Mercedes-Benz Forum banner

81 300D instantly losing prime

1K views 4 replies 3 participants last post by  cooljjay 
#1 ·
Hello all!

I run a general auto repair shop in Arkansas and have a predicament. I don't do a lot of diesel work, just enough to know how most of the systems work and replace parts here or there. I have a long time customer with an 81 300D that we do occasional work on. Beautiful car.

A couple of months ago, he installed an inline clear filter and the spin on filter. He drove it fine for about two weeks. One night he parked it, and it wouldn't start the next day. He had it towed to us. We pretty instantly determined it was losing it's prime, cracked the injectors lose, bled the system, and started it. Seemed like it ran fine after a minute or two. We pulled it around back and the next morning it wouldn't start again. I've checked and checked and checked and have found no external leaks. I'm not sure how to properly pressurize the system to check for other external leaks without the vehicle running (when it's smoothly going through the injectors). We can bleed the system out to get it to run (after extended cranking), and we can turn it off and immediately start it back up. However, if it sits for 5 minutes or more, it has the extended cranking issue again. If you wait an hour or two, you may as well crack the injectors back open after hitting the primer pump a million times.

The only thing I have noticed is that once it sits, I will sometimes get a few air bubbles coming out of the pressure relief valve up through the clear hose to the filter, but I've also seen a few bubbles come out of the filter in the other direction. It's not something I want to throw parts at, and it's been sitting out back on the reserve lot for over a month now. I've considered getting a rebuild kit for the pickup pump and getting a new primer pump to eliminate those possibilities, but again I don't want to just throw parts at it. I've also considered the possibility of a leaking injector or an internal leak in the injection pump, but I don't know how to test for this. I'm about this |-| close to getting it started and having him take it to a diesel specialist, but there is no one local that has a standing reputation for quality diesel diagnosis and repair. Any help would be appreciated.
 
See less See more
#2 ·
Does it have the mushroom style (unscrew and pump) primer or the later black cylindrical knob primer pump?

If it's got the old style you're probably getting air into the system through the primer pump. Replace it with the newer Bosch primer pump.

Dan
 
This is an older thread, you may not receive a response, and could be reviving an old thread. Please consider creating a new thread.
Top