shofast

Hello All,

I have been having a hard time getting the engine to start after it has been warmed up and it has sat for a little while. I have read over the threads of this common problem and I finally decided to tackle it, maybe my experience will be healpful to others with this same problem.


My first step of attack.

I got a fuel pressure guage and hooked it up to the fuel rail. When the car is running it reads about 35 psi steady, a little romp of the gas will let it rise to 45 or so psi. That seems normal from what I read, and the car seems to be running normal.

What doesn't seem normal is that the fuel pressure drops back to 0 immediately after the fuel pump turns off. When starting the car, putting the key in the start position will run the fuel pump for 2 seconds, for those 2 seconds I have pressure, after that it goes right to 0psi. Cranking the car will again turn on the pump, bringing pressure up to 35, where it stays if the car is running. When I turn off the key it drops right back down to 0.

I pulled up the fuel filler cover and clamped off the fuel return hose at the tank. Now when I turn the key to the on position I get 35psi-45psi for half a second, then the pressure goes up to 75+psi, but once again, when the pump turns off it goes right back to 0.

shofast

Since the fuel pressure was spiking up to 40something psi when I stabbed the gas while the car was running - this indicates that the fuel pressure regulator is working. I pulled the vacuum line off it for signs of fuel and there were none at the moment.

I think I have isolated the problem to front side of the pressure loop - I am being cautious because I know I might have more than 1 problem. Next I am going to pull the fuel pump from the tank and try to take a look at the check valve and the pulse dampner.

Since the pressure is dropping so quickly, instantly back to 0, I think it is something more than the injectors leaking down. Someone tell me if that is a bad assumption. What I have read about that is that the pressure drops gradually over at least a minute.

tombrammer

Sounds like you are on the right track, your problem sounds like the check valve in the fuel pump, but with the hard start it makes me think it's an injector. It's flooding one or more cylinders and making the hard start. Does it run rough and smoke after the hard start? That might be another clue to the look at injectors.

94z07fx3

If you want to isolate the fuel pump the best way is to rig a rubber fuel line directly from the fitting on the top of the tank to the gauge. Then you should see the pressure come up when the key is turned to on and stay there.

The fuel rail on an LT1 is easy to remove with the injectors attached. Disconnect the electrical connection from each injector by pressing the clip in and pulling gently on the plug. Unbolt the fuel rail's four (8mm I think) bolts and lift it straight up. With rags under the injectors if you turn the key to run and then off you should see a squirt from any leaking injector.

FWIW I use olive oil on the o-rings just a dab to make them slide in without rolling.

shofast

Is the check valve part of the fuel pump, or something replaceable seperate from the pump. I saw the pulse damper seems to be seperately replaceable if that is leaking.

Also, right now the tank is full, will any fuel spill out if i try to take the pump out? I am thinking about just waiting till I run some fuel out to check out what is going on in there.

94z07fx3

The check valve is part of the pump and therefore inside the tank. No fuel will spill out if you were to remove the pump. The pulse damper is simply a pipe with a fancy shape. If the pulse damper were leaking it would show outside just like any other pipe. I humbly recommend that you have a new access panel gasket in hand before you remove the pump. They are discontinued from GM but are available from the catalogs, TLD, MAM, Eklers, etc. about $10.

BTW I think you will need to unbolt the fuel line guides if you choose to check the injectors as I mentioned above. It will be obvious when you are looking at the rail from the passenger side.

shofast

Thank you, you have been very helpful.

I think after I get the issue with the fuel supply pressure drop I will be able to check the injectors. Those are what I was expecting to have a problem with but everything so far as pointed to the fuel leaking back to the tank through the supply. Like I mentioned before, I will not be surprised if there is more than just one problem.

shofast

I took the fuel filler off and disconnected the pressure line and attached the pressure guage to the top of the fuel module, the pressure did the same, pressurized when it was turned on and dropped back to zero when it was off.

I was reluctant to take the module out of the tank since I didn't have any replacement seal, but that seems to be a non issue since it came apart without tearing the seal that was there. I continued to remove the fuel pump since everything looked good and confirmed that I could blow right through it backwards.

A quick trip to the parts store and I had a replacement pump in hand and a new strainer since the old one tore when I took it off. I put it all back together and into the tank. I re-attached the pressure guage to the rail and now it is holding pressure after the pump turns off. So that one problem seems to be gone.

It has been about 15 minutes now and the pressure at the rail is still holding.

94z07fx3

That's good news, Shofast. Congrats on the fix! It was a nice move to replace the sock even if it had not torn on removal.

jisascended

I have a '95 with the exact same problem except....It starts fine when warm for the first 5-10 minutes but from 10-40 minutes it is VERY hard to start and really stumbles until I rev it for 30 seconds. It will start just fine if I wait over 40 minutes. I have a new fuel pump too. Did your new pump fix your car? I'm thinking about replacing all 8 injectors. Another thread said an O2 sensor caused this. I am replacing the 2 pre-cat sensors tomorrow for regular maintenance. I'll let you know if it helped.

fiftytwos

I have a 1996 Impala SS that is acting EXACTLY as you have described above. Can you tell me how this was resolved?

Thanks,

Rick