I've been tearing into the '85 I just purchased as it is almost entirely original, including the vacuum lines... As part of this I bought an adjustable fuel pressure regulator to ensure that I can get what I need out of any diaphragm I buy and now I'm about ready to get it back together.

I've got a question regarding setting the fuel pressure. The FSM for 85 says the pressure should be 34-39psi with the ignition on but engine OFF and "lower" with the engine running. Haynes and "the Internet" says this should be the pressure while RUNNING and that it should be 47-48psi with the engine off.

Any idea what best practice is here? I would usually swear by the FSM but maybe people have figured out better since it was printed

 

I would still listen to the FSM over ANY Chilton's, Haynes or aftermarket whatever. Most aftermarket manuals are written to cover more than one model or engine type and they lack sufficient detail on your specific model.

Replace every inch of rubber vacuum lines as they are important. There are plenty of 1985 owners on this Forum who will give you their thoughts. Your Corvette is the transition year when they started using the L98 engines in place of the 1984 Crossfire engine.

I have had a 1988 C4 for over 22 years and it is using a stock factory type diaphragm without any problems or issues. There are times where an adjustable Fuel pressure regulator is a necessity but in most cases I would stick with the factory type diaphragm. I installed a new diaphragm on my Corvette after fixing the EGR system on my Corvette.

Check to verify that your EGR valve is functioning as those are frequently ignored. A malfunctioning EGR can cause these engines to act strange and run hotter than normal. I strart at the EGR Vacuum Solenoid that is mounted near the thermostat housing on my 1988 C4. It gets it's PWM signal from the 2 wires attached to the solenoid and it activates the EGR using a vacuum signal. You can attach a hand vacuum pump to the tube from the EGR and see it move upon getting vacuum.

On my 1988 C4 I see 42-44 psi while idling when all is working properly. When you shut the engine off the pressure should hold somewhat and slowly bleed down. If the pressure drops too fast it might indicate a leaky injector.

 

I set my fuel pressure with the vac line disconnected and the engine running. Try removing the vac line, you'll probably find it drops about 8psi anyway

 

stock 85 should be 36-38 key on.....should hold for a few minutes with key off....get a gm fsm.....

 

I spoke to Gordon Kilabrew i.e. Mr C4, he said 34-39 PSI for the 85.

 

 

Thanks for the replies. I'm glad I bought the FSM for just this reason, but figured I'd ask around. My pressure drops way faster than it should so that will be something to dig into to.

I've been tearing into the thing pretty well. I've replaced most vacuum lines, the EGR valve, refurbed the unobtanium EGR temp switch and gave the COMPLETELY CARBON BLOCKED plenum and throttle body the best cleaning it's seen since it left the factory. I do need to check the EGR control solenoid. Just need to dial in the TPS, clean and reconnect the MAF and fire her up. And wait for that new brake booster check valve, but hey

 

The 85 has many one year only features and I think fuel pressure is one of them. Trusting the FSM is the way to go. The Haynes manual for the C4 covers many models including the 84 Crossfire, so it’s not reliable when making general statements about fuel pressure.

Anyway it sounds like you are taking the right approach and you have made some good progress. Good luck and enjoy.

 

Okay so I have everything back together and my fuel pressure is at 42 PSI on the lowest setting of the adjustable regulator. I can make it higher but not lower. Pretty frustrated as I was really looking forward to starting it up.

Is there anything that could cause too high fuel pressure? All the FPR is see for sale are 1985-1991 so I doubt this is an 85 specific problem.

I'd like to ask two questions

1) Is it a problem to run it at this slightly higher pressure?
2) Is there a part number for a non adjustable regulator you recommend that I can pick up from a store? Been having trouble finding one in the area as all the parts look up database don't work very well for the 85.

This is the regulator I used

https://www.southbayfuelinjectors.co...ator-diaphragm

 

'85 ran lower fuel pressure than the '86 thru ..... Reason being the '85 injectors were bigger - like 30's. That's why Lingenfelter used them. So say if you are using like Ford 24's the higher fuel pressure more closely mimics stock.

 

The FSM says I may have a restriction in my fuel return line, which I'll have looked at when I get the AC hoses replaced. Other than that I fired it up and took it out anyways. No codes even on the highway so I'm pretty pleased.

The 85 having specific pressures, etc doesn't surprise me considering it's "first year" status, but it does annoy me to see so many Corvette parts sites listing things as 85-91 when it's really 86-91

 

Congrats on getting the car on the road

 

You can disconnect the return line at both ends and blow it out with air. This may clear a restriction if there is one. Good to hear that you got to take it for a skid.

 

My 85 ran around 36 psi fuel pressure when she was standard, i do recall when i replaced the pressure regulator diaphragm it was in the 40 psi range.
Had to use the cover that came with it to run under 40 psi. the 85 has 24lb injectors hence lower fuel pressure.
They do like higher fuel pressure however will run rich at idle, i think 50 psi was a sweet spot bit the fumes from exhaust makes the eyes sting !!!

As for the fuel pressure holding when turning the ignition off, the injectors and fuel pump should seal for several minutes.
The usual culprit is the plastic pulsator between pump and sender piping cracking if the pressure drops straight away.
You can replace it with a piece of fuel hose and hose clamps, i have added a fuel pressure gauge on the schrader fitting to always show pressure.
My 85 will gradually loose pressure over half an hour or so, it does not affect restart from hot or overnight cold start as the pressure bumps up to normal right away when the fuel pump turns on.