Engine/Transmission (1994 - 1998) fuel press sux @ idle, fine otherwise ?????

[The patriot]

I havent driven my truck much since last year. Couple hunderd miles tops.

I did the fuel filter about 2 months ago. I had it out since then, but didnt notice this problem.

At idle, the fuel pressure goes down very low. Low enough to drop the RPM's at idle.

Thinking I may have a clogged pickup, I switched to the bed tank. Same thing.

I went over the fuel filter to see it it was tight enough, it was OK. I never tighten them bawl assed tight. It's now tighter than I ever had them on this truck. Checked the bleeder, t'was fine.

Pulled the overflow valve (orig part) and checked it. It's OK @ about . 550"

I'm going to try to get to the pre filter tomorrow to check it.

Any other ideas while I'm under there?

I saw no obvious fuel leaks.

I dumped the filter via the drain a couple times, it looked clear & clean, but slightly bubbly. I was running it when I dumped, so I'm assuming this may have something to do with the bubbles.

Any help is appreciated.

Eric



Oh yea, @ idle I'm running less than 8 PSI. At 2000 RPM's it's close to 30 PSI. I have two gauges to verify this.

 

[bmoeller]

If the pre-filter is OK, and no other restrictions on the suction side, it could still be the overflow valve. Could be the right length, but it doesn't mean that the spring couldn't be tired. Have the fuel lines (hoses) been replaced? Other thing would be, the lift pump.

 

[Joe G.]

The fuel heater, which is the part the pre-filter mounts to, may have a leaking electrical connection. If the heater element inside the heater burns up one of the pins in the connector can get loose and leak air. If it's a little leak it may cause the symptoms you have.

 

[The patriot]

Thanks guys, keep em coming.

I hope it's the pre filter possibly being loose. :{

Eric

 

[Smokin Ram]

Joe G's right, i had the same thing happen, but mine would not start. . took days to get to that one. 24 valve or 12 valve? if its a 12 valve maybe the lift pump it gone, like it wont put up the pressure at low end because its wore out, but if you spin it fast enough it keeps pressure, maybe the seals dried up form sitting? Maybe, if its a 24v then never mind.

 

[The patriot]

Awww crap

It's the fuel heater lines. Theyre fried. :{

I'm sure I can eliminate this, but does anyone have part numbers?

I need to do this on the cheap(at least for now), I havent worked in a year. :{

Thanks.

Eric



BTW it's a 1998 12 valve.

 

[Joe G.]

Eric,



You can take it off. The screw that holds it on is really an adapter. Take the prefilter off, take the screw out with a 10mm allen wrench. (I think that's the right size) Remove the fuel heater. The pre-filter will screw right in where the shiny screw went. I've been running that way for quite a while. My old fuel heater is a member of my failed part collection on a shelf in my garage.

 

[The patriot]

Thanks Joe. Cant get much cheaper than that!! :-laf

Any rough ideas on when these things are really needed?

The first year I got the truck, mine was disconnected from the factory.

It was down near 0 F a few morining and there were no problems.

I am however running fuel that ~ahem~ may not be up to the spec that fuel was. :-laf

Eric

 

[Joe G.]

I dunno. Maybe the folks living where their butt freezes to the ground every winter need it. If the heater fails and there is a problem it is an air leak not cold fuel. Of course, where I live the winters are not serious business.

 

[The patriot]

Not too serious here either. 0 to 10 below over night, maybe a few nights in a row at the most.

Gotta climb under there tomorrow. Even though I dont drive it much, I still like it ready to go at any time. Oo.

Thanks again.

Eric

 

[The patriot]

***update***

Awww poop!

I pulled the heater element. The wires were definetly toast, but I couldnt SEE anything wrong with the heater itself, even after I had it off.

The problem has not changed. Except for the fact it seems a wee bit more peppy on acceleration. This proves there MUST have at least some leaking here.

I'm now leaning towards the lift pump plunger being stuck in a partially plunged (???) position, thus it will not make sufficient pressure w/o the RPM's.

When I bring on the RPM's the pressure is absolutly fine.

I also drained the filter while it was running. It quit as soon as I opened the drain. It NEVER did this before. Well... . at least not this fast, which makes me think the lift pump even more.

Someone please convinse me out of pulling this off..... PLEASE!!!!! :{

I'm going to look at the fuel line hoses first, I dont think it's the O. F. valve, but I'll probibally get one anyway.

The return fuel line behind the fuel filter was done about two years ago.

Any more thoughts on this??????

Eric



Edit: wrench sizes to do this are, pre filter, 17mm, heater element 8mm hex wrench worked, but I think it's an SAE size I dont have. Maybe 5/16"?

 

[The patriot]

Any thoughts... ... ... ... ... ... ANYBODY????????????? :{

 

[The patriot]

Btt... ... ... .....

 

[The patriot]

I pressurised the fuel system today at the tank and searched for leaks.

None.

I tried the lift pump manually, and it seems to be in a full stroke.

Ideas?

Eric

 

[Lsfarm]

Tank venting?

Eric, try loosening the fuel cap on the tank, then see if there is any difference in your idle fuel pressure. Sometimes the tank vent fails, and the tank is running with a partial vacuum.



Hope this helps Greg L

 

[Dieselnerd]

When the lift pump fails on these trucks it's usually the check valves that fail. The symptoms are hard starting after sitting for a while, same symptoms you get with leaky return lines. When you manually pump you are not cycling the same piston in the pump that is driven off the camshaft. It's a separate primer pump made of plastic. If you don't have to pump it manually to get it started it doesn't sound like a bad lift pump.

 

[The patriot]

Thanks Greg, will try it.

Eric

 

[The patriot]

Ron, it starts in the 1st or 2nd revolution of the crank. No matter how long it's been sitting, except in very cold weather.

Guess I should keep looking... ...

Eric

 

[The patriot]

Eric, try loosening the fuel cap on the tank, then see if there is any difference in your idle fuel pressure. Sometimes the tank vent fails, and the tank is running with a partial vacuum.



Hope this helps Greg L

Come to think of it, I think I had the tank pressurised the other day while it was running, it didnt go up to normal pressure.

I know this dont have much to do with the vent, but you gotta think..... what the hell's going on... ... ... ... .

I gotta try that one again.

Eric

 

[Joe G.]

Eric,



I've been away from this thing for a few days. There is a last resort thing to do to check the pre-fuel filter system for leaks. Take it all off: steel lines, lift pump, and fuel heater/pre-filter. Put it all together and restrain the cam follower plunger with something so pressure does not pop it out of the pump. Presurize the whole mess with about 20 PSI of air. Put it in a five gallon bucket of water. I had to do that once to find the leak. It was the fuel heater in mine. Another guy found a bad fitting that looked good but many bubbles when it was in the bucket.