A few weeks ago, I bought a 1985 corvette and it has ran rather well. The past few day it has been taking longer cranking to start and it has a loss of power. The loss of power is really noticeable when trying to accelerate at WOT. I went ahead and checked the fuel pressure. 30 psi with key on, engine off. At idle only 23 psi, and idle without vaccum on FPR only 30-31 psi. From what I found that is low, so I replace the pump and strainer today. With the new pump the fuel pressure has not changed.

Before anyone asks, the pressure is not beading off. It will hold the pressure for a while. It does start a little easier after it is warmed up. I am absolutely at a loss.

 

Eh ... Is the fuel filter plugged ?

 

Nope. I should have mention i replaced that on the day I did my initial oil change. In other words the fuel filter is only a week old.

Also, If I crimp the return line at the tank the fuel pressure will shoot up to 100psi even at idle.

 

Ok yeah 23 is not right.
So here is what I would do next.
Install the fuel pressure gauge and start the car, let it idle. Then pinch off the return line somewhere you wont hurt anything, and watch the fuel pressure. If it comes up to something normal, or more likely way above normal, then you know the pump is capable of producing the proper pressure. As long as that's the case, then I would think the pressure regulator is malfunctioning. Which is a pretty darn rare occurrence (they frequently leak but that's different) but there's not really much else that could cause the symptoms you are describing. It is not impossible you have insufficient voltage/amperage to the pump.

 

Ha .. I just realized you were editing your post while I was typing. Sounds like you already did the return line pinch. As I stated above, it sounds like the regulator is messed up.

 

Hi
32 psi is the normal fuel pressure for an 85 corvette, the 85 has 24lb injectors so they run the fuel pressure around 32psi. 23 psi is too low, should not drop below 30 psi.

Check the fuel pressure relay, make sure the pump has a steady 12 volts. Check the fuel pump wiring connector and ground wire are good.
Check the 3 way connector is making a good connection.

Check the vacuum hose for any fuel smell, if the fuel pressure regulator diaphragm is torn you can loose fuel pressure and have it sucked into the engine. Check the oil for gas.

The later C4 runs around 40 psi with slightly smaller injectors.

Good luck, should be something simple

 

yeah, that was my though. My stepdad, who has been a mechanic for over 30 years, said he has never seen that happen. He said normally when a FPR goes bad it causes it to run rich because it leaks into the vacuum line. With what the pump is capable of when the line is crimped then what else could it be? I guess its worth a try, because I already miss the acceleration. I think I could be beat by a 1990s Camry at a stop light.

 

I'm with your step dad. Its a very rare occurrence for the regulator to not properly regulate pressure.
I guess the only other thing I would want to be certain about is that your pressure gauge is functioning properly, and if it is, then I would probably just throw a new regulator on it and keep my fingers crossed.
Whatever happens, report back to us here once you get it figured out. Its an unusual problem for sure.

 

If the diaphragm, which is rubber breaks, you are right. It would send fuel into the intake. OTOH, who knows why but a spring can go weak, break or whatever. Might not be likely but not impossible.

Lets try this. Crimp the return line and turn the key on but not crank. It should shoot up the pressure. See if it holds the pressure and for how long. Might be that the regulator is fine BUT IF the injectors are leaking that will lower pressure. Just to be through. Maybe something in the injectors is not closing all the way?

 

If the injectors were leaking then why is it not running rich, or flooding, or noticeable pressure leak off? It's not technically running lean, but it is definitely starving for fuel at higher engine speeds.

 

FWIW, in case someone else is wondering, below is a fuel pressure chart from Charles Probst's book on Corvette fuel injection.

For what it's worth, he says 34 psi is at the bottom of the scale for a 1985 Corvette...

Anywayz, I wouldn't rule out the fuel delivery just yet: static pressure is one thing, but delivery AND pressure is another.

Ordinarily, I'd be saying to check the filter. But, you've already done that. HOWEVER, that doesn't mean there isn't some blockage elsewhere, e.g., a kinked/pinched fuel line, OR the fuel lines full of crud. The pump too is part of delivery, but save that for last.

AND, kind of an after-thought, and I don't mean to veer off topic, a plugged catalytic converter will result in drivability symptoms very similar to fuel starvation. (Just sayin)

This chart is for your (and others) reference and reading pleasure...

.

 

Let me see if I can clarify. Multec injectors aren't built for ethanol in the gas. So either you ONLY use pure gas, assuming it is and you can get it easily all the time or it will eat at the windings sooner or later. If I were to ever get a C4, first thing I would do is dump all fluids and injectors if they were original. Second thing is with the ECM compensating, it might mask a few bad injectors. Thing thing is that sooner or later, it will go if you expose it to ethanol. So besides trying to make my own life more difficult, I'd replace it and the FPR at a time of my choosing as opposed to a time where it chooses. Most people don't want to do it because they would rather kick the can down the road till it forces them to. For me, that time usually has me on the side of the road at the worst time possible.

