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New variation on a long crank issue

1K views 3 replies 2 participants last post by  iragreenstein 
#1 ·
I have a 1992 300E 4Matic. There seems to be an issue after the car has sat overnight in that it takes a long time (5 - 10 seconds) to crank. Sometimes it just takes multiple tries. I have had it at my mechanics mercedes shop and they have tried everything i have found on this and other forums so far. They have checked the accumulator, EHA (replaced for other reasons) they put on brand new fuel pumps, checked OVP, Fuel Pump Relay and such. It seems that after the new fuel pumps were put on, the pressure looked good. The car started right up after 1 hour or 6 hours of inactivity. However, after sitting overnight (one of the techs drove it home and back to work in the morning), the pressure drops to zero bars. These guys have been around for over 20 years and have a good collective of knowledge. However, they seem to be stumped. They are not throwing in the towel yet, but i thought i'd poke the collective on this one.

Thanks!
 
#2 ·
Hi ira.

When you say zero bar, where do you mean? Supply side or control side?

I guess it's not on the supply side since you have new pumps. It could be in your return fuel line. Check the fuel pressure regulator in the fuel distributor. I don't think they mean the control side after the fuel distributer. One hardly bothers to check that out in a garage on your way to work, unless you're a real early bird idealist with some real good coffee. So the zero figure might be just an assumption. And remember that your cold start valve has the same pressure as the other injectors. The bad thing about the pressure sinking in the supply side while the engine cools off, is the vapour lock that occurs because of that.

When I diagnose this kind of thing, I start with battery, charging, and OVP. Then verifying no leak between the EHA and the fuel distributor. Then I check the accumulator. Today, on a 92, I would simply swap it, no matter what. It might seem fine, but can still cause problems like you have. Finding out if it works or not is easy. Finding out if it's giving you problems or not is hard. Then pump and one way or check valve. At last CIS injectors and cold start injector.

The best way to test an injector is to use a special jar like test aparatus. I use a new or good one, depending on who's the customer, and do the swap tango. Sometimes I'm lazy and take them in to a friend who owns an independant MB shop.

I'm not quite sure, but I think if control pressure drops, supply pressure should remain the same. And that only cranking can rebuild that pressure because the pump stops as soon as the pressure in the supply side is sufficient. I also think the cold start injector won't spray untill sufficient pressure has been established in all seven injectors, while the cylinder ones will. It may be hard for the fuel distributor to build up sufficient pressure while the injectors are releasing fuel.

In your case I would concentrate on the accumulator and injectors. It could also be the beginning of a failing fuel distributor. I think. It's getting late.


Regards

Geir
 
#3 ·
Geir,

Thank you very much for the reply! Your post was quite informitive. I just got back into town and spoke with my mechanic. Without any prompting from me, he told me that today they had put on a Fuel Distributor from another car and were going to try it tomorrow morning. However, the going rate for a Bosch reman is around $1800.00. Rather high. If it turns out to be the fuel distributor, i might hold off and put up with hard starts.

Thanks again!
 
#4 ·
Just to conclude this post, i picked up my car today. The mechanic had a Fuel Distributor off of one of his shop cars that was totalled last week. That seems to have done the trick. He tested it with two consecutive days of cold starts. The used FD was a might bit cheaper as well.
 
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