Drop in lift pump pressure on cold mornings. Bad pump or sending unit??

[Challenger-II]

Well..... here is yet another lift pump question for all the experienced and seasoned pump veterans. I have the Westach electric FP gauge and lately when the truck is cold and I start to drive my fuel pressure drops to around 5-7 psi idle and will drop even lower under WOT. It normally is always at 13-14 psi idle and never under 9-10 psi WOT. After a short time driving, it returns to its normal pressures. I NEVER see this drop when the truck is warm. No... it is not the slight drop that is seen when the heater grid cycles.



Could it be a bad sender or a filter issue or the signs of a lift pump going??

 

[Vaughn MacKenzie]

Challenger, diesel is basically a very light oil. It becomes more viscous (thick) and becomes harder to push through the filter media. There have been others observe the same phenomenon. I would suspect everything is fine with your lift pump and gauges. What kind of guy do you think you are anyway, hotrodding a cold Cummins TD? tsk tsk tsk

Vaughn

 

[Big White Beast]

Mine drops too.....

Hate these cold mornings, and heck I'm in SoCal. (Read: Wimp when it come to cold weather, 22 years in Chicago was enough!)



Garrett

 

[JHansen]

Mine does it too!

Got called out the other morning so I had to go quicker than I expected to (read Espar never got as much time to work as normal) and I noticed also that my fuel pressures were WAY down compared to normal. Once the truck warmed up to normal the pressures came back.



And my gauge is mechanical-so I think sender error can be ruled out



HTH



Jason

 

[BigDaddy]

When cold, I also see a 2-3 psi drop in pressure, but this is only for a minute or two.



Challenger-II, you are seeing a 7-8 psi drop This raises a question for me. Do you use a fuel additive?



I use Howes. Not excluding the possibly of a failing Sender/Lift Pump, could this fit into the equation

 

[Challenger-II]

Big Daddy... ... Yes I use an additive. I suspect that I may not be using enough though since I am filling up with several more gallons than the bottle of additive treats.



I also dont see the drop in pressure immediately after starting but after I take off running. If it is the lift pump, I wouldnt expect to see a complete recovery to normal pressures after a short time??

 

[Bigsaint]

I see the same loss of FP with the Mallory pump now that it's cold, takes around a 20-25 mile trip before the psi's are back where they were in the warmer months. The cold psi's equal around a -1 psi loss, pre and post filter.



If you see a loss at start up when cold, it's the grid heaters drawing amps. away from the pump. I have LEDs wired into my grid heaters and when they are both drawing current, my psi's swing from 12 down to 9. It's pretty cool to watch the grid heaters in action, they actually both kick on at start up or key on if unplugged, then they alternate sides for a few minutes ( I beileve 3 mins. , or until you exceed 18 MPH), then they kick off.



Scott W.

 

[Steve Campbell]

I agree with the 'fuel being thicker when cold " theory. I fashioned a piece of insulating material into a "U" shape and "attached" it to my filter with only the engine side open. My thought was to keep the fuel from geling in the filter using the radiant heat off of the engine block.

 

[Challenger-II]

Originally posted by Bigsaint

I see the same loss of FP with the Mallory pump now that it's cold, takes around a 20-25 mile trip before the psi's are back where they were in the warmer months. The cold psi's equal around a -1 psi loss, pre and post filter.



Scott... . I could see a -1 psi drop but I'm seeing around a -7 psi drop. It doesnt happen when the truck is warm, only after taking off after a cold start and not all the time!! Maybe I should change my filter even though it is not that old?

 

[Bigsaint]

I agree, -7 is a bunch, mine doesn't get low even with the grid heaters on and if it was your grid heaters drawing down the current/psi's, they would jump right back to normal after you went beyond 20 mph. I doubt it's the filter as the -7 reading isn't consistent. I'd just keep an eye on those psi's ( like your not ) and watch for a consistent loss or a sporadic fluctuation of psi's when at idle or at a steady throttle. If you start to see either, then you know it's new pump time, IMHO.



Scott W.

 

[claymike]

I had my lift pump replaced about three months ago. It always runs around 13 – 14 psi. In the mornings it is not that cold around 37 degrees my fuel pressure drops to about 7 – 8 psi and near 2 psi at WOT. It takes about 5 – 10 minutes for the pressure to return to normal. This only happens in the morning after the truck has set all night.



Mike