I have an issue with fuel pressure not holding, I've replaced the fuel regulator, pinched the return line and no luck. During the restoration of this 92 Z28 350 TPI, I replaced the fuel tank, fuel pump and sending unit. I did cut the fuel tank neck and attached a rubber hose when installed. I also have a new check valve. When I fill up the gas tank, I have to hold the gas nozzle up otherwise it will pop the nozzle out.

I've heard some things about after market fuel tanks or other fuel related parts that may cause that. Any ideas? This issue is causing long cranks and difficulty starting. Once it starts, it runs fine.

Thanks for the help guys.

 

One thing that I've seen before in this scenario with dropping pressure, is a bad injector or multiple bad injectors that are leaking

 

Perhaps it'd be worth checking out the pressure relief valve / fuel tank vent valve located close to the tank, under the vehicle.
I've heard a lot about that thing causing issues when it's stuck and the nozzle popping out does kind of point towards too much pressure in there.

 

The fuel tank has nearly nothing to do with the fuel rail pressure, except that it must have fuel in it. The pump is responsible for developing pressure and the regulator is responsible for maintaining that pressure at the set point. the tank would have to be under such negative pressure to inhibit the pump from developing rail pressure that the tank would collapse.

If rail pressure drops immediately when the pump shuts off, there may be a leaking injector, bypassing regulator, the discharge line from the pump to the tank bulkhead fitting is leaking, or the discharge check valve in the fuel pump is not holding.

 

thanks, I did check for leaking injectors by pulling up the fuel rail and turning the key on (engine off). Fuel pump worked no leaks. I have southwest injectors, only 3k miles on them.

 

Thanks, I’ve replaced the check valve (close to the fuel tank). I previously had an aftermarket check valve from summit, it didn’t fix the issue.

 

Thanks. Great info. The fuel pressure does drop immediately after the pump shuts off. I checked for leaking injectors, replaced the fuel pressure regulator and recently I replaced the o ring in the bulkhead fitting hose coming out of the fuel tank. One thing i haven’t checked is the check valve in the fuel tank, i bought the sending unit as one piece from eBay, how can i check if its working properly?

 

If the fuel pressure drops off in a matter of minutes,it should be of no concern. If pressure is ~45 PSI when the pump is priming and ~37 PSI when running at idle RPM, it's doing enough to start and run a stock system acceptably. If an injector or regulator is dumping fuel and making hot restarts difficult, that's a different matter, but it seems you've already addressed those.

FWIW, my '86 has been bleeding off pressure since at least 1994, and it starts/runs completely normally.

 

the fuel pressure drops instantly after the fuel pump stops, and the long cranks and difficulty starting is what I’m trying to solve for. And yes it runs at the low 40’s when priming and high 30’s when running. Before I replaced the fuel regulator the fuel gauge would fluctuate when running, that stopped when I replaced it. When you say injectors leaking, that means leaking on top of the intake manifold and not from the bottom of the injector into the block? I basically picked up the fuel rail with the injectors attached and I primed the fuel pump, there were no leaks.

if that’s not it, then it must be the pressure valve inside the tank because the check valve outside the tank is new.

 

People make a big deal about this "spec", but in reality, it is, as I like to say...."Worryin' about sh|t that ain't worth worryin' about". When you hit the key, pressure builds essentially, instantly and the car starts right up...so where's the problem? And who cares what the rail pressure is, 3 seconds after you shut it off? I don't.

 

No issue with your tank causing this. Before getting more advice from the board it might help to define what "long cranks" means. Maybe time it on your watch.

 

I agree with you but my problem is that the pressure drops right away causing it to take longer to start. If the pressure holds as expected, it should take less effort to fire up as the fuel gets to the injectors quicker.

 

ok, it takes a few cranks to start the car, even after its warm. I’d say between 5-7 seconds on the first crank, then 4-6 on the second crank, then it usually starts on the 3rd attempt.

 

Does that time and/or number of attempts, decrease if you hold the throttle wide open (pedal to the floor) while engaging the starter?

 

it doesn’t like it when I give it gas (pedal), it starts with key on, cranking the engine.

 

ALL the way to the floor?

ALL the way to the floor, does a function; cuts off fuel. "I give it gas" does the opposite function where it gives it too much gas. So for diagnostic help for you, and to be clear, going WOT during cranking makes it harder to start?? Or you haven't actually tried WOT during cranking?

 

I have a long-standing habit/quirk that developed when EFI engines became more prevalent. I turn on the ignition, listen for the running fuel pump, let the MIL flash on-off-on as it checks itself, and then hit the starter. About 99.9% of the time the starter will crank the engine about 180° (two cylinders worth)) before it fires and runs. The other tenth of a percent I will stop cranking, wait another half-second for battery and fuel pressure to stabilize, and hit the starter again for half a second. It's never failed me in doing that. Half a dozen cars, four trucks, two vans, and a Model A (SUV) included in that regimen.

If you need to crank 5-7 seconds, and then more than once, something could be wrong. A '92 is speed-density, and the lack of a MAF could be a hindrance, but that still seems excessive.

 

IDK...my '92 typically starts in about 1 compression. No MAF on that thing. After sitting for 6 months (all winter) it took 3 compressions, to start...



OP's cranking time is way too long. Now we need to identify, why?

 

I haven’t actually tried wide open throttle WOT, let me try that once I find the keys my son misplaced yesterday. appreciate the feedback.

 

Let us know the results you get.

 

Sorry for the long delay. I finally found the keys and I encountered another issue that I'm still dealing with.

However, I did try pressing the gas pedal all the way down and then cranking the engine as well as pressing the gas pedal all the way down while cranking the engine. It does not start. It only starts by turning the key repeatedly, about three or four times.

 

Sounds like it may not be getting enough fuel, then. Possibly.

 

Thanks,

That's what I initially thought as well, so I replaced the fuel pump with an AC Delco one. Fuel pressure reads about 42 when priming and about 38 once it's running.