Engine/Transmission (1998.5 - 2002) Low fuel pressure when cold, normal warm!

[MDale]

I just got my fuel pressure gauge hooked up again, and notice in the morning when the truck is cold around 30 deg. I only have 0-2psi for about a . 5 mile, then goes to 5-10psi for about 1mile, then goes to normal at 12-14psi. I've seen this for the last three days, and only happens in the morning or after the truck has sit for more than 5hrs. Has anyone else had this problem? Do you think it's lift pump going bad, or is the ecm cutting power to the pump for the first coulpe of miles? It works fine the rest of the time 14psi@idle, 11-12psi@cruise, and 6psi@WOT. Also is this hurting the VP44 I only drive light throttle until teh pressure comes up. Thanks in advance I hope to find the problem before it ruins something expense.

Thanks,

Matt

 

[College Cummins]

My truck has about the same thing happening, when cold the pressure reads about 3-4 lbs below what it normally runs, but after warming up it's fine until the truck cools off again I suppose. I would say that it's normal, but maybe my lift pumps on the way out too. I sure hope not. God Bless, James

 

[MDale]

My problem is alittle worse than that it only reads 2psi for a mile or so. I would agree that if only 3-4psi under normal on start is fine so your ok, but I have almost on pressure.



thanks

 

[DSpencer]

I would worry about the 2-3 pounds of pressure. Do you have a manual or electronic pickup for your fuel pressure guage? There may be a misreading of your fuel pressure due to low voltage as the manifold heater draws your trucks voltage down. See if the low pressure coincides with the low voltage and if so I would test the guage with a manual fp guage.

 

[BWillard]

I doubt that it is the lift pump because my FASS has only 8-10 PSI when cold and when warmed up it reads 20+PSI. This is with a mechanical guage also!

 

[MDale]

I have an electric gauge, so do I need to test the voltage at the sender or the gauge. Also it's a westach gauge if that helps.



Thanks,

Matt

 

[GHill]

I still have the factory lift pump in a stock truck. When I start my truck at that temp (25-35*F) the fuel pressure immediately goes to 12psi. After the truck is warmed up the fuel pressure at idle is about 15psi. I use a mechanical autometer gauge.

 

[DSpencer]

You should try and get a manual fuel pressure guage to verify your low pressure. I am not sure if it would hurt to run that low if only for a few minutes but maybe someone who has more experience could tell you. I have a westach guage also and mine will fluctuate some but usually not more than a pound or so. It'll read 15-16 at startup after the grid heater goes off when cold. In the summer my fuel pressure will read 1 pound lower. I can pull it down to 9 pounds on full throttle. I can also kind of tell when my tank is getting low because my pressures will be lower by a pound or two.

My first lift pump went out around 55thousand and my second is running pretty much the same pressures at 109k as it was at 55.

 

[Lightman]

As stated above when the grid heaters kick in your voltage drops so your lift pump runs slower= lower presure, my truck does the same even with a pusher pump when cold 15 lbs when warm its 22 lbs.

 

[MDale]

I've been watching the gauge and I think it might be that the fuel is thicker the colder it gets. It seem on colder morning it stays low longer, ex. 35deg, 2-3psi for 30sec then 10psi, at 20deg. 1-2psi for 1min then 10psi. Could this be the fuel starting to gel,then it gets heated by the fuel heater?



thanks

 

[WestTN]

MDale, you nailed it. Diesel fuel is more viscous when cold. Since you are taking the fuel pressure after the fuel filter the pressure loss is greater when cold. A 10 micron filter is pretty restrictive. The system fuel heater warms the fuel to something in the neighborhood of 165 degrees F after a time. That thins the fuel out and it flows through that restrictive filter much better. The key factors in how long it takes is ambient temperature and amount of fuel passing through the system.

 

[WestTN]

I forgot to mention I have an electric gauge with before and after filter sensors. Even when warmed up on a 90 degree day there is roughly a 2 psi pressure drop. My truck has never seen less than 30 degrees F, but the drop was around 4 to 5 psi drop then.

 

[ed9824v]

could the voltage drop be affecting the electric guage reading ??

 

[RJWEST]

Probably yes, but most of the fuel pressure drop is due to the low
voltage during the grid heater cycles.

I have a digital Voltmeter installed, The Battery/System voltage drops to 12. 4 Volts ( approx ) during grid heater cycle...
and coresponds to the fuel pressure drop.

As low as 2 psi with a holly blue. . Normal after grid heater cycling complete.

 

[jeepit]

My pressure is running the opposite. Higher pressure when colder 17-18 and lower when truck warms up 14-16...

 

[AFoulk]

My FASS puts out about 17psi when cold, and once I drive for a while and get the fuel heated up, I'm back up to around 20-21psi. Even after my intake heater is done cycling, I still have to drive it a while to get the pressure to come up, and I have a mechanical gauge (Isspro). I'd say its the viscosity of the fuel when cold causing your pressure to drop. How ever, it doesn't sound like your lift pump is to healthy anymore.

 

[SSolderitch]

my mechanical gauge reads 13 psi cold at 30 degrees and 16-17 psi when warm. measured at the pump. I have a fass pump with a inline filter on the suction side. It is also pushing through a 1/2" line from the tank to the filter housing on the motor. I was only reading 8 psi when I bought the truck then I kept adding antifreeze until I got most of the air out of the line between the isolator and the gauge. Check that line because air compresses more than liquids, hense a low reading gauge.

 

[CRead]

The grid heaters should shut off after around 18 mph so if you have a pressure problem after that I would say pump! I went through this for a few colorado winters. Installing a new pump helped every time. But I had that pressure problem with the factory pump setup and the worthless bully dog system did it worse. I installed the fass hpfp and all better no worries now. Always have over 12 psi in the cold or wot dont matter!

 

[DWeiler]

Airdog gives me about 16-18 warm,and then from 10 above to -45 Celcius will stay at about 14PSI until the fuel warms up. Which can take about 20 minutes of driving the highway speeds. After all the fuel pump problems of the past(stock/Carter/Fass/Product Engineering#@$%!#@$%!#@$%!)i would worry about every little pressure drop,thinking that this pump was on its way out too,now after 3 1/2 years of reliable pressure from the "Airdog" i don't worry to much now,kind of glance at the guage once and a while,i also cannot pull the pressure below 14psi when i put my foot into it as well. DW