the simple and cheap stuff first, fer sure. A new rack damper bolt ain't cheap.

I agree with what the others said concerning an Air leak. I had that same problem but not as bad. It turned out my Fuel Inlet Hoses were hard and while the clamps were tite you could still rotate the Plastic Filter.

For some reason it only let Air in when it was hot under the hood. Some SAE 5/16 30R7 Fuel hose that ever Auto Parts store sells (also rated for Diesel) took care of the problem for a few dollars.
But, also look under your Fuel Tank and see if the Hoses there are seeping Fuel. If so they need to be replaced also.

Also, is your Idle speed within spec.

I believe the Rack Damper was put there to take care of the extra shake when the Car was New and all the Engine components were in good shape. It was not intended to compensate for the Engine not running correctly or other problems like the Motor Mounts.

And, in fact I had to fix a list of stuff before the new Gold Color Rack Damper Bolt I had bought had and helpful effect.

Thanks Fellows.

I will check for the air leak in the fuel line first. I don't know how current the primer pump is, but I know that I have replaced it at least once. The cigar hose was replaced along the way too, but I don't recall how long ago it was. Long time, I'm sure. I Can't remember checking the fuel line near the fuel tank. As to the idle speed, I'm not sure. I'll check. I don't suspect engine mounts. I think new ones were put in when I had the transmission replaced at 319K miles. 25K miles ago.

The car always idles a bit rough when cold. After a minute or so, no problem. I replaced both fuel filters about a year ago, about 10K miles ago, or less. The little plastic filter is getting dark. I think I will replace that too.

Then I will look at costlier choices, if I need to go there. I guess I may become educated about what a rack damper bolt does if cheaper solutions do not pan out.

Thanks again folks. I'll report here when I get it fixed.

restoring to specified fuel pressure eliminated this for my '85

You are saying you had a lift pump failure?

Maybe he replaced the delivery valve? The spring is captive and cannot be removed and stretched in the 85's. Or he is referring to replacing clogged filters and or rebuilt or replaced the lift pump?

I'm curious too.

I have the same problem in my '93 and I will be installing new injectors tomorrow hopefully fixing this problem.

I also have white smoke at idle when cold and it runs really rough when cold.

When I give it some throttle it smooths out.

From some posts I've read on here the low pressure(?) situation that exists at idle will really expose worn/clogged injectors.

At higher RPMs the inefficiency of worn/clogged injectors is less obvious.

Somebody please chime in if I have given bad information.

I have this same problem.... :( My cold idle is fantastic and smooth, but if the engine is hot it is 90% likely to idle like crap. Every so often the idle will smooth out. Almost like an on/off switch. I have the new primer pump and rack damper bolt, but I didn't think to check for air leaks!! Will be doing that soon.

I always figured it was linked to the really bad knock the engine has (sounds like a rod knock, not injector). I've cracked each injector one line at a time, and the noise isn't associated with any cylinder. Weird. Would be AWESOME if it was an air leak. :D


I believe he adjusted the ball/spring that controls the pressure at which fuel can start to leave the IP and return to the tank. IIRC, you simply stretch the string out.

This is solid gold, I've had this precise problem with my car ever since I obtained it a year ago. It also sounds like I took the same backwards journey many of you did, starting with purchasing a gold rack dampener bolt only to find that it didn't help.

Diesel911 and snookwhaler, I was hoping make sure I understand exactly which lines you replaced. Both of you are talking about the two opaque black fuel line segments connecting to barbs and held on with hose clamps (compatible with 5/16" hose) that go between the hard line and pre-filter, then from the pre-filter to the lift pump, correct?

I'm wondering whether you two just replaced these opaque ~5/16" lines, or did you ALSO replace the fuel tank to hard line hose, and the three transparent (at least they were originally) lines that connect to banjo bolts:
1) lift pump to fuel filter
2) fuel filter to injection pump
3) (can't remember precisely) injector return line to fitting on filter housing, leads to cigar hose?


I really hope this does the trick, and am quite excited at the prospect of a calm engine. Thanks a lot!