81 300SD idles really rough

[francisweng]

I have read a recent post on a similar subject by caquino and now have a better understaning of the mechanics of my diesel. Since then I have noticed that initial rough idling gives way to a smoother idling when the engine temperature reaches 90 F. Short of changing the rack damper screw what causes rough idling at around 700-800rpm?
Thanking all who have an answer for me.

Francis weng

[darrellws]

Francis, you did not state the mileage of your 300SD but since it is an '81 model I am speculating that it has ~ 200k miles. First question is have the glow plugs ever been replaced? Ever so it sounds like the spray pattern of the GPs is now good and the combustion is not firing until the temperature is sufficient to burn the fuel. If this is the case you need to have the GPs checked for spray pattern and pop pressures.

Darrell W. Sigmon

[francisweng]

Darrell,
Car has 165, 000 miles in avaerage condition.
How does the glow plugs affect the spray pattern of fuel? The engine simmers down after a good 5 miles of driving as engine temperature climbs to about 90F.
Would the injectors be the culprit? If so why does the violent shaking of the engine diminishes eventually?
Any technical help always greatly appreciated.
Thanks,
Francis Weng

[darrellws]

Fran, my mistake in putting the GPs in the reply although if the GPs have a lot of carbon around them they will lose some of the heat from being insulated with the carbon. What I was trying to tell you is the spray pattern of the injectors. If the spray pattern is 'rich' the pattern is such that the fuel will not ignite until the combustion chamber is hot enough, whereas if the pattern in a fine mist it will ignite very easily in a cold engine.'
Sorry about my mix up with the injectors and the GPs. I think you are on the correct trail.

You might want to try some cleaner in the fuel to see if the injectors are carboned up which will mess the spray pattern up.

Darrell

[Johnson Chan]

First of all diesels run better hot. So a warm/hot diesel engine runs better than a cold one. I notice this on all my diesels, they are quieter and smoother when reaching normal operating temperature.

Now, do you notice any loud sounds (i.e. nailing)? When was injectors last replaced or serviced? They are recomended as replacement around 100K. You can clean them by using a cleaner called Lubro Moly (performance products sells it)

Glow plugs- when were they last replaced? i am assuming your light comes on and off normally. If you have a bad glowplug it can cause what your describing. Ditto about the carbon comment causing insulation.

[francisweng]

Mileage on this sturdy 300SD stands at 165,000 miles. Mr. Chan as for mysterious sounds like 'nailing' I did put my ears very close to the engine but the rattling sounds are more like an engine attached to an eager horse wanting to pull away. At moments just after the engine has been started the driver's side hood rattles like a piece of sheet metal caught in a hurricane. On closer inspection no loose pieces of engine parts are evident except for the air cleaner which in itself is subject to some freedom of movement. Would almost clogged fuel filters play a role in rough idling? Thanks,
Francis Weng

[darrellws]

Fran, check the motor mounts to see if they have 'bottomed' out and are hitting metal to metal. It still sounds like the injectors need some attention. At this mileage I think that a fresh set would be a good investment.

Darrell

[Marshall Booth]

Francis, Darrell has covered almost all the possibilities. The roughness that you are experiencing is probably due to SOME minor difference in the combustion conditions in one cylinder. All of the factors responsible for optimal combustion must occur simultaneously or roughness WILL be experienced. Most flaws WILL be exagerated when the engine is cold.

It could be as simple as one fatigued or coked glow plug or an injector that is suffiently coked or worn to alter the spray pattern or injection pressure. Both of thses conditions WILL result in POOR cold idle. It is also possible that one or two valves require adjustment (a valve that is slightly too "tight" WILL result in poor cold combustion and this commonly occurs as they wear - valves require regular checking and adjustment every 15kmi). In the early stages, these conditions virtually vanis as the engine warms up and will be worse at idle speed (700-800 rpm) that if the engine is run just a bit faster.

Those factors are the most likely causes of your rough cold idle, but there are other less common possibilities. If the motor mounts have not be replaced in the last 10 years or so, they HAVE hardened and/or collapsed and that will result in even minor imbalances being MUCH more apparent. A dose of a reliable injector cleaner (Chevron Techron and LubroMoly Diesel Purge are good) and an "Italian tune-up" or even just a long highway drive will probably result in SOME improvement, but if the valves or injectors are worn, only adjustment, calibration or replacement will result in a "cure."

Marshall

[Clifford]

Fran...I have had a experience with my 1987 300 D Turbo with rough idle. I had to take it to a MB dealer and they found the injector pump was slightly out of time. They timed it and it is fine now.
Clifford.

[myavar]

I have a 82 300sd. At cold start it is rough and if I don't keep my foot tone pedal it will die. Once it warms up it smooths out at 750 rpm. When in gear at a stop light; the idle is rougher than if I put it in neutral or part.

Where do I begin?

thanks.

[Marshall Booth]

I have a 82 300sd. At cold start it is rough and if I don't keep my foot tone pedal it will die. Once it warms up it smooths out at 750 rpm. When in gear at a stop light; the idle is rougher than if I put it in neutral or part.

Where do I begin?

When was valve adjustment last checked. That MUST be done every 15kmi or so and adjusted when they don't meet specs or the engine WILL run rough when cold.

The injectors will drift and wear with mileage and that will roughen idle (cold idle more than hot idle) as the car ages. Glow plugs become less efficient with age and probebaly need to be replaced (but far less often in a highway driven car than in a city driven car).

Marshall

[myavar]

Marshall thanks for responding. I have owned the car for 16 years and have put only 45000 miles on it. I changed the glow plugs when I bought it. Have never adjusted the valves.....did not know had to do it and drove the car 2-3 thousand miles a year. The next question is what tools and how to adjust the valves? I will search the forum to learn while I wait on your response.

Thanks....Mike