Engine/Transmission (1994 - 1998) serious loss of prime issues..please help

[Hootguy]

I've been chasing a loss of prime issue for the last 3 days now.

It all started with the fuel line steel inlet fittings on the top of the tank leaking,,I had a 2003 module from which we took the plastic inlet fitting unit and spliced it to the 96 inside module supply and return line,,took the module appart and cleaned up the filter screens,,,tested the inlet pipes to be sure our splice did not leak air and tested ok,,

While i was there,,changed the fuel lines from tank to pump,using a combination of braided line and a straight section of 5/16 and 3/8 steel fuel line. .

After all this work,,truck wouldn't start,,,would not prime at all,,so we pressurized the tank with a bit of air pressure,,,that got it somewhat primed and after some injector bleeding it started up,,but after letting it sit for 20 minutes it lost prime and would not start at all. ,

The filter was also replaced.

The next test i hooked up a small 12v motocycle fuel pump fed from a outside fuel container with both supply and return into it,,,and that would keep it running even when reved up... . then we disconnected the mootorcycle pump and just stuck the lines in the fuel container and reved up the mootor pretty good,,,a minute later the truck was dying almost stalled,,i kept a light foot on the throttle and it seemed to let the fuel supply catch up,,,but if i reved it up more it tries to stall...

This is making rip my hair outta my head,,which are hard to replace at my age,,

If anyone has any input i would GREATLY,,GREATLY,,appreciate it,,,

Thanks all. .

 

[Andy Redmond]

Sounds like your lift pump isn't working.



(lines in the can w/out the help of the 12v asst. pump). Could be the ck. valves in the lift pump.



Could be an air leak at the supply or return, that the pump you temp. attached overcomes.



Could also be the supply hose from the lift pump to the fuel preheater bowl, the fuel heater could be leaking causing loss of prime (air leak).



Overflow valve possibly stuck open or partially open, again your 12v pusher/assist pump may offer enough volume to overcome this. .



Let us know what you find,



Andy

 

[biggy238]

A quick test on the OF valve. Pump the lift pump manually with the truck shut down and see of you can here it break over. It will hiss audibly if it work properly. If it is stuck open I doubt it would hiss.

It's also a great time to add a FP gauge to help you out.



When you changed what you did, did you keep the antisyphon? That could have explained why it ran out after a shutdown.

 

[Hootguy]

Hi

I have all new fuel lines so i know thats not it,,

The fuel heater was removed 2 years ago,,,but the burnt out plug on top of the housing is still there,,but it was running fine until i repaired the fuel tank module with the fittings from a 2003 module i had,,,i only used the fuel inlet part,,we spliced the lines in with a small tube adapter we made,,,i took the module out again yesterday to check if that repair might have an air leak,,,but when i blow in in the inlet tube while blocking the other end i dont get an air leak,,i did take the bottom screen out and cleaned it,,also took the module appart and cleaned the othe small screen disc in the bottom of the housing,,,i'm pretty sure i reassembled things in the right order,,,,Does anyone know the exact way that bottom piece goes in. .

ANTI-SIPHON VALVE????? where is it,,and what could i have srcewed up???

Thanks for all the help,,,this problem is giving me fits...

 

[Hootguy]

I forgot to mention,,,about the overflow valve,,i took it appart,,checked the spring length,,13mm,,,all seemed good,,

I was never able to use the prime button on the lift pump to prime after filter changes,,i usually fill the filter as mmuch as i can,,spill a lot putting it in,,but it usually starts and runs rough for a few minutes,then it's ok. .

Note,,this is my second lift pump but it does have about 100,000 miles on it,,my original lift pump did work and i could hear the overflow valve hiss when i pumped it,,when i push the prime button i get no resistance what so ever,,,i never did open the bleed screw on top of the filter housing,,,does that make a difference,,,,,

Could my lift pump have been dead all this tiem??

Fuel pressure gauges aare something i would like right now,,,anyone have ideas as to what is the best way to go,,i would kind of like to have them under the hood to avoid the possibility of a fuel mess in the cab...

Thanks again,,much appreciated

 

[Joe G.]

You can check for air leaks with a five gallon bucket of water. Take the lift pump and fuel heater/pre-filter assembly off including the input and output lines to it. Restrain the pump push rod so it will not pop out. Pressurize the whole mess to about 20 PSI. Dunk it in a bucket of water. The bubbles will show you where the leak is.



You can test the lift pump by removing the curved hose. Attach a hose to the input to the pump. Attach another hose to the output from the pump. With this test there is nothing involved but the pump, no pre-filter, no truck fuel hoses. Put the input hose in a container of fuel and the output hose in a bucket. Jump the starter and watch what happens.



Too many times a fuel problem repair is attempted by just replacing parts until it goes away. Way better to find whats failing before replacing any parts.

 

[Hootguy]

Well,,

I guess the truck fixed itself,,,

It started up yesterday morning and seem to be running ok.

Could something logged in the overflow valve cause this,,or could there have been a airlock in the lift pump. .

I have to go to Road Atlanta in 2 weeks so i want to be sure the truck is reliable,,

Thanks

 

[Joe G.]

After I repaired the fuel heater leak problem I had low fuel pressure for a few miles. I think there was air trapped in the system some place. Don't know for sure. The problem went away about the time I was getting worried all over again.