Mountaineer

Fuel pressure is at 12 on start up. After the truck runs 5-10 mins it slowly starts falling off till hits 0 .
whats up ?

SIXSLUG

Bad Gauge, bad lp, fuel leak...

Comes to mind. I would try a test gauge on it and see what it says, then go from there.

Mountaineer

It's not leaking anywhere visible. I Have a Airdog 100 that I plumbed the return into the filler neck . I suspected the pump at first and took the fuel fill cap off while the truck was running and it seems to be cycleing to much fuel for it not to be working.
If it was a leak in the fp guage line or isolator , why would it read 12/13 psi on cold start-up?

dubc2323

what is the normal psi at an idle and wide open throttle/ and how long has it been doing that?

dubc2323

oh yeah....also is it a mechanical guage or electronic?

Mountaineer

Its like at 12 psi at idle and 12 at wot just after I start it but after it runs for a few minutes it just gradually starts falling off while its just sitting there idleing.
Iits a mechanical isspro and its been doing for a while but I dont use the truck very much or drive it very far but I need to take a long trip with it and want it fixed first.

klcammie

like Sixslug said check with mech. gauge. could be your gauge. mine did same thing, changed lift pump was no better. changed gauge and sending unit all better. if had checked with mech. gauge would have saved buying lp. oh well live and learn.

Mountaineer

So check my mech. guage with a mech. guage ?

SIXSLUG

Check it with a known working gauge, mechanical or electric. It will at least give you a yes or no and you can then delete pursuing that item.

Hard to diagnose without having it in front of me...just thinkin out loud.

dubc2323

yeah I agree with everyone above... if it still runs even with guage showing zero then I would def. think it would be a guage ...even though it showed 12 before warming up

Mountaineer

k, I'll check with another guage and report back.

Utah Dually

Your problem is probaly the isolator. Take the fitting off the gauge side of the isolator, use a pencil with an eraser and push the diaphragm back. Fill the line back up and you should be good to go. This happens with the ISSPRO isolators

Mountaineer

Update ... I think I've fixed it. On the guage side I think the line had a crack by the compression fitting . I cut it off and put on a new feral and so far so good.
I just dont know how it was still showing pressure on startup and the first few initial minutes of run time. but thats what it would do.

Thanks for the comments guys