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Varying Cold Start Idle

Varying Cold Start Idle

Postby Jamesef [OP] » May 29 2021, 12:10am

2018 Chevrolet Silverado 1500 5.3L
18k miles

During cold starts with the gear selector in Park, my initial idle speed is roughly 1100 rpm and then gradually goes down over the next 30 seconds. This is normal. BUT sometimes on a cold start, the initial idle speed starts at roughly 750 rpm. This does not seem normal to me

Also there is a slight up and down fluctuation in rpm up until the engine is warmed up. It is a small fluctuation but irritating nonetheless. These two things may be linked

What would cause one or both of these situations to occur?
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Re: Varying Cold Start Idle

Postby Farmguy » May 29 2021, 4:40am

The slight variation may be the ecm changing the electrical load and/or alternator charge command levels.

(they no longer use just a simple voltage regulator to control the alternator)
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Re: Varying Cold Start Idle

Postby Jamesef [OP] » May 29 2021, 1:35pm

Ok I see. So could the ecm be constantly changing the alternator charge command levels because my battery is in the beginning stages of dying? When I bought this truck, it had only 16k miles and was 3 years old. I am thinking the battery sat for a long while
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Re: Varying Cold Start Idle

Postby Chevy-SS » May 29 2021, 2:17pm

Well, it's hardly got any miles on it. I'm thinking something basic here. Is there a CEL? I'm guessing there is no CEL, otherwise you would have mentioned it (and the code).

I would start with the most simple.... add some dry gas and fuel injector cleaner to the fuel. Try that a few times. Also, maybe try a different gas station.

Additionally, look in engine bay, for vacuum lines and other items that may be broken or loose or fallen off.

Also, clean the MAF. Those things are super sensitive, it might be a tad contaminated.
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Re: Varying Cold Start Idle

Postby Jamesef [OP] » May 29 2021, 3:44pm

There is no CEL on.
It is a direct injection system so I thought it might of been dirty injectors but I added Royal Purple injector cleaner and the problem persists.
I typically get gas from top tier gas stations like BP or Marathon. I have switched gas stations and I put the recommended 87 in.
I already cleaned the MAF sensor and I checked for vacuum leaks. I will clean the MAF again and check for any leaks and let you know if anything changes. I also am going to clean the throttle body as a good measure to eliminate that. I will post an update
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Re: Varying Cold Start Idle

Postby GBSierraSLE » May 30 2021, 2:41am

To your question about the charging system... Yes, the BCM constantly monitors the battery state of charge and varies the alternator output so the battery will not overcharge or over heat. This is explained in your owner's manual. On cold starts, you should be getting full charging at 14+ volts to recharge the battery from sitting overnight and after starting the truck. GM started using this system about 2005. This would have very little impact on your idle speed. Using Tow/Haul cancels this function and puts the alternator in full charge mode constantly.

Other things, such as A/C cycling, could affect idle RPM's.
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Re: Varying Cold Start Idle

Postby Jamesef [OP] » May 30 2021, 5:28pm

Update:
I cleaned the MAF sensor and electrical connections for it
I replaced the battery and cleaned the electrical connections for it
I checked for any vacuum leaks and found none
I cleaned the throttle body

After I did all of this, the idle on both the cold start up and completely warmed up has improved. My initial idle on startup is 1100 rpm and not the 750 rpm. I also noticed that my oil pressure has gone up from my previous averages. Previously my oil pressure would be slightly over 40 while driving and close to 20 while idling at complete warm up. Now It is higher for both. Maybe this is contributed to the new battery
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Re: Varying Cold Start Idle

Postby GBSierraSLE » May 30 2021, 6:20pm

A new battery would have no effect on your oil pressure. The oil pressure is higher because your engine rpm's are higher.
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