Engine/Transmission (1994 - 1998) Fuel Pressure Corrected for Temp???

[MStearns]

I know all the specs for fuel pressure at an idle, under power, ect... But the specs don't mention anything about temperature and my fuel pressure is decidedly different depending on the temperature. Currently I'm starting it up here in the morning around 40 degrees outside and it idles above 30 psi... last summer when I was going through Vegas and it was 100 degrees outside I was lucky if it was idling with 15 psi. So is there a specific temperature at which the fuel pressure is supposed to be within spec??



In my mind the temperature really shouldn't make a difference as liquids are not really compressible. But the temperature obviously has an impact here. As the truck warms up the fuel pressure will drop a little but the biggest impact seems to be the ambient air temp... At least from my casual observations.

 

[JLandry]

Mark,

Are you taking the pressure reading pre or post fuel filter?

If taken before, it could just be the higher viscosity of the cold fuel having greater resistance to passing through the filter, thus increasing the measured pressure.

If taken after, then that would seem to blow my above theory!

John L.

 

[MStearns]

Yeah, unfortunately it is not that easy John, I am taking the pressure reading post filter. Probably should have included that in my initial statement. But yes, I tapped the banjo bolt exiting the top of the fuel filter on the way to the pump.

 

[JLandry]

Mark,



I'm still thinking it could be caused by a higher resistance to flow somewhere downstream of the pressure tap due to an increased viscosity of the fuel. But I have to admit that on my truck I generally see just the opposite. I get higher pressure readings with hot fuel and lower with cold fuel.



Good luck figuring it all out,



John L.

 

[Nyoest]

The compression if the liquid isn't the factor, viscosity is. Warmer, Thinner fluid pumps easier/ flows easier than cold thicker fluid.

I just installed a tork tek snubber for my in cab gauge to reduce flutter. Since the install and eliminating the old needle valve my pressure "shows" 5 psi less at idle and quite a bit lower at WOT. I also have the tork tek OV valve. I don't know which readings are accurate. I haven't had time to install yet another gauge to check accuracy.

 

[MStearns]

Ah, but thinner or thicker fluid should not matter because it is a regulated. The pressure regulator, the OV valve in this case releases excess pressure. My experience with hydraulic systems bear this out... in the airplane I fly our hydraulic system is regulated at 3000 psi and regardless of temperature it maintains 3000 psi. No matter how hot or cold it is. First thing in the morning a cold day... 3000 psi... at the end of a 2 hour low level through the desert in hottest part of the summer... 3000 psi.



I also have a Tork Tek OV valve, which I suppose could be the culprit but given the simple design of the OV valve I just don't see why it should be impacted by cooler temperatures. But otherwise it would almost have to be the result of some restriction in the injection pump... . Assuming of course the sending unit for the fuel pressure gauge is not being affected by the temperature.



I suppose if I really want to trouble shoot this right I need to plumb in a mechanical gauge to cross check my electric one.

 

[harrisonm3]

Dropping fuel pressure is a sign of a clogged fuel filter. Did you change your filter between summer and winter?

 

[Nyoest]

No arguments from me. But I know any engine with a real oil pressure gauge started on a ten degree day will have higher oil pressure than when started on a hundred degree day. Similar system, same symptoms due to temps.

 

[MStearns]

Unfortunately changing the fuel filter doesn't have an impact.

As far as the oil pressure analogy... Engine oil pressure isn't regulated. The oil pump just pumps oil to the top of the engine and bearings, then gravity collects it back in the pan. So as the oil heats up it flows more freely and doesn't require as much pressure to move and feed the top of the engine and bearings.

In my mind the fuel system, at least the portion between the lift pump and the OV valve, is a closed system. Thinking as I type here... If we consider it a closed system then I see 3 possible culprits:
1. The OV valve is allowing the thinner fuel to leak past.
2. A restriction within the injection pump... sticky plunger perhaps, something that frees up as the temperature increases, perhaps an internal screen or filter?
3. Or a weak lift pump. My lift pump is just about brand new and the fact that I can adjust the fuel pressure at idle to over 30 psi even with hot fuel tells me the pump is capable of holding pressure just fine.

If the fuel system isn't really a closed system but rather more of an open system like the engine oil, then my question is at what temperature do the fuel specs apply? Because that will impact the values. The spec at idle (17-22) I've been reading as a range but perhaps it is simply a rather sloppy minimum. The spec under power at full throttle (21) is obviously an absolute minimum value.

Maybe I am just over thinking this and I should just use the fuel pressure specs as minimum values regardless of temperature? But again, because the fuel pressure is regulated it shouldn't drop below the regulated value until the fuel output from the injection pump exceeds the capacity of the lift pump. And as long as the volume output by the lift pump doesn't exceed the flow capacity of the overflow valve then pressure shouldn't rise above the regulated value. In theory...