Help me diagnose strange fuel problem

[e9coupe]

I have a '79 280GE that had an '83 om617a swapped in it years ago. I haven't had the truck long but after driving it found a strange fuel related problem. I've confirmed with the previous owner that it's had the problem for years.


Symptoms:


With 2/3 to full tank of fuel the motor runs great with no issues. Once it gets below 2/3 (approx) it runs the same while it's cold, but once it warms up it starts to lose power under load, like going up a slight incline. It acts like fuel starvation.


This is very consistent and predicable as soon as the fuel gets below a certain level and the motor is warmed up. The motor swap was done a long time ago, they used the original fuel tank and the lines look like they're connected correctly. I've run the fuel all the way down near empty and the symptoms don't change once under the level that it occurs.


How should I go about diagnosing? I have a fuel pressure gauge and was going to start by checking the lift pump. Any other suggestions. I'm a newbie to diesels.


Dan

[Sugar Bear]

That is a classic clogged tank strainer symptom and possibly a clogged vent. Try running it without the fuel cap when it starves. Fuel gauge definitely accurate?

[e9coupe]

Originally Posted by Sugar Bear

That is a classic clogged tank strainer symptom and possibly a clogged vent. Try running it without the fuel cap when it starves. Fuel gauge definitely accurate?

Thanks. I may have tried the fuel cap already but I'll try it again to make sure. Seems like it would have the issue all the time if it were a strainer or vent. How do you think volume and/or temperature would affect it?

[Diesel911]

Originally Posted by e9coupe

Thanks. I may have tried the fuel cap already but I'll try it again to make sure. Seems like it would have the issue all the time if it were a strainer or vent. How do you think volume and/or temperature would affect it?

Just run it without the fuel cap when the fuel level is low and see if it runs better.

[Sugar Bear]

A clogged vent will get worse as the fuel level decreases due to suction. A clogged strainer will get worse as the level lowers due to weight of the fuel column lessening as the volume decreases and the strainer are a few inches tall and clog from the bottom up so the top is less clogged.

[e9coupe]

Originally Posted by Sugar Bear

A clogged vent will get worse as the fuel level decreases due to suction. A clogged strainer will get worse as the level lowers due to weight of the fuel column lessening as the volume decreases and the strainer are a few inches tall and clog from the bottom up so the top is less clogged.

I remember now that I inspected the sending unit a while back and drove the truck with the unit removed to test the suction issue. No change in the problem and the fuel level unit works correctly.


The strainer is interesting since it's a few inches tall, but the problem starts at the 2/3 level or so which I would think is well above the strainer. I think the weight of the fuel may play a role in why it does it with less fuel, either by helping it get through the strainer or helping the lift pump work better.


I'm getting ready to tear the whole truck apart and do a "refresh" on the chassis and drive train. I wanted to figure out the fuel issue first but I may just pull it all apart and see if I can find anything then. If I have to fix it after I put it back together, oh well.

[Diesel911]

Originally Posted by e9coupe

I remember now that I inspected the sending unit a while back and drove the truck with the unit removed to test the suction issue. No change in the problem and the fuel level unit works correctly.


The strainer is interesting since it's a few inches tall, but the problem starts at the 2/3 level or so which I would think is well above the strainer. I think the weight of the fuel may play a role in why it does it with less fuel, either by helping it get through the strainer or helping the lift pump work better.


I'm getting ready to tear the whole truck apart and do a "refresh" on the chassis and drive train. I wanted to figure out the fuel issue first but I may just pull it all apart and see if I can find anything then. If I have to fix it after I put it back together, oh well.

Put like a 2-liter container of diesel fuel under the hood and see if it runs fine off of the 2 liters of diesel.

Concerning the fuel pump pressure test you need to read up on that. Even with a liquid filled pressure gauge the needle vibrates too much to read. There are threads where there are 2 methods to deal with that.

Also, you need to know a place to hook up the gage.

[vstech]

Another issue especially on gasser conversions is the screen for gasoline is MUCH tighter and diesel really cannot flow well through it.

Also, if the return line and supply lines are reversed, it can starve for fuel when the level gets a little low.