Chasing the prime!

I have been reluctant to start a thread on this, but need help. I am losing prime on my truck in as little as thirty minutes or less. I have sealed any external leaks and checked all my lines. It seems to lose on the pre-filter side, meaning that if I hold the schrader valve open it takes several revolutions before I have fuel coming out. But wouldn't the truck start on whats in the pump and then die? In my case the it takes about ten seconds of cranking before I have fuel pressure and then it fires. This happens after just minutes of sitting. If you start the truck withing five minutes of it already having run it will fire on first revolution of engine. It also doesn't matter if tank is full or empty. I have been told it could be the check valve in the IP return nozzle? I am not sure if it is leaking down pre IP or post IP. That would be the first thing I need help determining. I also started having trouble with my tank switching valve about the same time. Coincidence? Fuel gauges started acting funny and after filling both tanks and going on a trip I stopped and decided to fill up the tank I was driving on (front) and noticed extremely good mileage, so I tried to put some in the other tank and sure enough it was down several gallons even though I had not switched to it? Okay, need ideas where to look for my prime, I'm tired of chasing it.

In my extremely amateur opinion I'd check the fuel lines in the rear and the fuel pump.

I have tried three lift pumps. I am thinking of redoing all of the rubber lines.

If I understand correctly, it sounds like you're probably losing fuel/ pressure before it gets to the engine. I think you're right in replacing the rubber lines. Particularly, the ULSD used now is really ******* rubber.

Dire, I am not sure where I am losing the pressure. If it is before the pump it seems my truck would start then stall. But I have to crank for awhile to even start?

I was having a similar problem a long time ago. Losing prime that is. I ended up removing the factory fuel heater in the top of the filter head and replacing it with a pipe plug. Fixed my starting problem. I think you are having a problem with your FSV though. Good luck with it!

CP, it is funny you mention the fuel heater. Mine was leaking badly. I pulled it apart and put a new o-ring and then sealed the top with permatex!

Mine leaked as well. I replaced the O Ring and no problems. However, I had no noticeable running problems when it did leak, but it did keep blowing my TCM fuse afterwards; so now move is unplugged.

I had the same priming issue and resorted to a 3/8" check valve between the pump and filter. I pressurized the tanks, pulled vacuum, replaced rubber, plugged the heater, eliminated the filter bowl, and just never definitively found the source.

BD, that is how I am feeling right now. I will probably try an electric fuel pump first?

An e-pump may mask the issue well enough to operate without problems. It's not clear if it's related to the FSV... I'd take that off and clean it out real good to help isolate the issue.

Another good test would be to feed the mechanical pump from a container of clean diesel. This will help you determine if the problem is from the lift pump back to tank, or lift pump up through filter. Try to keep the container down low, to keep gravity from helping out too much.

Try drawing vacuum through the pump inlet hose as well... a healthy pump "should" act like a check valve.

pastorjeep said:

CP, it is funny you mention the fuel heater. Mine was leaking badly. I pulled it apart and put a new o-ring and then sealed the top with permatex!

Click to expand...

Did you get the proper square o-ring? Its not just a regular o-ring. This heater port MUST be sealed tight or it will always give you prime issues. I have even been successful with using JB weld in a bind because it was loosing prime SO FAST, I couldn't stop for a meal on a trip because it would lose prime everytime. I like the 6.9 filter head that has no heater...and a smaller micron filter.

I'm having prime issues to new injectors lift pump and return lines. My ip is leaking not sure if that will do it? If a friend hits the valve on the filter head she fires?

Kevin, I did not get the right o-ring. I had to really shove the round one in. I went out and checked and it is leaking again, even with rtv in the top connector!

1/2 inch npt tap and 1/2 inch pipe plug. Don't run the tap down too far!

Plugs that thing right up.
Mine was also leaking up through the heater itself. Fuel was coming out around the electrical connector.