Engine/Transmission (1994 - 1998) Recovering Prime

[SSSmith]

What is the trick to recovering fuel system prime after replacing the fuel line hoses on a 95? Have I created a problem or is it operator error?



I have pumped the prime button on the lift pump 200-400 times, turned the engine 90 degrees and repeated, and pumped with the bleed bolt (10mm) on top of the fuel water separator loosened. I get no fuel at the bleed bolt. I believe new hose fittings are tight. I'll check that again.



In first attempt to prime, I ran fuel on the floor due to loose water sensor connection in the bottom of the new fuel filter. This tells me the pump primer is probably working. On tightening the sensor, the fuel leak stopped.



The original problem, prior to hose repalcement, is system losing prime after sitting for about 4 hours. Primer button was effective then in establishing prime to start it after sitting. Fuel pressure from lift pump was about 25 psi while running using homade pressure guage as described by signal73 and JoeG.



The old hoses were indeed cracked, and could be the reason for initially losing prime. Lift pump and pre-heater have not been removed since new. 157k mi on the truck now. I'm ok with replacing the pump and removing the heater, but I would prefer to find the source of the current problem first. I ought to be able to prime and start the engine as I could before replacing the hoses.



Thanks for any advice.

 

[Joe G.]

Try using the starter. Loosen the bleed screw. Crank it for 30 seconds and let the starter cool for a few minutes before repeating.

 

[SSSmith]

Try using the starter. Loosen the bleed screw. Crank it for 30 seconds and let the starter cool for a few minutes before repeating.

Thanks Joe. I did that and a little more, and got it started.



Double checking my previous work, I found the new hose fittings tight I dropped the new fuel filter to see if it had any fuel at all. It was full, but perhaps installed too loosely. I tightened it and pumped the primer. This time, bubbly fuel came out of the bleed screw, and the pressure gauge pulsed a little. Then your suggestion raised a sputter. A second try made a coughing start. It smoothed out and the test drive was fine. It starts instantly after shutdown now, as before.



Pressure at injection pump supply is 18-22 psi.



The test now will be attempting to start after it sits all night to see if the replaced hoses cure the lost-prime-after-sitting problem.

 

[Joe G.]

That fuel pressure seems to be a little low. I would remove the overflow valve and check the length of the spring. It should be just a little bit longer than 1/2"(. 550"). If it is not then stretch it a little bit. Be carefull, a little goes a long ways. If you stretch it too much it will wreck the tension in the spring.

 

[SSSmith]

The truck started instantly this morning after sitting all night. The loss-of-prime problem seems to be fixed with the new hoses. I took same cross section slices of the old return hoses. I could not find cracks running all the way from the outside of the old hose into the interior, ut some were very close. The notion that the cracks cannot be seen appears exactly right.



Joe,

Thanks for the pointer on the overflow valve. I'll follow your suggestion, but I have to run some errands with the truck first, now that I have it back.



I did check overflow valve and the lift pump by squeezing the new return hose from the injector pump. The pressure went as high as 60 psi, momentarily. I didn't want to hold it long. This tells me the lift pump is pumping fine, and the overflow valve is weak as you suggested. Have I got that right?



A question on the overflow valve spring. If the spring is shortened, hasn't it weakened permanently? Won't stretching it just be a very short term fix to a weakened spring? How long does the re-stretched spring generally last? A new overflow valve from Cummins is $97. 00!



Spike

 

[Joe G.]

Yep, that's the test for a weak overflow valve.



The overflow valve from TST Products is quite a bit less than that.



The spring stretch fix is temporary. Mine lasted about 180K miles before it gave up after I stretched it.