I have noticed recently that when my truck warms up the pressure reading will drop to 8 psi at idle. If I step on the accelerator at all it jumps to 14.5psi and stays there for a little bit and falls back to 8 psi. When I drive down the road it operates as usual - 12-13psi at highway speeds.

The gauge is prefilter. I will change the fuel filter but was wondering about the low reading. Could the bypass valve be sticking wide open and running the lift pump off the curve?

Any suggestions would be appreciated. The lift pump was installed 10K miles ago.

 

Is it stock?? Please tell us about your truck. Update the user cp section, and put info about your truck. I also am seeing something like this. At idle, fp is 11-12 psi, but cruising it will be between 12-13 psi. At WOT, over 2k rpm, it will run about 11,but never below 10, so I am thinking that the voltage to the pump is greater when engine is over 1800 rpm. Possible voltage regulator issue??

 

Are you using an electronic guage?
Voltage fluctuation might come into play.

I installed a mechanical gauge to avoid those gremlins.
I see 15 @ Idle, 12 @ Cruise (2000 rpm), 10 @ WOT.

If your sampling pre-filter, changing the filter shouldn't affect your readings.

 

I am using an ISSPRO mechanical gauge. I agree that changing the filter won't do the trick but I thought that I would eliminate that from any possibility.

I wonder if I am getting a vacuum at idle on the gauge in certain instances.

It seems that the low pressure reading comes when the truck is warm and will need more acceleration to get it back to 12 psi.

Could there be an electrical problem at the pump, i.e. the alternator fluctuating at low loads.

 

Sounds like the same symptoms my truck had. I believe its the ball in the relief valve of the lift pump hanging up. I would suggest you change the pump if it has more than 30 k on it before you end up with 2 PSI like mine did out of the blue.

Good Luck

Jason

 

I noticed mine doing the same thing when i installed my fp gauge and soon thereafter the lift pump quit all together, i'd go ahead and change the lift pump before it dies completely

 

He just did 10k ago.