Lift pump pressure ???

I posted in the 'what did you do to your idi' thread last week about replacing my mechanical lift pump with one from O'Reilly. It worked great for three days. During that time I added some JP5 jet fuel to my front tank and now have super low power and can barely drive it.

She starts and idles perfectly, just no power past idle. I figured I had a plugged fuel filter, so I changed that out and bought a 0-15 psi pressure gauge to test the pressure post fuel filter.

Was getting a max of 3.5 psi and some times dipped down to 2 psi just idling. Figured the new lift pump was bad and swapped it out for a replacement from O'Reilly. Now I'm getting 1 to 3 psi on that pump.

I swapped from front to back tanks and have the same issue. I don't use the back tank as it is rusted out around the top flange and needs to be replaced, but it still had a bit of fuel in it.

I visually inspected all the lines and don't have any kinks or dents.

It starts and runs so perfect at idle that it almost seems like some sort of limp mode, but the fuel pressure is wrong and feels when you drive like it's fuel starved but perfectly smooth.

Looking for suggestions on what could be the source of such low fuel pressure before the IP with two new lift pumps ?

Cheers, Pete

Here's a video of it starting and running.


Let me know if this is weird or if I have something else going with the IP?

Cheers, Pete

Just a thought, but typically the "shower head" in the fuel cell is disintegrated on these trucks......maybe the pick up in the fuel cell is clogged?......

New lift pumps are garbage.
Run a hose from a fuel can to the lift pump and see what it does for pressure then.

The pumps are only specked for 4-5 psi and thats in a hypothetical dead head. The fact that fuel is moving and there is pressure drop across the filter 3psi is not unreasonable. Besides you just need to move fuel to the transfer pump in the injection pump and it will boost pressure to what the injection pump needs. If the injection pump had a large bypass of fuel but the transfer pump was still in good shape that would lower the fuel pressure out of the lift pump.
A much better way to test the lift pump would be run a free flow test out of the lift pump while cranking, and also deadhead into a gauge.

The mechanical pumps pump off of engine RPM. The higher the RPM, the faster the pump pumps, and the higher the volume and pressure of fuel. At least to an extent. That's probably not the case at high RPM and/or load. You didn't have either one of those so my guess is that it's probably working OK.
I'll also second what Black Dawg said.

How much is "some" you are talking about dry kerosene?right?
First, it's thinner, second, it's probably going to have less lubricity.
Let's see how / what the alternative fuel guys say...

Clb said:

How much is "some" you are talking about dry kerosene?right?
First, it's thinner, second, it's probably going to have less lubricity.
Let's see how / what the alternative fuel guys say...

Click to expand...

It's a blend of #2 diesel and Jet A ~ JP5 Not exactly sure of the ratio then added to a 1/3rd more #2 that was in my front tank. Long story, but my buddy recycles tractor trailers and the filtered mix runs great in all the big rig Peterbilt's and Kenworth's.

My truck is going to become a farm truck up near Butte, MT at 6000' elevation and I was testing a winter fuel mix for those times in years past where it might get down to -35 F.

Thought I had it sorted at first, but now have what looks like some fuel tank and possibly fuel tank transfer valve work to do

Cheers, Pete

Just got a test with a cup running directly down to the lift pump and got 4 - 6 psi after the filter.

Now, where to start ???

Drop the tanks and get it done or work back from the tank selector switch?

Quite the job trying to get a full tank emptied into buckets without making a mess. My slip tank I use for fueling excavation equip is full with that mix

Thanks for your guys help !!!

Pete

If running it from a fuel can fixed the problem, next stop for me would be looking at the tank valve. Did it on both tanks, right?

Black dawg said:

If running it from a fuel can fixed the problem, next stop for me would be looking at the tank valve. Did it on both tanks, right?

Click to expand...

Yeah, but didn't get enough time on the rear tank to be 'perfectly' sure.

Any suggestions for a new tank valve or can I just clean it?

Cheers, Pete

There was a write up several years back on how to rebuild one. I think it was by Oldbull88. I may be wrong on that part though.

Check each leg of the journey from the lift pump back to the tank...

R.i.p. Bill

Clb said:

Check each leg of the journey from the lift pump back to the tank...

https://www.oilburners.net/members/oldbull8.11026/

R.i.p. Bill

Click to expand...

Thanks for the link !!!

try pinching the return line at the top of the filter head.