Rough Idle, Cold Start W123 300D
 

I have a cold start issue that has become progressively worse. My car is currently in for service. The shop (dealer) first checked glow pugs as I would have suggested. The resistance test was good for all glow plugs.

Next was a valve adjustment and had minimal effect. They then proceeded to test the injectors and reported that they looked perfectly clean, tested OK.

Could a glow plug have the correct resistance and not produce enough heat?

Any thoughts on other potential issues?

Yes.

I've always had trouble testing the ohms of the glow plugs. I usually pull them out and give them 12v. Then I can see that they work or that they don't. You might also check the glow plug strip fuse. If it's cracked you could be getting low voltage to all the glow plugs.

Other thoughts are:
1. Are you running any biodiesel? Biodiesel gels at a warmer temp than diesel and you'll have trouble starting not matter how good the engine is.

2. Compression, have you checked the compression. Cold starts are rough idle are signs of low and uneven compression.

3. Rack Damper bolt needs adjusting. What year is you're 123. The later models have a rack damper bolt that if mis-adjusted can cause hard starts.

4. Filters, have you changed your fuel and air filters lately. The tank strainer could also be clogged.

No to the Bio-diesel. They did check the compression and told me that one cylinder was low at 25 bar, one at 28 bar and the rest were at 29 bar.

Don't think I have a rack damper it is an 81, normally aspirated. Tank and strainers were cleaned 3 months ago.

Also, the rough cold start was getting progressively worse, with a good bit of smoke until warm.

Um, what is a 'rack damper" and does the '83 300D have one?

Yes yours does have one. Do a bit of searching here, there is lots about it. But it's on the rear of the IP. Look down between the oil filter housing and the IP and you'll see it sticking out of the rear of the IP. It can be in need of adjustment or need replacing.

That's interesting. What would be cause?

I'm having problems too. I just checked the glow plug system this am and found that they all have good resistance, the relay seems to be working fine and shows 12v at the pins on the relay, but I'm only getting about 9v at the plugs themselves. I do also notice that the glow plug strip fuse gets quite hot, with even a tad of smoke, and it did also melt last year, when I scavenged one off my 80 300SD and put that in. It worked for a long time, until recently, when it no longer will start.

I don't know if it's due to issues with the glow plugs, though. Maybe it's a fuel related issue. But wanted to see if there are any thoughts on my 9v issue.

Thanks

You want to measure the voltage at the glow plugs, i.e. both + and - meter leads at the glow plug body. This will tell you if you have problems with + and - wiring. You should get 10 to 11 volts at the glow plug. If you get lower than 10, clean all battery cable and ground connections (don't forget ground strap under the car) and also clean the glow plug fuse and the glow plug connectors and make sure they have sufficient spring tension (you can take the connector apart and crimp each contact for a better connection).

Answer
 

Post#9 of this thread
http://www.peachparts.com/shopforum/diesel-discussion/141112-symptom-1-2-bad-glowplugs.html

More data on this topic
http://www.peachparts.com/shopforum/diesel-discussion/137732-glow-plugs-link-thread.html#post1019018



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I hope I'm not hijacking someone's thread, but I'm also having some cold start issues.

Glow plugs were replaced in September and the valves were adjusted a few months ago.

The car will start on the first crank almost instantly, but will not run unless I keep my foot on the pedal a little bit. Using the cold idle switch helps too. If I remove my foot after a few seconds, the car shakes and vibrates badly. After running for a minute, everything is fine. This happens below 45 degrees.

My only guess is that my 15w-40 non-synthetic may be too thick.

Any comments?

Sounds like air in fuel.

What causes this?

I've been wanting to work on my injectors as I am unsure of their condition.