fuel pressure

[bigguy69]

I have a strange problem. When I first crank my truck it will hold 8 pounds of pressure. After it warms up it moves to zero. The truck still runs and will go down the road. New fuel pump that is a 110 gph . Blazer tank in the rear. Changed the fuel filter 3 times. Still does it. Can a mechanical fuel pump lose pressure after warming up? What else do I need to check?

[71K20chevy]

Almost wonder if the pump's bad or something..

[bigguy69]

I was wondering the same thing. Will the fuel pump if it is bad slowly lose pressure?

[derotoreut]

Do you have a liquid filled fuel pressure gauge? If so, try draining the glycerine from the gauge. Some of these liquid filled gauges malfunction when warmed up. I had the same thing happen to me. When I drained the gauge out it worked fine.

[bigguy69]

Yep it is liquid filled. I will have to give that a shot.
Posted via Mobile Device

[RHestand]

Originally Posted by derotoreut

Do you have a liquid filled fuel pressure gauge? If so, try draining the glycerine from the gauge. Some of these liquid filled gauges malfunction when warmed up. I had the same thing happen to me. When I drained the gauge out it worked fine.

Dumb Question??
I've never heard of draining a gauge. How do you do that?
TIA, Ray

[derotoreut]

It's not a dumb question. Many of the liquid filled gauges have a rubber plug on them where they added the glycerine. You just remove your gauge, pop off the rubber plug and let it drain. It will drain slowly as this is somewhat thick. You will also have to tip the gauge back and forth to get all of the fluid out.

For some reason many liquid filled gauges have that problem. I read it on other forums and also heard it from a guy at Summit or JEGS (can't recall which) But when they heat up, the needle won't read properly. Mine did the same thing that happened to Bigguy69. It actually went down to 0 psig, but the engine still ran fine. Once I heard about the liquid filled gauge problem, I drained it out and mine is working fine.

[RHestand]

Originally Posted by derotoreut

It's not a dumb question. Many of the liquid filled gauges have a rubber plug on them where they added the glycerine. You just remove your gauge, pop off the rubber plug and let it drain. It will drain slowly as this is somewhat thick. You will also have to tip the gauge back and forth to get all of the fluid out.

For some reason many liquid filled gauges have that problem. I read it on other forums and also heard it from a guy at Summit or JEGS (can't recall which) But when they heat up, the needle won't read properly. Mine did the same thing that happened to Bigguy69. It actually went down to 0 psig, but the engine still ran fine. Once I heard about the liquid filled gauge problem, I drained it out and mine is working fine.

Thanks for the info. Dan. I'll definitely stash this in my memory bank.
This question came to my mind because several years ago I bought and installed 2 small liquid filled pressure gauges (Fuel and oil, side by side.) in my 98 Dodge/Cummins. (BTW,both of them seem to be working fine.) After finishing the install I then noticed one gauge is a little over !/2 filled with liquid, and the other a little over 3/4??
More questions:
1. What is this liquid? Mine looks like some kind of clear oil?
2. Are they supposed to be filled to the top?
3. Since some gauges seem to work better after draining, what purpose does the liquid serve?
TIA, Ray

[derotoreut]

Originally Posted by RHestand

Thanks for the info. Dan. I'll definitely stash this in my memory bank.
This question came to my mind because several years ago I bought and installed 2 small liquid filled pressure gauges (Fuel and oil, side by side.) in my 98 Dodge/Cummins. (BTW,both of them seem to be working fine.) After finishing the install I then noticed one gauge is a little over !/2 filled with liquid, and the other a little over 3/4??
More questions:
1. What is this liquid? Mine looks like some kind of clear oil?
2. Are they supposed to be filled to the top?
3. Since some gauges seem to work better after draining, what purpose does the liquid serve?
TIA, Ray

1. Usually the liquid in them is glycerine. Some liquid filled gauges have silicone fluid in them.
2. Typically the gauges are not filled completely to the top. I think it should be more than 1/2 full though.
3. The purpose of a liquid filled gauge is to absorb vibration and pressure spikes. They dampen out the gauge from shock and make it easy to read.

Typically a liquid filled gauge is more expensive and works better, but I am finding they have their limitations. The fluid installed in them is only good for a certain temperature range. I have heard that if they reach a temperature of around 140-degrees the fluid will have a tendency to break down. I also read that you can vent the liquid filled gauge because as the temperature increases, the pressure increases. A liquid filled gauge is typically not vented, whereas a standard gauge dial is open to atmospheric pressure. I heard you can poke a small hole in the rubber plug to allow it to vent, but you need to be sure that this plug is in the twelve O'clock position otherwise you will lose all the fluid. I just decided to drain mine and try it that way. Since then I have had no problems with it.