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Engine Revs to Redline on Cold Start 6.2L

Denny NWO
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Denny NWO Enthusiast

Denny NWO

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For the last week in outdoor temperatures below freezing, my 2019 AT4 6.2L has occasionally revved to redline on cold start after pushing the start button with my foot on the brake. This has happened 3 times in the last 4 days. I consistently use premium fuel only from the same gas station and cannot figure out why this is happening. Before i bring it to the dealer, who will want to keep it overnight, I was just wondering if anyone else has experienced this? 

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Denny NWO
Denny NWO

It's been a week with the monitor connected to the OBDII and connected live with GM TAC and my truck has not revved to 3500RPM but has coughed and fluttered upon starting but it recovers immediately.

Denny NWO
Denny NWO

My truck went to the dealership on Tuesday and GM gave me a rental to drive for the week while the tech took my truck home. It started normal the first few days but acted up Friday morning when he sta

SPASierra
SPASierra

Right, but even drive by wire TPS sensors are essentially a potentiometer that sends a voltage based on position of the throttle plate.   None of this is likely to be the cause of his issue, but the

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mafd2 Senior Enthusiast

mafd2

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Wow that's a first! Has nothing to do with the fuel you're running. Has to be something in the BCM or TPS. Definitely a trip to the service dept. Keep us updated.

SPASierra Enthusiast

SPASierra

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Posted

Do you have anything like a K&N air filter installed? I wonder if your throttle body could be sticking open briefly.


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mafd2 Senior Enthusiast

mafd2

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24 minutes ago, SPASierra said:

Do you have anything like a K&N air filter installed? I wonder if your throttle body could be sticking open briefly.


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The throttle body controls air flow and not fuel like older throttle body injections. These are DI engines (injector per cylinder direct into the cylinder) controlled by the BCM.

SPASierra Enthusiast

SPASierra

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Posted
The throttle body controls air flow and not fuel like older throttle body injections. These are DI engines (injector per cylinder direct into the cylinder) controlled by the BCM.

Is the TPS not on the throttle body? I’m well aware of what direct injection is. I’m not aware of a late model gasoline engine that doesn’t use it. A/F ratios at startup are a closed loop function, but the BCM needs to air flow reading to determine injector duty cycle. A gummed up throttle body can certainly throw off the reading of a TPS and a MAP/MAF sensor by not allowing the throttle plate to fully close.


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mafd2 Senior Enthusiast

mafd2

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Posted
16 minutes ago, SPASierra said:


Is the TPS not on the throttle body? I’m well aware of what direct injection is. I’m not aware of a late model gasoline engine that doesn’t use it. A/F ratios at startup are a closed loop function, but the BCM needs to air flow reading to determine injector duty cycle. A gummed up throttle body can certainly throw off the reading of a TPS and a MAP/MAF sensor by not allowing the throttle plate to fully close.


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I understand but, the TB controls air flow with the amount of acceleration/fuel and the acceleration isn't controlled by the air flow. The air flow is adjusted according to fuel flow (rich/lean) and vice versa. The more air the more fuel needed then the BCM will compensate for  more fuel which you already know but it won't control acceleration. i.e; if you'd manually open the TB flap at its widest point, the engine won't accelerate and rev to its limit. 

SPASierra Enthusiast

SPASierra

Posted
Posted

Right, but even drive by wire TPS sensors are essentially a potentiometer that sends a voltage based on position of the throttle plate.

 

None of this is likely to be the cause of his issue, but the signal for wot is coming from somewhere.

 

 

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calgator73 Senior Enthusiast

calgator73

Posted
Posted (edited)

If there was a mismatch between pedal position and throttle position you should get a code for that.  It's interesting you saying it's going to wide open throttle as soon as you start it because these things are regulated and limited to 2,500 or 3,500 RPMs when in park.

 

With these things being direct injected I wonder if there's a possibility of one or more injectors sticking like a diesel when it goes into runaway.

Edited by calgator73
Denny NWO Enthusiast

Denny NWO

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On 11/15/2020 at 10:59 AM, SPASierra said:

Do you have anything like a K&N air filter installed? I wonder if your throttle body could be sticking open briefly.


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I have the factory cold air intake. The dealer had to re-flash the ECM as a dealer installed option.

Denny NWO Enthusiast

Denny NWO

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On 11/15/2020 at 5:50 PM, calgator73 said:

If there was a mismatch between pedal position and throttle position you should get a code for that.  It's interesting you saying it's going to wide open throttle as soon as you start it because these things are regulated and limited to 2,500 or 3,500 RPMs when in park.

 

With these things being direct injected I wonder if there's a possibility of one or more injectors sticking like a diesel when it goes into runaway.

I know the RPM's are regulated in Park and could not tell exactly how high it revved because the speedometer and tachometer needles were doing their start up reset. It hasn't done it since and I'm recording every time i cold start it. I know if i bring it to the dealer, they will want to keep it overnight outside so they can cold start it in the morning. Now it is starting and almost stalling

calgator73 Senior Enthusiast

calgator73

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On 11/16/2020 at 9:00 PM, Denny NWO said:

I know the RPM's are regulated in Park and could not tell exactly how high it revved because the speedometer and tachometer needles were doing their start up reset. It hasn't done it since and I'm recording every time i cold start it. I know if i bring it to the dealer, they will want to keep it overnight outside so they can cold start it in the morning. Now it is starting and almost stalling

If you have an obd 2 scanner with live data capture you should be able to get the truck set to run without starting, connect your scanner, set to record and start the engine.

Denny NWO Enthusiast

Denny NWO

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The truck started normally for the last 10 nights and the issue happened again last night. The engine went straight to the regulated full RPM when in park and held until i shut the truck off after about 3 seconds. I restarted the truck and it started normally. I thought it could have been a fuel issue but have since filled up at other locations. Dealer hasn't heard of this issue, guess i will have to find an OBD2 scanner with live data capture and figure it out myself.

Edited by Denny NWO
ShadyOakBrewer Enthusiast

ShadyOakBrewer

Posted
Posted

Haven't had this happen personally (2020 AT4) but I seem to recall a review either on this forum or on the Pedal Commander website of a T1XX revving on startup. Never saw a solution posted...

Denny NWO Enthusiast

Denny NWO

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Tonight I decided to start video recording when I left work. I was able to capture the event. I pressed the brake and start button and the engine sputtered then went straight to 3500 RPM (Park Redline) and stayed there for 10 seconds then went to idle. Check it out.

 

Anyone hear about this happening to anyone else or have a possible cause? I now have something to show the service manager at the dealership.

2019 AT4 Cold Start High RPM.mov

Edited by Denny NWO
CamGTP Senior Enthusiast

CamGTP

Posted
Posted (edited)

I don't have much to add but I will say this. The BCM does not control anything air, fuel, throttle, lean/rich or whatever else.

 

BCM stands for body control module. PCM stands for power train control module or otherwise known as the ECM to some.

 

These trucks are torque based, end of story. You are requesting a torque value when you press the accelerator pedal, that then gets thrown in to a giant calculation and it will open the throttle, give it fuel and the timing needed to meet that torque value.

 

If you want to read until your eyes and brain hurt you can read the logic behind a torque based computer.

 

http://www.hptuners.com/help/vcm_editor_tuning_how_to_advanced_e78.htm

Edited by CamGTP
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