5.7 Vortec intermittent long crank --> crank no start, backfires, clunking

[slow_c1500]

Hey all, I started a thread almost a year ago about this issue that Ive been inactive on because of college, but now that im gonna have time to work on the ol girl again, I figured Id start a new thread with a quick recap of what we found out in the old thread. Anyways, its a 96 5.7 vortec. New GP Sorenson MPFI spider/regulator, new fuel filter

Findings/symptoms:

- Normal start when sitting less than ~15min or overnight.
- Hard start when sitting over ~15min but less than a day.
- There seems to be a fuel leak somewhere, but i havent noticed any drops on the ground at all
- 60psi fuel pressure when pump is running, drops to 57psi 2 sec later when pump shuts off, continually drops slowly after that.
- I also tested clamping the different fuel hoses to see if the pressure drops differently:
With either the inlet fuel hose clamped, or both hoses clamped, as soon as I let go of the pliers, the psi instantly dropped about 12 psi. With the return hose clamped, when I let go of the pliers, the psi didn’t drop. It basically behaved the same as if nothing was clamped. I did the calculations and the psi dropped the fastest when either the inlet hose or both were clamped. For both of them, it took ~30-35 seconds to drop 1 psi (not including the instant 12psi drop after releasing clamp). The psi dropped considerably slower with nothing clamped, or only return hose clamped. Bottom line: whatever is leaking, it leaks faster when you pinch the inlet hose or both hoses.

Any suggestions or guesses as to what my issue is? Thanks

 

[Frank Enstein]

Test the coolant temp sensor. If it's got too much resistance the computer thinks it is much cooler than it is and gives too much fuel at startup leading to long crank times. The clamp/pressure drop is puzzling but it may be a dribbling injector. prime the pump a few times and see if any of the injectors are dripping.

 

[slow_c1500]

Test the coolant temp sensor. If it's got too much resistance the computer thinks it is much cooler than it is and gives too much fuel at startup leading to long crank times. The clamp/pressure drop is puzzling but it may be a dribbling injector. prime the pump a few times and see if any of the injectors are dripping.

I’ve tried priming the pump with the injectors pulled out and they didn’t seem to be leaking but I could try again at some point. How would I test the CTS?

 

[docstoy]

So has it been sitting a year or you've been driving it ? Fuel pump prime 2 seconds ? Then pressure drops off with no external leaks ? Either leaking past regulator or at fuel pump in tank is my way of thinking , by having new stuff installed better chance of fuel pump or fuel pump connections in tank as you don't mention any fuel smell .

 

[slow_c1500]

I drive it every month or so, and the issue is always there. I guess it’s possible there could be an external leak at an o ring or something that isn’t big enough to leave fuel on the ground. I have actually noticed a fuel smell sometimes come to think of it.

 

[docstoy]

What's making this hard is you say you drive it every month or so, are you adding any fuel stabilizer when you stop at the gas pump ? Reason being gasoline goes bad and it depends on where you refuel if it's just here and yonder you don't know what your getting and this could be an issue with old fuel . What is your refueling practice?

 

[slow_c1500]

top tier gas station, and I refill at the same one every time. I still had this problem even when I consistently drove it.

 

[docstoy]

So what happens if you clamp just the return fuel hose ? Because your using the fuel pressure connection on fuel rail at throttle body area for fuel pressure gauge ? Does pressure decrease after fuel pump quits running ? So if it decreases after pump quits running and you clamp after leaves fuel tank is that before or after filter ? And pressure holds steady ?

 

[slow_c1500]

When return hose is clamped, it does drop, but very slowly. Also, the psi doesn't instantly drop after I release the clamp on the return hose.
However, with the inlet or both hoses clamped, it drops faster. And when I let go of the clamp(s), the psi instantly drops about 12 psi.

During this test, no matter what, the psi drops, but it drops faster when the inlet or both hoses are clamped.

When the truck is running it holds at 50 psi.

KOEO I get 60 psi when pump kicks on, then it shuts off 2 sec later and it goes to 57. Continually drops slowly after that.

Yes i am using a fuel pressure gauge at the test port on the fuel rail above the intake manifold. I am clamping it on the rubber sections past the fuel filter.

 

[docstoy]

So if you are completely crimping hoses it looks like you have 2 leaks one at fuel injectors/pressure regulator and at fuel pump . Is fuel pump O.E.M. or has it been replaced ? Really thinking it's your fuel pump but hate swagging and throwing parts but also hate walking without my walking shoes . And it's not easy to inspect either .