The Perp: '84 300D Turbo
The Situation: Goes BUMP-BUMP-BUMP at idle when at operating temp

WTF?

Car cranks & idles clean when cold. Just replaced & adjusted rack damper pin & did valve adjustment last Thursday. Driven down the road, she runs liked a cat shot in the a$$ -
THEN, when up to operating temperature, you come to a stop & she idles like a cylinder is dropping out; nasty rough idle. Put 'er in Neutral & rev it up, revs clean, then settles back to rough idle...

Fuel is fresh, injectors clean & proper spray pattern. New filters installed last week with rack damper pin swap, tank strainer to be pulled & replaced tomorrow as PM, but have tested fuel and no apparent algae/fungus/gunk issues. (such as those seen in photos of other board members' "fuel tank discoveries"). Injector bypass hoses new & no leaks -

Only appears to occur when hot - I could chase my posterior on this & eventually find the problem, but I chose to present this issue to the wise diesel Shaman on the MB hill of knowledge -

bnc

The telling is in the tale.

Sometimes that pin is a little tricky to set properly. If it does not contact the rack, the engine will shake. If it is jammed solidly against the rack, the engine may also shake and give you starting problems.

So, give it another try............turn it in 1/4 turn at a time........once it contacts the rack........until the idle smooths out...........not more.

have you checked the valve clearance lately? a tight valve will be tighter when the engine is hot.

good luck

tom w

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[SIGPIC] Diesel loving autocrossing grandpa Architect. 08 Dodge 3/4 ton with Cummins & six speed; I have had about 35 benzes. I have a 39 Studebaker Coupe Express pickup in which I have had installed a 617 turbo and a five speed manual.[SIGPIC]

..I also have a 427 Cobra replica with an aluminum chassis.

Have you done a compression test? A cylinder thats low at idle can sound fine when you crank it up.

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1985 Euro 240D 5 spd 140K
1979 240D 5 spd, 40K on engine rebuild
1994 Dodge/Cummins, 5 spd, 121K
1964 Allice Chalmers D15 tractor
2014 Kubota L3800 tractor
1964 VW bug

"Lifes too short to drive a boring car"

Thanks for the feedback Brian/t walgamuth/Stevo -

We swapped the rack damper pin to a new one due to the shake; old RDP had a bit of wear & plunger was weak - we (my 22+ year MB tech Dave & I) discussed the shake at idle & opted to replace the pin due to no known history of replacement... Dave has always done the tasks I don't have tools/time to perform - Master certified dealer trained tech at his own shop now. And to Brian's comment, after glowlight goes out, engine fires right up in just a few revolutions of starter.

Valve adjustment was performed at same time/day that rack damper pin was replaced - 2 valves were slightly out of adjustment, but each valve on a differing cylinder. Dave didn't recall which cylinders they were on.

Compression test not performed yet; next on list -

Curious, but my line of reasoning on this issue is that any other time that I've worked on an engine with a weak cylinder/low compression, it showed itself as off or misfiring/rough-running throughout the RPM operation range of the engine, whether hot, cold, or in between tempwise & regardless of idle or WOT - This engine cranks & runs beautifully, with crisp throttle response, until it warms up to just above 80C on temp gauge. Then it begins it's rough idle...

Any comments on this line of thought are respected & more than appreciated -

bnc

If you have ruled out the valve lash and the rack pin, the only thought that comes to mind is an attempt to pin down the cylinder with the problem.

It won't be easy, but, if you crack each injector line, you are looking for the group of four cylinders that run the best. The cracked injector is the culprit cylinder. The remaining four groups will run worse than "the best" because they all have the bad cylinder.

Naturally, the test is difficult because "the best" is not so great to start with because only four of the five are online.

This won't pin down the problem, but, if it works, it gives you a leg up on information........especially if you plan a compression test for it.

Naturally, if you can isolate the problem cylinder, you can perform the tried and true injector swap.......but.......I'm doubting the benefit of that because the engine runs perfectly fine when cold.

I have one question: Is the rough idle evident by looking at the engine? Is it shaking noticeably more than usual? Or, is the engine steady.......like it would normally be.......but you feel a rough idle in the cabin??

I've got a set of 5 injectors going through pop test & cleaning this week; may swap them in for S&G -

The engine shakes like a dog with a tremor - vibrates through whole car, but engine doesn't shake excessively or hop on the mounts - When it does idle rough, the idle will drop ever so slightly, then come back up, sounding almost like it's fuel starved for a moment - almost like you pinched off the fuel & then released it. And I'll add that you do feel the shake in the cabin; I'll throw the car in Neutral at a stop & rev it up, then let it drop back - it clears up for a minute then resorts back to a shake.

bnc

How about the o-ring on the secondary fuel-filter fixing bolt? Guess it would shake all the time, though, huh?

My wagon was doing the same thing and the rack-damper adjustment made the shakes go away.

Your car doesn't have an alcohol problem, does it? D.T.'s are a drag.

Jay.

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On the road, currently:
'83 300D (German spec.) 168k mi. - - Wolfgang (tucked snugly away for the winter!)
'87 300D 375k mi. - finally went over to "The Dark Side"
'87 300TD 225k mi.
Cannondale 66cm CAAD5 w/Campy/Jamis Aurora
Hujsak Custom Fixie/Landshark Track Shark Custom/Ahearne Custom 29'er

No Jay, the car doesn't appear to have an alcohol problem, but I'm starting to suck air on this whole issue & develop an alcohol problem of my own -

Probably gapped the glow plugs incorrectly - wait a sec, aren't they pre-gapped from the factory?

Clearly, that's one cylinder dropping out.

Now, if you can isolate which one it is......................


Make sure it's not something stupid like one of the 17mm nuts is not fully tight..........either on the injector or the IP.

I can't imagine a compression test providing any useful info for this situation. Anything internal to the cylinder won't be an intermittent problem.........to the best of my knowledge.


How many miles on this engine?

Roughly 197k without going out to look at it

One thought that Foster's (TM) just gave me was to drop in that Buick Grand National Turbo 6 & automatic & call it a night...

I'll check the injector & IP lines in the morning - Mr Foster is empty & I guess it's time to turn in -

SIDEBAR:
Brian - did you receive my message earlier re the coil spring tool deposit?

bnc

Have to think more on the entire situation.

I did get the PM and confirmed it via return.