Hi guys/gals. New member first post. Bought my 98 Z28 new inSeptember 1998. It’s ran like a champ. Never an issue. Finally, it failed me. 18 years, 2 months and 14 days. Could be a record for a daily driver?


Anyway. Now the issue. Fuel pump. Driving on interstate it started cutting out. Seemed to me like a fuel delivery issue. Got it parked and checked the fuel pressure at the rail and it was <25psi and would drop out to nothing. Towed it home. New pump installed (Delphi). New fuel filter. Had a bit of an issue with the supply line hose on disassembly. One o ring and backer came out and the o ring was split. Got a new o ring 3/8 x 1/16 (can someone confirm?). It’s not leaking so all seems good.


Now with the new pump <5psi. Strange. I can hear the pump start and run for the 2 second prime. I check all lines and no leaks. I barely get any flow at the check port onthe rail with the 2 second prime. I pull the pump to verify all is okay. All looks okay and I reassemble/install. This installation was very easy compared to the first. No issues with lines and all was good.


Check pressure at rail and <5 psi and drop to zero after 2 second prime.

Pull the clip side of the fuel filter to check for flow and tons of fuel pouring out. Standard amount from the supply side but the downstream side of the filter is just pouring out fuel. Somehow siphoning from the tank?While plugging the filter hole I have the key turned for the 2 second prime and the pump supplies plenty of fuel (albeit with zero backpressure) to supply side.


Questions:
1 I’ve changed the fuel filter plenty of times and never had it pour fuel like that and it didn’t on the install of this filter (but the pump was out so no siphon available). Anyone ever heard of this?
2. Any thoughts as to why no pressure? Thought it could be astuck injector but it builds nothing AND the car will start and run for about3-5 seconds and it runs fine until it’s starved.
I’m pretty confused. All the technical stuff for this fuel system is in/on the pump. Bad pump? Bad regulator? Blocked filter? Flows really good the wrong way!!!


Next I thought of trying to rig a line to check pressure atthe fuel filter, but with fuel pouring out I cannot change the filter withoutdraining the tank.

 

Follow up: Chasing the fuel lines I see there is a Tee right after the filter with a line going back to tank and another going up front. Assume the one going back to tank is the "second" pressure line that feed the pressure regulator.


This would explain how it was able to drain 4+ quarts of fuel opposite flow back thru the filter. Now, Why the back flow? I would think there would be a check valve in the system to prevent this (as this likely is not supposed to happen for many reasons). Also if a bad regulator, I can see why there would be no pressure at the rail as it's going thru the tee right back to the tank.

 

When you checked the fuel pump assembly, did you verify the flex hose was securely attached to the pump and to the sending unit pipe? Did you check to see there were no cracks in the flex hose? A loose connector or a crack in the flex hose could be submerged in the fuel allowing fuel to flow freely out of the tank to the filter. Did you relieve tank pressure before you opened the connections at the filter?

Your assumptions on the "tee" line are correct. Might be interesting to block off the return line and measuring pressure at the rail when the pump primes. That would tell you if its a problem with the regulator. Not familiar with the details of the 98/LS1 fuel system, so not sure how hard it would be to block off the line from the tee to the sending unit.

 

Thanks Injuneer,


Indeed. I did check fittings and for cracks. Basically anything that could cause a leak. All seemed fine. Fittings were good and tight, no cracks, cuts, or splits that I could tell. It was well lit and I looked it over pretty good. I pushed on each clasp and all seemed solid. I will pull it back out assuming it needs replaced. I will triple check it before committing to returning.


Fuel tank was at atmospheric pressure.


As far as blocking, that would be conclusive. If I had a plug or end of line that size (flared) it would be easy enough (tee is female threaded all around). I assume it's flared. Unsure if it's metric thread or SAE and I don't want to guess wrong.

 

Conclusion. The new fuel pump was bad allowing a loop right back to the tank. My guess, the pressure regulator was installed backwards.