Randomly loosing Prime overnight

[Diesel_Junkie]

So i have searched the Forums and cant seem to find any answers, I am hoping someone can help.



The problem is intermittent. Once in a while, after sitting overnight or a few days, the pump looses it's prime, and i have to manually actuate the lift pump in order to get it started. Seems as though air must be getting in some how.



Its a '92 almost stock in the motor, just a little extra fuel in the excentric and a 16cm2 housing.



Seems as though the problem is very random in nature, but the following things increas the chances that the truck wont start without a manual priming:

1. Sitting for many days rather than just over night.

2. Lower amounts of Fuel in the tank.



Sometimes i can leave it for a couple weeks with an almost empty tank and it will start just fine though.



I am wondering if this failure model works. It might be the lift pump. If the engine stops with the pump actuated one of the check valves may be mechanically held open and the other one may have a bad seal, allowing a slow drain back into the tank. This fits the randomness of the failure because the engine does not always stop with the lift pump actuated. Also supports the theory where the lower the fuel level in the tank, the faster the drainback.



Any Ideas???



Thanks a bunch!

 

[Greenleaf]

Yep...



Test and troubleshoot the fuel supply system. The fuel from tank to lift p;ump is in a vacuum. Test with a vacuum gauge at the lift pump.



The fuel from the lift pump to the injection pump is pressure. Test this with a low pressure gauge, say 15 or 30 psi unit.



You will need the proper fittings to adapt the gauge(s) to the fuel lines.



Specs of system operation are found in the FSM. Ask for specs if you don't have a FSM, and then... ... ... ..... GET ONE!!!!



GL

 

[JLEONARD]

A leak (sometimes) in the fuel lines somwhere may also be the culprit. I have found leaks by slightly pressurizing the system using an air gun into the fuel tank using a rag to seal the opening... . you only need (and want) a few psi. While a helper pressurizes the tank you can look (and listen) for a leak.

Good luck.

Jay

 

[bgilbert]

Yep check your fuel lines from the tank to the lift pump. Any leak or small crack will allow air to enter the fuel system. Also the fittings/connections on top of the fuel tank which you pretty much have to drop the tank or remove the bed to service, these can crack causing air to enter.



Then again it could be those big tires and big lift causing the no start condition??

 

[JLEONARD]

Then again it could be those big tires and big lift causing the no start condition??

:-laf :-laf

Yea... it's an altitude thing.

 

[Diesel_Junkie]

Thanks guys, I will go on the hunt for a leak.



I doubt its a lift and tires though, (smartass) This is where i wish i had a picture of the time i pulled my buddies Chevy Up a hill in 3 feet of snow while he was hitched to his 38' fifth wheel toy hauler. Only a cummins, a lift, a locker, and 8psi in the load range E's was gonna do it. Huge rooster tails of snow off all 4.

 

[Greenleaf]

I would parke the truck inside, out of the sun and weather elements. Keep it washed and well waxed. The ultraviolet rays are what is causing the ... ... ... ... ... ... ... ... ... ... ... ... ... ... ... .



Oooops. You said Prime not PRIMER :-laf :-laf



Sorry

 

[1stgen4evr]

Loosing prime

I have had this problem on all my trucks to some extent. In every case, it is less of a problem if I keep the tank near full. From some other comments on the forum, I suspect that there is an o-ring in the injection pump associated with the diaphragm that responds to boost pressure that is the problem. It is more pronounced in cold weather. As you mention, it manifest more significantly with longer periods of down time. On the road, I have on occasion parked with the front up on an incline. This makes the problem worse. At home, I can keep the tank full and park with the front low. I never have trouble when I do that.



There is also some interaction with the lift pump as, if it is in good order, the fuel doesn't leak back toward the tank. The valves in the lift pump are not cam actuated and only respond to fuel flow. If there is a problem, it is wear in the pump or trash. There is no filter BEFORE the lift pump and any trash can be a problem there.



And, I will repeat my mantra on these matters which I regard as always sometimes never problems. If the problem is only 'sometimes', and I can 'always' get the truck to go, I 'never' worry about what might be the cause. If it every got bad enough so I felt that I was abusing the starter, I would hunt it down and kill it. In a million miles behind a 6bt in a variety of trucks, I have only had to be towed twice. My fault both times.



But use the methods suggested and report what you find. The answer will be helpful to more than just you.



James