OM617 idles very bad - now with compression test results
 

I just picked up an old 300GD. Some previous owner swapped the NA OM617.912 with an OM617.952, which is a good upgrade for the heavy, big car.

But - there is a problem. The engine idles really rough, shakes like crazy and shakes the whole car. I feel some "puffing" in the exhaust which can seem like one cylinder is not running at idle. Everything seems ok when driving, all cylinders fire and power is pretty good.


I have:
* Adjusted the valves (they were all too tight)
* Used diesel purge directly connected to the filter/return
* Changed the intake/exhaus manifold (was leaking)
* Replaced both diesel filters
* Replaced fuel return lines

The clear diesel fuel lines look good. No bubbles or foaming except when letting go of the gas down from high rpms. (normal or not?)

Sometimes, it will idle a little bit better, but far from smooth. Problem is exactly the same with engine cold, warm or hot. Engine starts easy all the time.

What should I suspect? I'm just hoping it's not a problem with the compression, like burnt valves etc, but the OM617 is almost bullet-proof, so.. Maybe something with the fuel delivery? But if it was air, it should

Any suggestions on what to check out? I've had a few diesels before, but none of them ever had a problem, so I just don't really know where to start.

Did it run the same before the Valve Adjustment?
Is the Idle Seed set with in the limits on the Emission Sticker?
Is there any Smoke and if so what color is it?
How does it run when you are at higher speeds?

Try loosening the Oil Fill Cap at idle. If the rough idle goes away your Crankcase Breather system could be clogged.

Check the EGR Valve to see if it is function properly. Apply Vacuum to it at idle and see if it causes the Engine to stumble and lose rpm.

Get some longer 5/16" ID Fuel Hose and run it to a separate container of Diesel Fuel and see if it runs better like that. If it does you have a Fuel Supply issue to the rear or an Air leak.
Loosen and tighten one Fuel Injection Line at the Injector at time. This cuts the Fuel off to that Injector. Cutting the Fuel off to a good Injector causes the Engine to run even worse.
If you cut the Fuel off to a problem Injector the Engine there will not be much reaction to it not working. But, that only identifys a problem cylinder. To find out if it is the Injector you swap the position of the Suspect Injector with a good one. If the problem follows the Suspect Injector you know it is the Injector.
If the Problem stays with the Cylinder that now has the good Injector you have a problem in that cylinder of the Engine.

For More info:
DIY Repair Links
http://www.peachparts.com/shopforum/diy-links-parts-category/

http://www.peachparts.com/Wikka/DoItYourSelf

Thanks! :)

It got better after the valve adjustment, but is still far from good. I will try the injector "trick" later today. Is there a heat shield or something between the injector and head that should be renewed before reinstalling the injector like on the old VW/Audi diesels?

The engine has got no PCV valve, nor EGR as is it an European engine. The breather from the valve cover just goes directly to the intake, and it's not clogged. I ran the diesel purge from a bottle, but it didn't make any difference.

Will try loosening one and one fuel line to find the troublesome cylinder. If air is the problem, I would think it would just be running rough, not the same cylinder all the time, right?

Engine mounts need inspecting.

Yeah, the mounts are more or less shot because of some old oil leaks. The rubber has disintegrated and they will be replaced. Problem is that the car is missing - easy to see on the grey smoke and the "puffing" in the exhaust which clearly indicates a missing cylinder.

I'm thinking about pulling the injectors, have the nozzles replaced and the opening pressure adjusted. Problem seems to be mostly from cylinder #5, but #3 is also not 100%. The rest seems relatively ok.

Is the breather line connected before the turbo or after? On the turbo models, it should connect to an oil/air separator connected before the turbo.

If the Euros have a rack damper bolt (pin), check that too.

Yes, there are heat shields between the injectors and head.

