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Old 03-30-2000, 11:36 AM
Posts: n/a
Thanks to all who have given me advise. to
recap, my 81 300D (240k mi) idles terribly rough. Jumps, misses and blows smoke when idling only. Smells like unburned fuel to me, very strong. When on the road, it runs smooth as silk. It just seems to take spells running rough at idle, but it is getting worse and more frequent.

Ok, I've taken out the injectors, had them cleaned and checked at a diesel joint. The man tells me they are fine! I was hoping something was wrong with at least one, to clear up my problem. Oh well.

What's my next step? I would think if something major was wrong with it, such as pump timing, etc. it would run rough continuously? Why only does this at low rmp idling, and not all the time idling, unless it's something minor?

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Old 03-30-2000, 03:24 PM
Join Date: Mar 2000
Location: Gainesville FL
Posts: 6,844
Did you check the compression with the injectors out? That is the first thing I would check with an idle only problem.

The reason that compression is more critical at idle is because compression is what you got after all the air is squeezed into the space at the top. If you have leaks this pressure is reduced. If you have leaks time is the next factor. At higher rpm the same size leak has less time to affect things.

The same concept follows for pump element check valves inside the pump. If the column of fuel leaks back into the pump between strokes different fuel quantities are delivered. Since this is a leak and time makes it worse it affects idle more than at higher speeds.

Steve Brotherton
Owner 24 bay BSC
Bosch Master, ASE master L1
26 years MB technician

[This message has been edited by stevebfl (edited 03-30-2000).]
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Old 03-31-2000, 06:04 AM
Join Date: Jun 1999
Location: Quezon City, Philippines
Posts: 79
The key and/or key slot on the auto-advance drive shaft could be damaged. This is the shaft which couples to the injection pump. The auto-advance drive gear is mounted on it too. The auto advance gear is a slip fit on this shaft, and develops excessive radial play over time.

You could also have a bad valve guide to valve stem fit.

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Old 07-07-2000, 04:10 PM
Registered User
Join Date: Jun 1999
Location: Los Angeles, Calif, USA
Posts: 521

Did you fix the problem?

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Old 07-10-2000, 11:41 PM
Dennis Taylor
Posts: n/a
I have been plaqued by the same issue. I note that until the Rpm is raised above 1400 the engine is rough and feels like its miss firing on one or more cylinders. At 1400 Rpm I can hear the turbo starting to wind up and the engine is instantly quieter and smooth.

If the ALDA is adjusted at idle, screwing it out quietens the engine but allows it to smoke. Conversely screwing it in harshens the engine and makes it rock worse. The rack damper has no effect.

The question I have for the group is:-

Can the injector pump go out of calibration at idle?
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Old 07-11-2000, 08:39 AM
Posts: n/a
Hi David and Dennis.

I had the injector pump adjusted/timed. It was out of timing pretty bad. It did seem to
help the idling but did not clear up the roughness, smoking and missing completely. But it runs far better, that's for sure. It used to get so bad it embarrassed me at a redlight, but if I rev it just a little it will clear up most of the time now.

I do notice that it gives the most problem when the engine is cold, OR after it's been running hard (70mph) for a while and come to an idle.Just running around town it doesn't have the problem that much as it used too.

I still haven't checked the compression as Steve suggested. That's probably next when i get the time. But if it's compression problem just guess I will have to live with it. It actually runs so good on the road and fires right off at start. I may get around to checking out the key that Jake suggested, but that will be down the road for sure.

Have a new problem, guess I'll post that on a new thread.

Thanks, Phil
81 300D 247K
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