anyone ever had a lift pump fail slowly? (79 300td, 195,000)
 

Hello:

If anyone's ever had a lift pump go south slowly, I'd love to hear about how it happened.

For starters, it's a vegetable oil car with 20,000 veg miles, and though this may have been the root of the problem (especially several exposures to cold veg oil), it's now acting as though it's fuel -starved on diesel, so I think the vegetable side of things is now somewhat irrelevant.

Symptoms: More problems in hot weather, and when I've been running for a hundred or so miles. (would a failing pump like this perform better cold?)
More problems on vegetable oil (presumably more resistance in the filter, etc.)
Lot of cranking, plus waiting, plus revving required to get the prime back. (Initially after an episode, it revs fine but won't idle. Minutes later, it will idle fine but die under load).

I've cracked the return line several times, though not enough to say for sure, but I think the return stream is pretty minimal, and even non-existent when it's recovering from an episode.

Also, filling the diesel tank all the way seems anecdotally to help quite a bit (better hydraulic head to assist the pump?)

I'm travelling, in California from Massachusetts. If anyone has any ideas about how to take care of this problem cheaply in the Venice beach, CA area (or knows of a mercedes mechanic there who is particulary competent, I'd appreciate hearing about it).

Many thanks,

Brad

Brad,

Its unlikely.

How much of this stuff are you familiar with?
http://www.peachparts.com/shopforum/diy-links-parts-category/142399-diesel-fuel-delivery.html

there are quite a few simple issues that can cause what you are describing.

Your 'full-tank' syndrome says alot. Next time it happens, check the size of the air bubble in the clear prefilter. If air is more than half the volume it probably is not the fault of the lift pump.

sounds to me like a pretty standard air or restriction issue.

Replace all the filters, if you can't get to the in tank screen, for the interim you can swap supply and return lines for now and run the fuel in the opposite direction.

I hope it's an air leak or a lift pump problem. Your symptoms are consistent with problems that have appeared here after a person runs on vegetable oil. The first symptoms of a failing IP are inability to run on diesel while still running on WVO. The thick WVO can still be pumped but the thinner diesel cannot.

A fuel restriction in the line from the tank or the strainer. If the tank is full & you remove the fuel line from the primary filter, fuel should flow out, if not you have a blockage. If it does flow out, you may have a bad valves in the lift pump.try operating the primer pump, you should be able to hear the fuel flowing through the system, if not, the valves may be bad.

The wagon tanks are lower than the sedan tanks so the lift pump has to do more work. If you're not getting return fuel then there probably is a delivery problem.

You might take a peak inside of your Lift/Fuel Supply Pump.
If the Pump Piston is scored and scratched up it would not work as well hot as there is no Piston Seal it is just a metal to metals fit.
I hav also seen the Sping break.

It takes a 30mm to remove the big nut from the Pump to access the Piston.

The valves are not shown in this pic but they are mad of a fiberous plastic material. You can rub the Valve sealing face acrossed some 400 grit emery paper and clen it up a little. Also I have seen the little Valve Springs rust through before.
Fastlane sells a Lift Pump Valve kit with the Valves and Valve Springs. But, I would check the Piston first.

Or just get another lift Pump from the Junk Yard.

Fuel pressure relief valve adjustment bypass valve
http://www.peachparts.com/shopforum/diesel-discussion/234609-fuel-pressure-relief-valve-adjustment.html

Some long overdue follow-up...
 

Thanks for all the opinions.

It turned out to be something wrong with either the lift pump or the primer pump attached to it. Sorry I can't be more specific, but I was on the road and just replaced the entire assembly without any attempts at forensics.

Thanks again.

Great to see you got it going !!
Also great that you gave us all feed back on the outcome.
I am sure we will all be happy to give you future help.

Good Luck with your car !!

I had a similar situation. The engine seemed to gain 10 hp whenever the tank was filled. Replacing the two valves (and their associated springs) in the fuel pump made an amazing difference in performance.