Hunting down a slow start when warm issue on this new-to-me '96 LT4. 40k apparently easy miles, everything appears stock and original.

The car usually starts immediately when cold, but often requires a few seconds of cranking when warm. Based on various prior threads I checked fuel pressure (with vacuum hose connected) and obtained the following results:

1. Engine cold. Key to RUN-->pressure spiked very briefly to ~43PSI, then stepped almost instantly to ~38PSI. Then it was dropping slowly--but faster than I had expected--dropped below 20PSI in less than 30 seconds, and was continuing down.
2. Key to START (before the fuel pressure tanked altogether), turned over a couple times and started. 38PSI idle, wandered up and down a little with reving to ~3K (stationary), sometimes up to about 40PSI, sometimes down to 37.
3. Key to OFF. Dropped slowly at first, down to <30PSI in 25 seconds, <20PSI in 60 seconds, then dumped to effectively zero (off the scale) within 80 seconds.

Interesting observation: If I return the key to RUN shortly after I have shut down and the pressure has gone to zero, the pressure will shoot back up to 38PSI--so the fuel pump is actuating in that case, with key in RUN and not engaging the starter. But if I wait awhile, e.g., half an hour, turning the key to RUN results in no fuel pressure until the engine is cranked a few times--so the fuel pump is not kicking on in that case.

My Inexperienced Analysis: Looks to me like I'm dealing with two issues, a faulty fuel pump relay, and a fuel system leak. Maybe a leaky injector?
The relay puzzles me though--why would it work when cold or hot/recently shut down, but not when warm/resting awhile?
Thinking to just replace the relay, that's easy...unless there are other ideas. Then I can chase down the leak.

Questions: (1) Is my analysis sensible? (2) Is my plan sensible? Or, what should I do/check/measure next?

 

I wouldn't be concerned about a 2 second crank when warm.

 

Seconds like 8 - 9 turnarounds and smoke after starting ?
I have the same and think its one of the injectors. I can also hear some ticking.
If it is a valve, pump or relais ... you should have more problems when the car is cold

 

I have the same car and if you look at the wiring diagram in the FSM you will notice that the fuel pump does not receive power until the engine oil pressure sensor indicates there is oil pressure. So just turning the key to run should not provide power to the fuel pump and you shouldn't see any fuel pressure from a cold start.

 

Thanks for the replies!

Agreed, but it is longer than that, usually 6-8 seconds--not brilliant. Besides the fact it's hard on the starter, I find myself frequently apologizing to my passenger.

Interesting. I took a listen to my injectors. All of them are clicking away, but the cylinder-one injector has a different sound, less of a sharp click, more muted. This feels like reading tea leaves but I'm wondering if that one is my leaker. If so, that may be enough to explain the rapid pressure drop. But there's still this interesting question on why the pump apparently doesn't kick on sometimes when the car is warm and is turned from OFF to RUN. I'm more and more convinced that has to be the relay.

I'm under the impression it will also receive power for two seconds on the transition from OFF to RUN (aka "ON") as stated in 96FSM-6E3-A-39,I think what I'm going to try next:
(1) Swap horn and FP relays, I think I remember reading on the forum somewhere that they are the same. I'll see if that solves my warm start problem. I have a fresh relay inbound but it's a couple days out.
(2) Seek the source of the pressure leak using a modified GM procedure (from 96FSM-6E3-A-41ff): Pinch the fuel supply and return lines at the tank sequentially, see if either of those stops the pressure leak--that will help me narrow it down. Will report back here.

 

Yes, you are correct. The PCM is involved in the starting mode as you have indicated. In my FSM, Book 2, it is pages 6-736 & 737. Page 737 under "Fuel Pump Electrical Circuit" it explains the operation of the circuits involved. As a backup system to the fuel pump relay, it states " If the fuel pump relay fails, the fuel pump switch and engine oil pressure sensor runs the fuel pump. An inoperative fuel pump relay can result in long cranking times, particularly if the engine is cold". That is not your condition, so it doesn't look like it is the relay and the backup system is working.
According to the FSM, page 738, fuel pressure should be 41-47psi at the test port with the engine ON and the engine OFF (zero vacuum).

"I'm under the impression it will also receive power for two seconds on the transition from OFF to RUN (aka "ON") as stated in 96FSM-6E3-A-39,I think what I'm going to try next:
(1) Swap horn and FP relays, I think I remember reading on the forum somewhere that they are the same. I'll see if that solves my warm start problem. I have a fresh relay inbound but it's a couple days out.
(2) Seek the source of the pressure leak using a modified GM procedure (from 96FSM-6E3-A-41ff): Pinch the fuel supply and return lines at the tank sequentially, see if either of those stops the pressure leak--that will help me narrow it down. Will report back here.[/QUOTE]

 

Okay, I accessed pinchable rubber fuel hose (supply and return) at the fuel tank and did a little experiment:

• OFF-->RUN without starting the car. Pressure jumps up to normal range, then bleeds down to zero in about a minute.
• OFF-->RUN without starting the car, fuel supply line pinched closed, same result.
• OFF-->RUN without starting the car, fuel return line pinched closed, same result.

I believe this means a leaking injector. Anyone think I'm jumping the gun on that, or does that sound right?

Assuming that's the conclusion I'm thinking to buy a set of injectors, probably rebuilt ones from Jon/FIC. Should I steer clear of rebuilt and just buy the new ones? Here's what I'm thinking:
rebuilt: https://fuelinjectorconnection.com/c...l-injector-set

 

Leaking injectors would be my conclusion

 

More than likely a leaking injector. A leaky injector would also contribute to a long warm start due to one or more cylinders getting loaded with fuel. I just did all new rebuilt Bosch III injectors(from John @ FIC) in my L98 and they work great. Might as well replace your fuel pressure regulator while you're in there.

 

If it’s loading up with gas causing long crank time on warm start, try opening the throttle when cranking. It may start faster then since you’ll be letting more air in to somewhat balance the heavier fuel load.