Problem Starting when Warm

[PJones96]

96 GMC with 5.7 Vortec (250k). When the engine is cold it starts normally, idles and drives fine. When it's warm, the engine turns over fine. I hold the key to crank the engine a couple seconds and when I let go of the key the engine kinda slowly sputters to life. Gains rpm until it is at idle speed. It does usually start and once it starts it idles and drives fine. Need help knowing where to look first. Is this timing chain, ignition, or compression related?

Thanks for your input.

 

[Schurkey]

1. What is the fuel pressure? How old is the fuel filter? Is this truck using the old poppet-style spider injector, or the newer/upgraded electronic injectors?

2. How old are the usual "tune-up" items and procedures? Distributor cap, rotor, plug wires, spark plugs? PCV system check? Ignition timing check? EGR system check?

3. Connect a scan tool, verify EVERY sensor and computer output. I'd be paying special attention to the coolant temp sensor.

 

[PJones96]

1. What is the fuel pressure? How old is the fuel filter? Is this truck using the old poppet-style spider injector, or the newer/upgraded electronic injectors?

2. How old are the usual "tune-up" items and procedures? Distributor cap, rotor, plug wires, spark plugs? PCV system check? Ignition timing check? EGR system check?

3. Connect a scan tool, verify EVERY sensor and computer output. I'd be paying special attention to the coolant temp sensor.

1. I will test the FP. The fuel filter is old, not even sure how old to be honest. It's running the original spider injector.
2. Usual tune-up items were all replaced this past summer. EGR solenoid and PCV valve are new. Have not checked ignition timing.
3. I'll connect a scan tool and check all the things.

Thanks

 

[movietvet]

@Schurkey has covered the known basics that I was thinking of but that CTS is also what I focused on as soon as I read the question. Try unplugging the coolant temp sensor and see if it starts better when warm. If you do, it likely will set a code.

 

[Schurkey]

If he disconnects the coolant temp sensor, the computer will think it is -40 and it'll start hard when warm. I'm thinking the sensor or the wire harness for the sensor are defective, and the computer already thinks that it's -40 out.

 

[movietvet]

If he disconnects the coolant temp sensor, the computer will think it is -40 and it'll start hard when warm. I'm thinking the sensor or the wire harness for the sensor are defective, and the computer already thinks that it's -40 out.

If you say so....

 

[Schurkey]

If you say so....

You must be registered for see images attach

The more resistance in the circuit, the colder the computer thinks it is.

An open circuit has infinitely high resistance. The computer will think it's AT LEAST forty below zero, colder than that if it's programmed to recognize greater resistance than a hundred-thousand ohms.

 

[movietvet]

I was not disagreeing with you. I do understand that an open circuit will have the high resistance. Who says the problem is not in the PCM circuit for that....It is an option that is worth a shot, or not.

 

[PJones96]

I hooked up the scan tool and the coolant temp sensor seems to be working fine. I drove multiple times with it hooked up and the coolant temp sensor always gave an accurate read when cold and as the engine warmed up. No issues there.

I replaced the fuel filter. Afterwards I went to start the truck to check for leaks one either side of the filter and the starter solenoid went BOOM. I then replaced the starter. For a day or two I was thinking maybe the starter was the issue the whole time but it is still being slow to start when warm.

Just to muddy the waters a little more, during a cold start, the engine barely has to turn over and it's running.

 

[movietvet]

I hooked up the scan tool and the coolant temp sensor seems to be working fine. I drove multiple times with it hooked up and the coolant temp sensor always gave an accurate read when cold and as the engine warmed up. No issues there.

I replaced the fuel filter. Afterwards I went to start the truck to check for leaks one either side of the filter and the starter solenoid went BOOM. I then replaced the starter. For a day or two I was thinking maybe the starter was the issue the whole time but it is still being slow to start when warm.

Just to muddy the waters a little more, during a cold start, the engine barely has to turn over and it's running.

Is there supposed to be a heat shield for the starter?

https://www.autozone.com/batteries-starting-and-charging/starter-heat-shield/p/gm-genuine-parts-starter-heat-shield/1251499_0_0?searchText=starter+heat+shield