Concerning the Injector Heat shields; while the should be changed for test purposes they do not need to be changed.
Also When I did the Injector Rebuild on my Volvo Diesel I did not realize there was heat shields that needed changing. I drove the car like that for 1-1/2 years until I managed to drop a Screwdriver into the Timing Belt of my ideling Engine.
When I got the Head Gasket set it came with new Injector Heat Shields. At that point I realized they should be changed.
re-using the Heat Shields caused no issues during the 1-1/2 years I drove it.
In post #4 in the thread below there is a thumbnail pic. The Idle Fuel goes through the #14 and #15 holes. My Mercedes shook a lot at idle and rebuilding the Injector removed about 1/2 of the Shaking issue.
I attribute the Shaking to the fact that those holes were completely plugged with carbon.
If you end up pullling an injector/s look and see if you can see if the hole is plugged.
http://www.peachparts.com/shopforum/diesel-discussion/284501-complete-remanufactured-injectors-nozzles.html#post2541395

I would start with a compression check. You have to find out where to start. Or establish some sort of baseline before going onward in my opinion. You want to work your way upward systematically. There is no sense in attempting to repair a house on top of a bad foundation.

Without even knowing what the engines general compression is like. Any another method of trying to move forward could be very frustrating.The harbour freight compression gauge model is about 25.00 unless you can borrow one.

I am not suggesting the engines compression is the issue. Until you test it though you just do not know. Starting by establishing a baseline is probably the cheapest method. Otherwise you will just spend money usually not knowing what results to expect. Could waste money in fact. I really do not know how to express this properly.

Certainly you could do two free tests first in my opinion. Substitute the fuel supply to the injection pump as suggested in a prior post is basically free. Plus close the return line from the injection pump off temporarily if the substitute fuel supply did not change anything.

Letting off the throttle and all kinds of air showing in the fuel line indicates there might be some difficulty there. I do not think that is normal.

There is a chance you could also have multipal problems. That is why the systematic approach is both cheaper and best if the two free tests do not indicate anything substantial.

You already state there seems to be some issue with #3 and #5 cylinders. I could list probably half a dozen causes of this apparent difference. Without a compression check after the two prior free tests it is just guesswork. Too early yet to have things like the injectors pulled and checked as well. You have no firm indication they are bad yet.

If the engine has been run on alternative fuels to your knowledge make sure to mention that as well. I base a lot of my assumptions on many examples of this vintage engine type having not recieved proper maintenance over the years. Many things like the tight valves you found. As long as the engines ran in some fashion the owners did not care.

Even diesels have sub systems that should be checked and maintained from time to time. If your compression proves to be in a reasonable range the other things can be chased down in a systematic way. This is the cheapest and fastest method by the way in my opinion.

All current indications are not bad either. You state the engine starts and catches fast. That is a positive thing.

I agree with a compression check, rack damper bolt and new motor mounts.

Not all turbo models had an oil separator in the PCV system. The early turbo 617's in the W116 had a hose going directly from the valve cover to the U tube at the turbo inlet.

ES is in Norway!

Before you go to much further expense and digging into other possibilies, it might be a good idea to double check your valve adjustment. When I first got mine, they were too tight as well. The first adjustment helped but I read a thread, here on this forum, that recommended running it about 100 mi. and doing it again. The second check and adjustment did wonders for smoothing out the running and idle.

Thanks for all the tips! :)

I will double check the valve adjustment, and yesterday I bought a diesel compression tester on Ebay. (weird that buying the tester from the US is about 1/5th of the price of a tester here) I think and hope that with new nozzles, rack damper bolt and another valve adjustment, things will smooth out. And yes, new engine mounts too. It's just that W460-specific parts tend to be rather pricey.. ;)

I made a video of a cold start today. It starts easy, fires up on all cylinders, but then starts intermittently missing. When it warms up (I will make another video), one cylinder misses all the time, I can hear, see and feel it in the exhaust pipe. Will do a compression test, but something tells me the compression should be ok when it starts easy and immediately runs on all 5, but then later starts missing. (just have to find out if there's room for the glow plug adapter for the compression gauge, as I don't have the correct socket for the injectors here..)

Any ideas?


Check the video:

http://www.youtube.com/watch?v=4kFA4Fpqa3c

a "correct" socket is not needed on these injectors, standard deep full depth 27mm or 1-1/16 socket works fine.
it sounds to me like you have an intake valve not opening properly. in addition to checking the valves, verify you don't have a flat lobe on one of your intake lobes.
you are getting a puff of smoke on one stroke of the engine... low compression could also do this.

oh, one more thing...
post some pics of this vehicle and it's powerplant installed!!!