Engine/Transmission (1994 - 1998) Erratic Fuel Pressure

[Deezul1]

I have just returned from a 5000 mile trip and noticed some strange doings on the fuel pressure gauge. It has been running about 18-20 lbs pressure most of the time for quite a while so on the 1st leg of this trip (not towing anything this trip) running 80-85mph for hours at a time I noticed the pressure was down to 10lbs or a little less and knew I was going to have to change the lift pump before heading home. On another 300 mile leg The pressure was back up to 20lbs again. On the last leg heading home with same high speeds for 10 hours the pressure was up to 22 or so. Now that I am back home its back to its normal 18-20lbs again.

I wonder whats up with the 10 psi reading? I dont want to get stranded towing the trailer some day. Power is good and the filter has about 10,000 on it and the overflow valve is a year old.

TIA

John

 

[The patriot]

Does the pressure vary to the low side when youre running low on fuel?

The fuel recirculates, so it gathers heat. I suspect it's losing some viscosity toward the end of the tank.

Just a thought.

Eric

PS You running an electric, or machanical gauge? My electrics (one is pre, other is post filter) vary quite a bit.

 

[KLockliear]

Change out the overflow/return valve. Very common to see low pressure with some mileage on those. A P-pump needs 20psi minimum to work with.

 

[illflem]

If you're running a electric FP gauge a failing sender would be my first choice as the source of the problem.

Most intermittent problems on any vehicle usually come down to electrical, something our 12 valve fuel systems thankfully are lacking.

 

[FATCAT]

From what I have experienced ...

The drop in presure is due to the warm/hot fuel. Being thined out, the presure drops.



My truck cold, idles between 20-22 psi. When hot, it idles around 12 psi.



(Measured from the bleader screew).

 

[The patriot]

I've noticed that in the winter I regularly see 30-32 psi on the fuel pressure.

Now that it's hot out, I never see over 25 psi.

I dont trust the electric gauge like I would a mechanical, but I didnt have enough room on my post mount for 4 gauges, so I did one electric gauge with a switch to two senders. One B-4 the filter, one after.

Eric

 

[Deezul1]

The overflow valve is about 1 year old. The high and low fuel pressures occured under almost identical conditions on different days, which were 700 miles a day across desert at close to 100 degrees at 80-85 miles per hour for almost the whole day. The low was the 1st day the high was the second day. I have a place I can tap the line at the sending unit and install a mechanical gauge also. Have to try that and see if the 2 gauges are reading close. The last time I tested them they were but who knows.

Thanks for the input, keep the info coming please.

John

 

[illflem]

I also see FP differences according to the temp but never down to 10 psi even after running all day at 100°. Minimum has been around 18 psi.