I'm in the process of detailing the intake of my L98, repainting the plenum, runners, etc. While I have it all apart, I figured I'd install an adjustable FPR. I already have Bosch 3 injectors that I purchased from FIC some time ago and I remember when I spoke to Jon at the time of purchase, he said an adjustable FPR would wake up the motor with those injectors.

Anyway, before I started taking everything apart to paint, I wanted to do a baseline on the fuel pressure. I relieved pressure and installed my fuel pressure gauge, I pressurized the system and it read 31 psi. I started the motor and the 31psi stayed constant at idle. Removing the vacuum line from the FPR, the psi went up to 41, also at idle. I was thinking this was a little low, but then remembered that the stock FPR on the 85 is an odd duck in that its lower because the injector rating was larger. With that said, is this within spec? I'm going to get another gauge and see if my gauge is off, just to be sure. By the way, the car runs, starts and idles fine.

My other question is there seems to be a leak down. With the car off, vacuum hose on the FPR connected, the pressure starts off at 31, drops to 20 almost immediately and then slowly bleeds down. No fuel is present in the hose of the FPR. The last time this happened, it was the fuel pump and new pump solved the problem. I changed that pump around 6 years ago. If an injector was bad and leaking, what's the best way to test that? I was going to remove the fuel rail to reinstall the runners and was thinking of maybe pressurizing the system for a moment, relieving the pressure and then seeing if the bottom of the injectors were wet.

Anyone have any suggestions, comments, further tests to try, etc? Any help is greatly appreciated.

[UPDATE] I tried another fuel pressure gauge and the pressure with key on, no start is 38, it drops after a second or two to 30 then goes down to 20 fairly fast. From 20 psi, it slowly loses pressure. Within 5 minutes, it's almost 0 psi.

 

Fuel pressure for the 85 should be 34-39psi with the ignition on, not running. There is a fuel system diagnostic here:

https://www.corvetteforum.com/forums...post1601902897

 

My c4 really likes 50 psi, however it will idle rich as but great performance.

 

I was looking for that diagnostic, thanks! Everything points to the fuel pump so I'll be replacing that tomorrow.

 

it's been a few years since I changed my 85.....I think they were stock 24# but jon at FIC said I need 22# ones and then raise the fuel pressure to 44 #........so I put in a adjustable pressure regulator to compensate.....haven't had any problems....

 

Hi
32 to 34 psi was std for the 85 with 24lb injectors, the later years ran 38 psi with 19lb injectors.

As for loosing pressure the biggest issue is the pulsator, (between the pump and sender assembly).
That is a plastic part that can crack, its a common issue with rapid pressure drop.
You can replace the pulsator with fuel hose and hose clamps, that should keep the pressure stable for longer when engine is shut off.

I find the fuel pressure will bleed down over time, even with new injectors and fuel pump.
My fuel pressure holds for quite some time only loosing a few psi over half an hour or so, however i run two pumps.
One in the tank the other inline as a booster pump for the Vortech FMU, that raises fuel pressure around 8 psi per psi boost.
To force more fuel through the injectors, to keep the blown engine fed.
Have never worried about it even after sitting for a week and showing zero psi, it starts right up the prime is so quick.
Don't forget about the 9th injector the cold start injector, check that is holding pressure.