Engine/Transmission (1998.5 - 2002) Heat Affecting Fuel Pressure?

[JFitzgerald]

I put in a fuel pressure gauge a couple weeks ago and I'm getting used to it. Normally, it runs about 12-15 at idle, about 12 cruising on the highway, and 9 under hard throttle.



The odd thing I'm seeing is that when I leave work at 5PM, the pressure runs about 3 PSI lower than at other times. It has been in the mid and upper 90's here. My truck is black and sits in full sun all day on fresh black asphalt. It's quite toasty when I get in, so I can imagine the fuel is pretty hot too.



Is this caused by the fuel having lower viscosity at the higher temps? Does anyone else see this? Am I goofy? Don't answer that. :-laf

 

[CORed]

Diesel fuel gets thinner with heat... up to a point. Your tank is shaded all day under the truck, and although it easily reaches outside temps by 5pm, is still cooler than it is after you have run the truck and recirculated the fuel for an hour.



3 lbs seems a bit excessive though and it could be related to a weak LP or weak batteries or maybe a bad grid heater controller that is turning your heaters on even when not needed. They take a lot of amps and this reduces the energy available to the LP... thus lower pressure.



My pressures are around 1 lb lower in the morning or afternoon after sitting for awhile. It seems they are best after about 30 minutes of driving. It is maybe 1. 5 lbs lower in the winter until the fuel warms up.



But I have larger banjos installed and that may reduce the difference a bit.



Just my experience.

 

[SnoKing]

Mine goes down about that amount when the grid heaters are active. Do they cycle when the truck is that warm? SNOKING

 

[JFitzgerald]

Thanks for the replies. I had not thought about the fuel being warmed while running (makes sense), but this seems to be the reverse. I have no indication the grid heaters are coming on. Going by the voltmeter, it comes back up to 14 volts right after starting. No wait to start light either. I know from winter time how the truck acts when the heaters are on, and I'm not seeing any of that. Also, the truck sat in the barn for two days over the weekend, and no grid heaters Monday morning. It has been hot here for at least a week, and I only see this lower pressure when leaving work.

The parking lot has new black sealer and it radiates a lot of heat during the day. My thinking is that the heat is rising up, warming my fuel tank. Plus the sun beating on the black truck. Last night for instance, I saw this when I left for work, drove an hour to my daughter's ball game, left about two hours later, and the pressure was back to the normal higher readings. Air temps had cooled during this time of course and I had parked in the grass at the ball game. It's not a big deal, I was just wondering if anyone else saw this in similar circumstances, and also a little curious if my LP is not up to par... .

Fueling is stock other than the Geno's large banjo fitting for the FP guage.

 

[SnoKing]

Maybe based on battery temp the alternator is cutting back the voltage a bit and as a result the pump is not putting out full voltage? SNOKING

 

[CDenton]

That's bull I WORK IN DEATH VALLEY 117 yesterday no fuel pressure change 12 full throtle 15 normal it's something else.

 

[JFitzgerald]

All I can say is that I only see this after parking all day at work. I've had the guage in two weeks now and it's been doing this consistently. I had 13. 5 PSI cruising at 70 mph this morning on the way to work. Experience says it'll be around 10 when I leave this afternoon. I'll just keep an eye on it. Thanks for the input.

 

[BDMiller]

One thing I have noticed which has not been mentioned-

I have Westach gauges in a black gauge pod on the top of my dash. When they bake in direct sunlight all day the gauges become less accurate. Remember the temps can get really hot in there & electricity flows differently under different temps. You can get a really cheap mechanical fuel pressure gauge from NAPA & mount it under the hood as a back up. I did this & found out that a lot of factors affect what the Westach will read, though it is usually close enough.

 

[JFitzgerald]

I understand what you're saying, about electrical guages. My guage is a liquid filled mechanical from Jeg's. It's mounted on the bottom of the dash near the shifters.

 

[rbattelle]

Are you using the A/C in the evening, but not in the morning?



-Ryan

 

[JFitzgerald]

Ryan - Yes, I do that sometimes, but I drive a couple miles with the windows down to cool the cab off a little before turning the AC on.

Now I'm having battery troubles, see other post. I don't think these are related, though, because it only happens after work.

 

[rbattelle]

Now I'm having battery troubles, see other post. I don't think these are related, though, because it only happens after work.

Anything that affects the vehicle electrical system (in particular the batteries) will change the lift pump speed. For example, my LP is loud enough to hear in the cab and when I roll up my windows I can hear it's RPM change slightly as the power motors draw current away from the system.



Your LP problems might be more related to your batteries than you realize!



-Ryan

 

[SnoKing]

Anything that affects the vehicle electrical system (in particular the batteries) will change the lift pump speed. For example, my LP is loud enough to hear in the cab and when I roll up my windows I can hear it's RPM change slightly as the power motors draw current away from the system.



Your LP problems might be more related to your batteries than you realize!



-Ryan

I think I said that a few post back also. SNOKING

 

[JFitzgerald]

I hope you guys are right. I'll install my new batteries tonight, so we'll find out next week when I come back to work, if it's hot. Have a great holiday weekend! We're going camping/Jeeping/fishing..... Oo.