Rough starts in the Morning

DRAGONSCULLFIRELORD

MEMBER
2006 GMC SIERRA
5.3L
V8
4WD
AUTOMATIC
212,000 MILES

So I rebuilt my top end 6 months ago or so and ever since then I’ve had rough starts in the mornings but it has gradually gotten worse or this time period so now in the mornings or after work it takes 1-3 tries to get it started after it gets started it runs fine. I tested my fuel pressure and first starts it’ll bounce from 40-50psi while trying to start, when I turn the key to accessory it has a normal fuel pressure reading of around 55-60psi so my first thought was maybe fuel pump so I replaced the fuel pump but the I still have the same issue the fuel pump did help a little though so I guess my just wondering where to go from here also my engine does not have a fuel pressure regulator on the fuel rail, I also replaced my coolant temp sensor and my camshaft position sensor cause it was leaking I’ve also tried running royal purple injector cleaner through and it has not seemed to help either I purchased all my parts from Napa and had Napa clean and machine my heads and other parts like throttle body valley pan and valve covers I also replaced my knock sensors any ideas?

5 Replies

JACOBANDNICKOLAS

MECHANIC
109,885 POSTS

Hi,

That is odd that normal pressure is obtained when the key is in the accessory position. Do you mean the run position?

On a cold start, if you cycle the key 3 or 4 times (engine off) between off and the run position, does it make a difference?

As far as the fuel pressure regulator, it is actually at the tank and part of the fuel sender assembly, but I don't feel that is the issue. It sounds like the pressure drops off too much after sitting.

One other thought, does it start easier if you hold the throttle to the floor, and have you checked the fuel pump relay? If you haven't switch it with a different one with the same part number to see if it makes a difference. If there isn't one, here is a link that explains how to test a relay.

https://www.2carpros.com/articles/how-to-check-an-electrical-relay-and-wiring-control-circuit

Let me know.

Joe

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Friday, October 8th, 2021 AT 8:47 PM

DRAGONSCULLFIRELORD

MEMBER
5 POSTS

Good morning. Apologize, I do mean the run position.

Pressing the pedal to the floor does not seem to help and cycling the key does seem to help a little bit, but not really.

I have not checked the fuel pump relay; I will do that today and I’ll let you know what I find. Thank you!

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Saturday, October 9th, 2021 AT 5:58 AM

JACOBANDNICKOLAS

MECHANIC
109,885 POSTS

Hi,

Let me know about the relay. The PCM is what controls the fuel pump relay. It should prime for a short period of time, but the PCM will shut it down if an ignition reference isn't made. What is confusing me is why you would have lower, erratic pressure in the start position. It should be the same. The same relay provides the signal, so that is making me question if there is an issue with the ignition switch.

Let me know what you find.

Joe

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Saturday, October 9th, 2021 AT 5:26 PM

DRAGONSCULLFIRELORD

MEMBER
5 POSTS

Hey. Yes, the relay tests good and the fuel pump does kick on for a few seconds when turning the key to the run position with no ignition reference being made.

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Saturday, October 9th, 2021 AT 8:10 PM

JACOBANDNICKOLAS

MECHANIC
109,885 POSTS

I ran through schematics. The only thing I can think of is if the battery is weak causing the lower voltage to make its way to the pump when the starter is engaged. The same power supply to the pump is from the same wire on the ignition switch. What isn't making sense is the idea that you have good pressure in the run position. That should be enough to prime the system and start the engine.

By chance, can you check what the voltage drops to when cranking? I'm sorry to have so many questions. It just isn't a typical issue.

Let me know.

Joe