I hooked up a fuel pressure tester to my 85. On start up I read 49psi !. When I shut down the pressure goes directly to zero. I think I have 2 issues. Bad fuel pump in tank and bad regulator. AmI on the right track

 

Unplug the hose to the pressure regulator and cap it off when you take the pressure. It should read about 36-39 PSI. If the pressure drops to zero immediately then the check valve in the pump is bad.

 

OK I just changed the Fuel Pump assy includes pump, fuel gage sending unit, and bag filter. The reading is still 49 psi and still goes to zero after I cut off the eng.

I am thinking 2 things, either a stuck fuel injector or bad regulator The reg on the was changed about a year ago, the injectors are Bosh. I have the Corvette service manual and it points to Fuel Reg

 

Just found the vac line going to the Fuel reg was off. Hooked back up got 39 PSI with vac attached, 49 vac disconnected. Still have problem with fuel pressure dropping to zero after eng shut down. New assy in tank

 

Hello there,
I had a leaky injector which allowed the pressure to drop to close to zero but it took up to 1/2 hour. If what you have is a "leaky injector" then you must have a bunch of them.

Your problem is probably not an injector bleeding down but someplace else in your fuel system.

When I replaced my fuel pump, I replaced the Pulsator with a piece of rubber line designed for immersion in fuel. I do not recall ever having seen a "check valve" in the fuel system. Do I even have one on a 1988 C4 Coupe? If I do have one then it must be built into the fuel pump.

After I replaced my Injectors, Fuel pump,Fuel Pressure Regulator, EGR valve, EGR solenoid and fuel supply line I no longer have pressure issues like it used to. The pressure stays high for several hours and even longer. This makes the car start right up when cranked and my idle is now smooth as glass.

I had a blockage of rust form in my fuel supply line which required a line replacement. When this happened the pressure was right but there was not enough flow. This was a pain to diagnose, after 3 separate fuel pumps could not make the flow I realized that I had a fuel line problem. The newer gas with the ethanol makes the fuel absorb more water and that wreaks havoc on the fuel system if the car sits for any extended period.

I wish you the very best in diagnosing your cars gremlins!

 

Thanks for response. I have new injectors (BOCH) , new fuel tank assy,(Pump. fuel level sensor). It is a Delco from Rock Auto. I have my EGR disconnected and have removed and installed the replacement pully assy deleting my smog pump. It is registered as Historic, in MD and does not require emission testing. I still have connected the ninth injector for cold starting. It might be stuck open! The fuel reg reading at idle is 39psi. When I increase throttle the pressure increases. I will keep looking

 

Original '85 low pressure & bigger injectors.

 

I don't understand what you are saying. Do you mean I need to raise the pressure for my Bosh injectors?

 

Hello again,

I believe the the earlier poster was referring to the fuel pressures are different than the 1988 on cars.

Here it is: "The stock fuel pressure for 1985 to 1987 C4 engines with TPI was 36 to 39 psi with the fuel pressure regulator vacuum hose connected, and 47 to 48 psi with the vacuum hose disconnected. For the 1988 to 1996 C4 corvettes, the stock fuel pressure is 40 to 42 psi with vacuum hose connected and 47 to 48 psi with the vacuum hose disconnected."
Source: It Still Runs

I have the 1988 version and it runs slightly higher pressure.

Good Luck!

 

Thanks. Looks like I am ok with 39 psi with Vac hose on. The only problem is that the fuel pressure go to zero very fast. I put in a new gas tank assy in pump, sending unit and bag. The eng runs good, should I be concerned the pressure is going to zero?

 

I had a similar issue and replaced my fuel pump with a delco unit and it still dropped to zero. Turns out the check valve in the new pump was bad and allowed back flow. Put in a new pump and the problem stopped.

Have you run the pinch test with your return and supply lines back at the top of the sending unit assembly? Just didn't want to post the whole procedure if you have already done it

 

My 85 ran about 30 -32 psi standard with 24lb injectors, when i replaced the fuel pressure regulator diaphragm it went to 39 -40 psi. Could not get the lower pressure diaphragm they only have the 86-91 type which run the higher pressure with the 19lb injectors.

There is a plastic "pulsator" between fuel pump and level sensor pipe, most replace that with a small piece of fuel hose and two hose clamps. If the pulsator is cracked it can loose pressure quickly, depends on how hard the car starts when hot if the injectors are leaking or pressure to build up.

My 85 holds the pressure for around 10 minutes, before slowly dropping down.
The original injectors did leak down quickly but no ill effect with start up, i went to bosch 3 24lb injectors which help with smoother idle.

 

[QUOTE=gerardvg;1596473821]My 85 ran about 30 -32 psi standard with 24lb injectors, when i replaced the fuel pressure regulator diaphragm it went to 39 -40 psi. Could not get the lower pressure diaphragm they only have the 86-91 type which run the higher pressure with the 19lb injectors.

There is a plastic "pulsator" between fuel pump and level sensor pipe, most replace that with a small piece of fuel hose and two hose clamps. If the pulsator is cracked it can loose pressure quickly, depends on how hard the car starts when hot if the injectors are leaking or pressure to build up.

My 85 holds the pressure for around 10 minutes, before slowly dropping down.
The original injectors did leak down quickly but no ill effect with start up, i went to bosch 3 24lb injectors which help with smoother idle.[/QUO

 

FIRST - all above relevant and on the money !!!

SECOND - The '85 ran lower fuel pressure due to larger injectors. LPE used the '85 injector in his '383' because it delivered 24# @ around 40 psig.

So if a stock '85 injector you need less pressure. Change injector to new 1986 thru 1996 then use higher FP like the 40 psig.

 

I had the same problem on my 86. It took me a little time but; it turned out to be my 9th injector leaking down. I never thought about that one.

Just my experience.

 

......same here, on my 85 the cold start injector was the baddest leaking one. So I deleted the sucker. No problems at cold start.....

 

Thanks for the responses

The next check is the pinch test. How do you delete the 9th injector. Is it as simple as capping off the fuel rail?

Thanks Art

 

Let's do the pinch test first and see what to do. Sure you can cap it off. Some have reported harder starting and some not. With that done you can see if it is the injectors or not.

I really don't like to bypass the cold start injector unless I have someone rewrite the EPROM to make it have more pulse width for cold starts but that's me in WI driving in winter.

 

There is a TPI cold start valve delete kit on Ebay.

 

If we find it is the injector, Jon at FIC will be able to fix or replace it