Mysterious symptoms
 

Hello all,

just wanna throw this out to the forum to try and get a fresh batch of ideas for potential solutions. I'm having a slight problem with my Benz.
'78 OM617, 123.150

It's like a fuel system "gremlin" that only comes out AFTER high speed driving. No problem on start-up or regular driving or even during high speed driving but after, say 100 miles I-77 @ 70mph, pull of an exit and its like it wants to "chug down".. hit the pedal and it chugs away from the stop but it regains power after say 7 seconds. Then, the "chuggin" symptoms will continue after pulling away from stops for the remainder or the trip. Get home, shut off, cool down, next day.. symptoms will not reappear until AFTER a bit of full speed driving.

Long story short.. i've tried several things trying to cure this problem but with nothing obviously glaringly wrong i'm running out of ideas. i've changed both fuel filters, checked fuel lines for cracks and leaks, tightened all hose clams, visually inspected the fuel flow by looking at the filter, checked the air filter, and like i said... nothing obviously amiss.

one more thing. I have a two tank grease car system installed for years no problems. Using Just pump diesel in the "grease" tank, with the water heat shut off, there are NONE of the above described problems. So, based on that, I've isolated the problem to the stock fuel system.

Thanks for wrapping your brains on this for me. i'll appreciate any and all cements.

P

Two suggestions:
1. adjust valves. if you have a tight one or two they will expand when hot on a trip and not seal as well as when cool.
2. Treat your tank to a good dose of Biocide in case there is bacteria clogging your tank screen.

Do you have an old style or new style primer pump? I had similar symptoms with a leaky old style primer on an 81 or 83 SD.

Sixto
87 300D

Thanks. Valves were adjusted in December 2011 by my trusted MB mechanic. I have considered the biocide thing. Do you have a recommendation for a good product for that? I have also considered the fuel tank inlet strainer as the cause of my problems. Is there a method to test that without completely removing the tank and all?

Doesn't valve clearance actually increase when the engine is hot? The lack of sealing occurs when the engine cools down.

MB recommends GrotaMar 71.

You can remove the strainer from underneath the tank, it requires a large socket (46 mm).

i have new style but i did have some curious happenings with that pump. One of the times i was pumping on it it seemed like i heard a little "pop". could have "blown" it?? it doesn't leak fuel that i can see.

Have you cleaned the tank breather?

Is that the tube coming out from the tank and terminating in front of the left rear wheel? Whats that tube coming out the top of the tank and down the trunk side??

The tank strainer is brass, so if some veggie gets to the main fuel tank is can make a mess of the strainer. Even if no veggie it is most likely time to clean it anyway. After draining my fuel tank the strainer was still holding back about 3-4gallons. It was that clogged.

drove for a bit with the fuel tank cap off to try and get some evidence to support the tank breather idea. Car ran like a Champ! bobbin' and weavin' on I-85. Pushed it up to +80MPH a couple of times. No problems until i pulled off the exit ramp. Chugged away from that stop, i thought it might stall. then switched the tank. Drove the rest of the way home with no problem at all. so, i'm narrowing in on the tank strainer.

Clean the tank breather. Changed the tank strainer.... no luck.
Checked the fuel pressure valve.

Revisited the "New Old" lift pump that i had put in there a few months ago. Got is straight of a '85 300 and put it in mine.

Got the fuel valve rebuild kit for the fast lane and and did that job. BOOM!!

Much better!!

Seems like the valves inside the lift pump can become sticky with black crud stuff. nasty.