GregMN

Sudden onset of a no start condition. It was cold, high of 5. It had been starting and running fine in that weather. Wife drove it to work and at the end of the day it would not start.

A month prior it would not start for her. I drove to it and gave it a jump and it started right up.

New battery in the last month.
2/3rd's tank of fuel. So same fuel the last few cold days.

We had it towed home to my heated garage ( about 45 degrees ).
It would not start after sitting overnight, so I don't think the fuel was jelled.
The fuel pump is humming.
Fuel is coming out the line after the filter.
No CEL or codes.

We left town for a week. Still no start when we returned.
After some research, I ordered a crank position sensor yesterday.
Today it started.

Can a crank position sensor be an intermittent problem?

Any other ideas?

I am leary of driving it until I know what the problem is.

Plutoe

The major driver concern today is the intermittent issues such as yours where there are no faults---it sets up situations where you really have to know the engine systems and have the test equipment to patiently diagnose all the potential areas such as fuel and electrical----plus you have another negative---which one of the local butcher shops knows anything about an MB diesel!

If you want to guess, yes a CPS will cause issues such as yours, but how are you going to test that theory, without wasting money and time!!

GregMN

That is the dilemma. I drove my wife to work (65mile round trip) and made 2 stops on the way home. It worked fine.
I would be happy if it was jelled fuel. The only thing I did after I returned from my trip was to remove the fuel line, after the filter, to confirm fuel flow to the high pressure pump. The next day it started.

It seems as though something shut off the injectors and now they are back on.

At least it didn't happen at a rest stop out in the middle of nowhere NM.

For $36 I have a CPS coming. If it happens again I will check for a signal.

I think that I own over 1/2 of the MB's in the county. The nearest dealer is about 200 miles away.

Plutoe

If that was my situation, I would slowly invest is a DAS Xentry system,especially if I was a diesel owner!!

GregMN

I have found that the fuel pressure relief valve is not holding pressure.
When running the starter, the pressure is around 2000psi.
I unplugged the electrical connection and then plugged it back in.
When I ran the starter, the pressure initially spiked to 6000psi then dropped back to about 2000psi.
I used a heat gun to warm up the valve end of the fuel rail.
I unplugged the electrical connection and then plugged it back in.
When I ran the starter, the pressure initially spiked to 9000psi then dropped back to about 2000psi.
The spikes show up on the gauge for much less than a second. Not long enough for a cylinder to fire.

I seems like the pressure relief valve is not available separate from the fuel rail.

Has anyone ever heard of a pressure relief valve going bad on a cdi ?

mersum1es

Yes, sometimes they do fail. Either spool jammed or o-rings of the valve body broken. Try to find second hand valve from breakers... MB used to sell valve as a separate but nowadays you need to buy whole rail

Fuel Quantity control valve (located in hp-pump flange) is also controlling fuel pressure, I've read they have failed more often. Then leak in any place of the fuel system (injectors, hp pump) cause pressure drop during the cranking, but I think it would not even give pressure peak then...

Good luck!

GregMN

No fuel being release from the rail by the pressure relief valve. So it is not leaking pressure.
When I unplugged the power to the pressure relief valve on the fuel rail, the pressure stayed at 64psi. (I think that 64psi is the at rest pressure reading for the sending unit.)

I am thinking that it is the valve on the high pressure pump. I unplugged the power to it and there was no difference in rail pressure (2000psi +/-).

GregMN

I took the valve out of the pump.
The o-rings looked fine.
I squirted some WD-40 into the valve.
I jumped power to the valve. on/off, on/off many times.
More WD-40, more on/off cycles.
I reinstalled the valve.
After a few attempts, that cleared the air out of the pump, it started.
When the fuel pressure got over about 2700psi, it started.
Fuel pressure at idle is around 3000psi
Fuel pressure at high rpm, no load, is around 13,000psi.

mersum1es

Like Plutoe said, star has test procedure for each component...

GregMN

The pressure in the rail was being released by a faulty injector.

I had the car at a MB dealership for diagnosis. They could not find the problem. To be fair, the car would always eventually restart for them in the shop. It is a 3 hour drive, so the car was fully warmed up and the shop is warm. It was driven right into the shop when I got there for my appointment, so it never cooled down. I drove the car home and it would not start the next morning, just like before.

I finally pulled the injector return lines and installed a 6" piece of clear tubing on all of them. During the start attempts, only the faulty injector's tube had fuel in it. It never triggered a fault code.