Loosing Prime Overnight Sporratically
 

So once in a while the ol 92 looses its prime overnight, but it is still random in nature.

Definitely takes 8-12 hours or more to loose prime, but still only happens once in a while. Some days it starts strong, others it will barely kick and run ruff, and some it just wont start at all. Of course if i hit the manual priming lever on the lift pump 40 x it will fire no problem.

I pressureized the fuel tank and cannot find any leaks anywhere.

I am down to these:
1. Leak in the return or pickup lines in the tank. Need to tear the tank apart to get to them. Seems to be more common when the tank is closer to empty.

2.Leaky lift pump check vlaves, due to the sporratic nature it depends on what part of the lift pump cycle the engine stops at.

3. Leaky injector letting air in and the feul out of the line. But the truck runs fine.

4. Faulty vent system in the tank. When the tank is lower on fuel it seems to cause the problem more often. Also the tanks seems to be getting pressurized more often and i have a problem filling it, i have to fill it at the slowest setting on the nozzles and angle the nozzle just right or the thing shuts off. I used to be able to fill with the truck stop nozzles at full speed.

What should i chase first??????????????????. Injectors and lift pumps are expensive but quick to swap.... The tank woudl take forever, and this is my only daily driver.

The lift pump would be my first choice after checking for leaks.

Same here, throw a lift pump at it. They last a long time, but not forever. If its sporratic it could be some crud in the lift pump holding the check valves open every once in a while.

Cool maybe i will try that....
Is the Piston Lift pump the way to go?
Is there an ideal model or part number to hunt for?

Maybe i will add a pressure gauge at the same time.

The piston lift pump for the first gens. seems to be the way to go judging from other's opinions. It will take a little customizing to install it properly. From what I have read, the sound insulation on the lifter cover needs to be excavated slightly for clearance and there may be some plumbing mods as well.

I had the exact same problem and replaced the lift pump and problem went away I thinkthat is your problem

For what you have in your sig it's not necessary, the truck will run just as well with a stock type replacement.

Part #'s can be found in the "tech facts" section at the top of the page.

Thanks guys..

At what level of fueling would you recommend cutting over to the piston pump??? Does it have to be pretty extreme?

Right now i have the AFC system maxed out, and a 16cm2 housing, and no muffler. Pretty mild.

I think i will likely get the Prince of Darkness injectors, and fiddle with the full load screw. Dont know how much more it might do.

I think once you swap to bigger injectors the piston pump is a good idea.

Update:

I installed the piston lift pump sucsesfully, and i think my sporratic air leak or prime loss is gone, but i am not sure, its only been 4 days, and it started a little weak this morning, it ran at about 100rmp less than idle for about 2 seconds, similar to what it used to do when it needed a prime.

The funny thing is that before I swapped the lift pump the truck was smoking a lot, i think more than it did a few years ago, especially at full load and high RPMs. Now that the piston pump is in there the smoke level seems to have dropped by 80% at least.

I am wonderinig, if i was leaking air while the truck was running, would that cause excessive smoke? or could it have been a weak lift pump that caused that?

I have to wait and see, but i might still have a leak somewhere. I read about a guy that found a leak in the tank sending unit that would seal up when the engine was running and pulling fuel, but would drain back when the suction of the lift pump was dropped when the engine was shut down.