123 200D Starting Problem

Hi! Im driving a '76 123 200 diesel Mercedes. Despite its age, the car has been performing well, judging from the performance standards set in the manual. Its a beast of burden. According to the manual, our driving conditions are characterized as 'severe' (stop-and-go traffic, tropical temperatures). I drive it everyday in these conditions!

The problem is, after reaching operating temperatures (more or less 80C), the car will not start if you turn off the engine. That is, there is no problem if you start it when the engine is cold, but if you try to start it after driving for about 30minutes, the engine will just turn slower than usual and will not start. The only recourse is to wait for about 30minutes for the engine to cool down. Or you can push-start it. Either way, its been really irritating.

So, trying to solve that problem, we 'overhauled' the engine (Yep, overhauled it according to specs!). We replaced all bearings and gaskets, but not the piston rings, because they're quite alright and are expensive. We installed a stronger battery, replaced all fuel filters, had a change oil and filter. However, despite these efforts, the same problem arose, though this time, it seems that the engine cranks more easily.

What could be causing this? My car still uses the old type of injection pump which doesn't use vacuum to turn the engine on or off so it aint probably vacuum related (it uses the cable-type to push or pull the 'lever').

I haven't had the injection pump calibrated, so that might be it, though I do not know what might cause it to fail.

Could it be the starter motor? Why?

I need help. I usually do the repairs myself, if I have the tools for it. Im on the verge of a nervous breakdown! I love this car and I am not about to sell it or give up the ghost! Thanks!

[franklyspeaking]

My money's on the starter motor. Engine condition is best judged by cold starting. Even with a warmed up engine, if the starter is not turning the engine over at a good clip (I have heard 100 RPM on this forum), it won't start. Even though the starter motor is at rest it does soak up engine heat, and I suspect you have a heat related problem with starter motor.

I agree with the starter. Had the same thing on VW diesels. Suggest new starter with new armature as well as bearings and brushes. I know people who put new bearings and brushes in and cleaned up the starter only to find minor improvement that diminished quickly. If you have already re-built the engine-cost of a new starter is worth it. One other remote possibility first. I had a VW diesel that intermittently exhibited the same problem you have. Sometimes it cranked properly and sometimes not. New starter did not cure problem. The positive cable from the battery to the starter was corroded inside the cable so you could not see the corrosion until the sheathing was cut open. The high resistance of the wire caused the problem. Suggest you try new positive cable- don't know if your car uses a ground cable also - if so change it and clean the point where it bolts to chassis. Then apply a film of vaseline to the connection to retard further corrsion. Hope this helps

Thanks! Sure hope your suggestions work. Also, I've tried starting it with 2 batteries in a 24 volt pattern to perk up even a probably defective starter. Of course, the engine cranked really quickly but it still wont fire/run if its hot! What do you think?

Diesels need three things to work - heat, compression and fuel. If you have overhauled engine and it is mechanically sound - you have compression. If you whip it over with 24v - you have heat buildup from the repeat compressions. The only weak link left is fuel. Check your fuel shut off circuits - also check for an air leak in your fuel delivery as it could cause fuel drain back and the pump will have to try to build up fuel pressure again. The leak may be present when the engine is hot and not cold but enough fuel is delivered while the car runs and when turned off - fuel drains back causing hard starting. When the car won't start - check if your injection pump is actually putting out fuel. Other than that- did you change the fuel filter? If the engine is turning over like a banchee and won't fire - you are not getting fuel - find out where. That is all I can suggest.

[franklyspeaking]

but if you try to start it after driving for about 30minutes, the engine will just turn slower than usual and will not start

I based my conclusion of the bad starter motor on this statement in your earlier note. Slow starting RPM = no start; Problems could be poor connections (gound and battery, broken wires, etc), bad starter motor, or weak battery. Battery and connections are less likely to be related to temperature.

The only weak link left is fuel. Check your fuel shut off circuits - also check for an air leak in your fuel delivery as it could cause fuel drain back and the pump will have to try to build up fuel pressure again.

Hi! I did notice, after installing a semi-transparent hose that leads to the fuel filter, that the fuel did form bubbles and appeared to be moving back to the tank as soon as I turn off the engine. Is this not normal? How do I check the "fuel shut off circuits"? How do I check for an air leak? Thank you very much!

Did you install the hose to the fuel pump BEFORE the problem started or AFTER the problem started?

The problem occured even before I installed the semi-transparent hose, but I only discovered the "bubbles" and the movement of the fuel away from the (main) fuel filter after installation.

[psfred]

Don:

You have a fuel leak or defective hand pump. The old style hand pumps will go bad and leak air into the fuel. So will a bad fuel line -- the black fabric covering can hide deteriorated rubber.

On the W123 the fuel will siphon back into the tank rather than leak out -- if you can, park the car on a steep hill, engine down. This will cause the fuel to leak OUT rather than back into the tank, allowing you to locate the problem.

On a warmt start, the glowplugs don't operate until you run the starter, and if you have fuel deliver problems you can have staring problems.

I just replaced the glowplugs in the Volvo for this reason -- bad cold starts, but simply had to crank forever to get it going after it had been sitting for a while. New glowplugs fixed that, but I think you may have a fuel problem.

Wouldn't hurt the check the glowplugs, though!

Peter

Thank you all! I believe my problem is a mixture of all these factors, and you guys provided some very good insights. Thanks!

Don - glad it all worked out. One thing I learned working on cars is to take the problem down to the simplest circuits - only scutinize the circuit involved - and it becomes less intimidating. Just remember -a car is a whole bunch of simple circuits working together - as a whole they can be scary - as simple circuits - not so bad.