Photomania

My 1990 coupe is hard to crank when it's cold. In fact, if it's parked for 15 to 20 minutes or longer, it is hard to start.

I ran a pressure check today with a fuel pressure gauge. When the engine is running, the pressure is around 35. But as soon as I turn off the engine, the pressure drops to 10 and then 5 and then 0 -- all within a few seconds. Isn't some pressure supposed to remain in the system under the hood after the engine is shut off?

When I turn the ignition key without starting the engine, I can hear the fuel pump running. The pressure spikes up to about 42 and drops to 35. It stays there a second or two and then drops to 10, 5 and 0.
I bought a NAPA premium fuel pump and installed it three years (almost 30,000 miles) ago. A mechanic friend told me that he never uses after-market fuel pumps. Only the car maker's fuel pumps. He said he'd had a lot of trouble with after-market varieties.

I'm wondering if the fuel pump's check valve is leaking fuel back into the gas tank.

MK 82

1. Check Valve
2. Pulsator
3. Regulator
4. Injectors

FICINJECTORS

first thing you have to do is eliminate if the fuel pressure regulater is bad. To do this you have to pinch the return line with a small c clamp or vice grips and 2 1" square pieces of wood cut from mixing sticks so you don't cut the hose. Clamp the hose and do the same test. If it still leaks down it is either the check valve or more likely the injectors. Do you get some smoke out the exhaust when it finally starts? Also, if the check valve is bad and you pressurize the system before cranking it really should start fine. A lot of forum members have reported loss of fuel pressure through the check valve.

Frizlefrak

Easy answer. The pressure can only go three places (assuming no leaks in the lines).

1. Back to the tank through the regulator and return line.
2. Back to the tank through the pressure line via check valve or pulsator.
3. Into the engine through the injectors.

Do the above test. If the regulator tests good (pressure still leaks down with return line pinched) then pressurize it again and pinch off both pressure and return lines. Turn off power to pump. If it still leaks down rapidly, you have an injector leak. If not, the issue is in the pump or pulsator. Fortunately, very easy to check and correct in a C4. Good luck.

Photomania

To answer the FICINJECTORS supporting vendor's question, no, the exhaust seems to be normal. I'm assuming a lot of smoke would indicate leaky fuel injectors.

Also, I can turn on the fuel pump once or twice and the car will start up. While watching the fuel pressure gauge, I turned the key to crank the engine while the pressure spiked up to 42, and the engine cranked up beautifully, like nothing was wrong.

I'm beginning to think it's the fuel pump. But I'll do the tests involving clamps on the fuel lines.

Thanks for the suggestions. And a special thanks to a vendor. I'm sure I'll need some fuel injectors someday because gas everywhere has enthanol in it. And I understand ethanol is bad for C4 injectors.

FICINJECTORS

without the pressence of smoke ,its not loading up. also now that we understand it does start ok if you build pressure, then, now I am leaning to fuel pump also. I would still check the regulator with the pinch test. Its free and you will know for sure its not that

93lt1fun

Add a 4th place, the gas cap. Pressure can escape from the fule tank throught he gas cap. Get the gas cap checked or simply buy a new one for about $10.

AGENT 86

DarkBlue88

Gas Cap???? Anyway, I bought a NAPA pump for my 88 and it lasted 1.5 years before it started doing the same thing. Bought another one(I was in a bind and couldn't get a delco so I could get home) It did the same thing. They wouldn't give me a refund or even store credit and I was pissed, haven't bought anything from NAPA since. Put a delco pump in, FIXED! Napa pumps are CRAP, and I'm not even sure they have check valves. You can look into the outlet of a Delco pump and see the check ball.

MK 82

Beam me up Scotty!

Frizlefrak

I'm a long way from a master mechanic, but explain to me how fuel pressure can escape through the gas cap? Vapors can escape from a faulty gas cap, but I'd hate to see 42 psi of fuel spewing out of a cap.

Sorry bud, gotta disagree. Even having no cap at all wouldn't affect pressure on the rail.

Midnight 85

lltrevino

on my 86, it was the regulator diaphram. HOWEVER THIS WAS MY SCENERIO!! dO THE FUEL LINE PINCHES, QUIK AND EFFECTIVE WAY, although, if it were me, and it was the regulator, all the intake has to come apart, I would change injectors anyway, (I did on mine). The car ran like it just came off the assembly line. You can not beat FICINJECTORS prices either, not to mention the customer service!!!!

mk842766

Here we go again....IF you have a Holley or an aeromotive regulator, THEY WILL NOT HOLD THE PRESSURE! Many of us have gone through this with these regulators, and when you call the tech reps, they will tell you the regulators are not designed to hold any pressure.

Now, that being said, if you don't have one of these regulators, the you would be best advised to follow the advice the others have given...just wanted to let you know about the other regulators....



Cheers,
Michael

93lt1fun

What do you think is released and that noise when you undo the gas cap?! It's vapor pressure in the tank. Remove the cap and see what happens to the fuel pressure that is directly related to overall fuel system pressure.

Also note Step 2. in the factory manual (1993 LT1) instructions for relieving fuel pressure to work on the fuel system:

1. Disconnect battery negative cable assembly terminal to avoid possible fuel discharge if an additional attempt is made to start engine.
2. LOOSEN FILLER CAP TO RELIEVE TANK VAPOR PRESSURE.
3. Connect gauge J 34730-1 to fuel pressure connection. Wrap a shop towel around fitting while connecting gauge to avoid spilling.
4. Install bleed hose into an approved container and open valve to bleed system pressure. Fule connections are now safe for servicing.
Drain any fuel remining in gage into an approved container.

If the tank pressure and cap didn't have anything to do with the system fuel pressure then why is it the first step when working on the fuel system?!

Regardless of who is right or wrong we can all agree that the FMS is the source for reliable diagnosis. Although problem is on an L98 maybe the LT1 FMS will help.

MK 82

Because it may cause fuel to leak out of an open system! Nice try but the fuel cap does not cause the fuel pressure in a closed system to drop abnormally!!