Engine/Transmission (1998.5 - 2002) LP pressure fluctuating with temps

[mhenon]

After moving my LP down to the frame and installing Vulcan's Big line kit my fuel pressure came way up. I kept the in-tank pump as a pusher to feed the OEM LP and was getting 26psi @ idle and 22psi @ cruise. Nice!

Lately though I'm seeing pressures on cool mornings down to 10 or 15 psi. After work, when temps are in the high 90's, it's right back to 26psi. What's going on here?

Just for a test i parked the truck in the shade after arriving at work. Went out at noon and started the engine and had 13psi. I turned the truck around so the drivers side was into the sun. An hour later I tried it again and pressure went right back to 26psi. This really looks like a temperature related problem.

Any idea's?

Mike

 

[mhenon]

It gets weirder. Started the engine this morning and got 12psi. I turned off the in-tank pump (toggle switch in the cab) and was now running on the carter only..... 14. 5psi. WHAT... . how did the pressure go up? Turned the in-tank pump back on and got 17psi. After running for 1/2 hour the pressure finally came up to 25psi.

Either pump running alone does what you would expect. Cold temps or hot the in-tank pump puts out 7psi and the Carter about 14psi. When they're running together (in series) you just don't know what pressure you'll get 'till everything warms up. Then it's 26psi and impossible to pull less then 22psi.

I quess I'm not too concerned as the lowest pressures are way above what the in-tank did alone. I just can't figure out the interplay between the two pumps with cold temps (cold fuel?).

Mike

 

[mhenon]

I'm thinking now that the Carter is crapping out. It's almost as though it seizes up in the cold and the motor can't turn fast enough to build pressure. Once it warms up everything returns to normal. Can't believe the pump will hold up when the weather turns really cold. Luckily. . changing a pump is now a piece of cake. I've got a spare but want to see how long the Carter's going to last. The in-tank is maintaining a steady 7psi and will be my back-up. Surprisingly. . it's doing this while pushing fuel through a 20 micron filter, the Carter pump and a 2 micron filter. Maybe it really is a low pressure/high volume pump!

Mike

 

[mhenon]

Well... I swapped LP's but the problem was still there. Before tearing into the whole fuel system (again)) I checked the fuel pressure gauge and lines. Nothing but air in the feed line from the VP to the sender. Got the system primed and put it back together and what do you know... ... . 25psi!

I'll keep the old LP as it may be OK. The message here is to check the fuel gauge first before getting out the power tools.

Mike