That said, It seems like you can hold pressure when the return line is crimped off. I have less doubt that it is the pump being unable to deliver. If it can push it up to 100 psi, I think it will deliver. Also, another test, like I said, would be to pressurize the system and see what it does at key on engine off. See if it holds. Someone also suggested a WOT run to see if the pump delivers. With your test being able to do 100 psi, I suspect the pump will deliver and the circuit holds pressure even at the pulsator. Doesn't hurt.

 

The fuel pressure regulator is cheap and easy to replace. I would try that first and if the pressure and or the power does not improve, you may have a bad injector or two or three that is/are not spraying properly.

 

Well it has been awhile, but I finally got the time to work on the fuel pressure and I have positive things to report. I went ahead and replace the fuel pressure regulator with an adjustable Holley unit on. At first, with the regulator fully adjusted i could only get 50 psi, when the product advertises up to 65 psi. I decided to pop the fuel rail loose and look for injector leak. After about 10 min or so all of the injectors leak maybe a drop of fuel. At some point in time I will get a set of rebuilt injectors, but it is not that urgent, and I couldn't leave it apart where it was. I took the fuel rail and fuel filter off. I blew threw the fuel line at the fuel pump and blew everything out through where the fuel filter would be, then did the same for the line that goes from fuel filter to fuel rail. I made sure the fuel filter is not stopped up and put it back on. Then used my mouth (best way to feel the flow) to blow though the completed line with filter in place. Honestly it flowed rather decently, so if there was a restriction in the line it has to be gone. Also the inside of the fuel rail looked clean. I put it all back together and adjusted the regulator to 40 psi with no vacuum. It looks like the regulator has plenty of adjustment left, so I may have had something in the line, but I don't know for sure.

Now the car starts with just a bump of the start, NO MORE CRANKING for 20 seconds!! It doesn't try to stall when taking off, and it seems to have gotten it's performance back. I have not gotten to drive it much, since I had a flat and the tire is ruined. I had a bald 225/60 laying in the back, so I stretched that onto the wheel so I could get it off the side of the road (spare tire is missing). Going ahead and replacing all the cracking old tires.

While at the upper plenum and runners of I tried to clean the carbon and gunk from the inside. The best thing I found was LOTS of carb cleaner and a toothbrush, but it was not perfect. Does anyone have an idea for a better solution?

 

Is your pump capable of turning out 65 psi? Would your pulsator in the tank leak and relieve the system of fuel at 50 or would it hold 65? That is what Holley said it would do. Give you UP TO the stated 65 psi. If your system is not capable of it, the REGULATOR will not add another few psi. I really wouldn't go too far since it won't help on regular driving. The ECM will detect the richness and cut the pulse width and bring you back to square 1. MAYBE in WOT mode, it might enrich it a little, hard to say how much effect it has alone.

The true test would have been to see what the volume and spray pattern is. It might hold pressure but if the volume is low or the pattern is poor, the fuel mixture might not be right.

That isn't the best way. The best way would be to hot tank it. Faster and cleaner if not as clean as with the toothbrush and way less hand fatigue. TB is harder to hot tank. For that one, I would take the top off, take the IAC solenoid off, take the IAC housing off. Clean the IAC pintle with a GENTLE spray of brake cleaner and hose down the passages and the rest of the TB gets the toothbrush. Obviously new gaskets are needed.

 

I am not running the fuel rail above 40 psi as recommended for a stock engine. I believe I said that. I was just curious at to how high I could get it to go and I just thought I would mention it. A few post back I stated the pump would do 100 psi or better when the return line was crimped. All that maters to me now is that it runs good and starts good. Got to break in the new tires this weekend.

also aklim, I do have Bosch injectors, they are not multec.

 

That's good news.

 

Now I am curious as to why it couldn't do 65 since you achieved 100 before I agree it does not need to go above 40 but since the system is capable, it should have gone to 65 as per specified

The injectors are probably dirty and it is as cheap to throw them and replace with reman as cleaning unless you need original

 

It might reach 65 now that I have blown through the lines, but I have not messed with the regulator since I set it to 40 and put it all back together. Honestly, not too worried about it at the moment. I will be getting a get of rebuilt injectors in the near future. They don't leak bad enough to effect anything, so it all good for now.

 

Just curious since I had trouble with the regulator that came with the Holley Stealth Ram and the replacement they sent me. Fast bleeding down Somehow the Kirban unit seems to be holding for a while after shutting down the engine