Startup stumble.

[mars636]

I am having a problem with the start up on my 2012 sierra 6.2L. at startup it high idles for about 2 seconds then drops and stumbles then it catches itself before stalling then smooths out. It has stalled out once. This only happens on the first start up, if i start it up again after this happens it starts normally.

When i park my truck outside overnight it is not as bad as the truck high idles for a bit longer due to the cold temps. The times it happens where it is the most noticible is when i drive home in the morning after night shift when it is cold outside then i park in the heated garage, Then when i start it in the afternoon when the temps are warmer that is when it happens. My tahoe with the 5.3L never has this problem, and the tahoe is parked in the garage all the time.

I have been using a scan tool to try and see what is happening at start up and one thing i notice is when the ambient air temp sensor is reading a lot lower than the intake temp sensor the more noticeable the problem is. I am pretty sure that the ambient air temp sensor has nothing to do with start up but it is something that i seen, the DIC temp shows what the temp actually is. when i started it today coolant temp sensor, and intake temp sensor both read 24 celcius as did the dic was showing 24 but the scantool was showing the ambient temp at 6, i started the truck and it did its stumble then he ambient temp jumped up to 24 as the truck smoothed out.

With tuning my car i know that ambient temp is only used for the climate control but this is the only thing i see different The truck is stock right now i just want to get everything running right before i start tuning it.

Is there something that could be causing this, or something i should be logging during startup? the truck runs fine with no codesSorry for the long post but there seems to be a few people with this problem but no answer.

Sorry for the long post but i am hoping that someone here will be able to point me in the right direction

[mars636]

Well i took it into the dealer as it is still under warranty, they said it had a tick when starting it cold so they replaced the camshaft, actuator and lifters and said that was the problem with the stumble, so today i started it and it still stumbles and now i have a few leaks that looks like engine oil. there was 3 big puddles under my truck only one day after i brought it home. So back to the dealer it goes.

[schpenxel]

Get a log of it happening. Start logging before you start it.

Sounds like they did a shitty job on the R&R and now you have oil leaks because of it. At least it's under warranty.

[mars636]

Get a log of it happening. Start logging before you start it.

Sounds like they did a shitty job on the R&R and now you have oil leaks because of it. At least it's under warranty.

what PID's should i log that would help?

I just did a quick look under the truck to see if i could see where it was leaking from and it is from the front cam cover, you can actually see the gasket sticking out.

[schpenxel]

what PID's should i log that would help?

I just did a quick look under the truck to see if i could see where it was leaking from and it is from the front cam cover, you can actually see the gasket sticking out.

I wonder if the gasket is flipped around the wrong way? It looks like the gasket lines up with some features pretty well then that one part just sticks out like crazy. That can't be right

[schpenxel]

check out the timing cover for an L76.. looks like the gasket is backwards to me. See how there's that lump on the left side of the cover in the picture? Looks like that lump is on the wrong side. That would be two obvious oil leaks. Can't believe they screwed that up.

[mars636]

check out the timing cover for an L76.. looks like the gasket is backwards to me. See how there's that lump on the left side of the cover in the picture? Looks like that lump is on the wrong side. That would be two obvious oil leaks. Can't believe they screwed that up.

Wow, that must be it, hopefully they can get this sorted out and i can get my truck back.

[mars636]

The dealer has no idea what is making the truck stumble. they have started to change parts, They changed the MAF sensor with no change. so now they are calling gm technical for help

[10_SS]

this wont help you much, but my Camaro did this after I did some mods.. added Whipple SC.. etc. Mine sound like it was going lean, and the scan also seemed like the injectors werent squirting much fuel when it stumbled, as the injectors increased quite a bit as the stumble corrected itself.. anyway I added 25% more fuel to the open loop RPM vs. IVT table in the coolant temps this was happening... for me around 60-85F.. now problem is 100% gone.

Point is, it may be lean for some reason... If 100% stock, I would suspect low fuel pressure, etc. How are the LTFT's when driving around, and at idle? If they are far away from 0 then could be vacuum leak, or could only be a problem during startup with low fuel pressure even.

[mars636]

this wont help you much, but my Camaro did this after I did some mods.. added Whipple SC.. etc. Mine sound like it was going lean, and the scan also seemed like the injectors werent squirting much fuel when it stumbled, as the injectors increased quite a bit as the stumble corrected itself.. anyway I added 25% more fuel to the open loop RPM vs. IVT table in the coolant temps this was happening... for me around 60-85F.. now problem is 100% gone.

Point is, it may be lean for some reason... If 100% stock, I would suspect low fuel pressure, etc. How are the LTFT's when driving around, and at idle? If they are far away from 0 then could be vacuum leak, or could only be a problem during startup with low fuel pressure even.

LTFT were around 3% once the truck was up to temp, also the truck starts fine when it is colder out as it will hit the high idle and stay there. on another forum someone had a similar problem and he thinks the dealer reset the alcohol sensor. i am pretty sure that our trucks dont have a sensor anymore. but he said that this fixed his truck. I am also pretty sure that i seen that PID and mine was reading 0, hopefully the dealer can figure this out soon and not have to rip into the engine again, because my confidence in their abilities is pretty low after the screw up with the gasket

[mars636]

well so far the maf and the throttle body has been changed, when they changed the throttle body they said it was fixed, i picked up the truck and immediately noticed a smoother idle. but the next day it did the initial start stumble. It is booked back into the dealer. Is there anything i can check myself, I have been doing datalogs of the startup stumble, but are there specific PID's i should be looking at? And can anyone explain the absolute throttle positions b,d &e

[schpenxel]

Have you or they tried the same on another of the same truck to see if it happens?

How cold does it have to be for it to happen?

I'd log fuel trims, MAF airflow, ETC (throttle blade), coolant temp, O2 voltages, MAF hz, VE airflow, dynamic airflow, MAP, IAT, Commanded AFR, RPM, ignition timing, any sort of idle spark adapt or anything like that you can find.. pretty much max out how many things you can be logging and see if anything sticks out

Can you log other vehicles without having to license them? I can't remember if you can or not. If you can find another truck that isn't giving any issues, you might could try logging one of those during a cold start and see if you can find any differences

[mars636]

Have you or they tried the same on another of the same truck to see if it happens?

How cold does it have to be for it to happen?

I'd log fuel trims, MAF airflow, ETC (throttle blade), coolant temp, O2 voltages, MAF hz, VE airflow, dynamic airflow, MAP, IAT, Commanded AFR, RPM, ignition timing, any sort of idle spark adapt or anything like that you can find.. pretty much max out how many things you can be logging and see if anything sticks out

Can you log other vehicles without having to license them? I can't remember if you can or not. If you can find another truck that isn't giving any issues, you might could try logging one of those during a cold start and see if you can find any differences

I have tried my 2012 tahoe and that starts up no problem, the sierras stumble is not normal, half of the time it actually stalls out. it is worse when the ambient temps are warmer, as when it is cold it will hit high idle and stay there. I will have to wait overnight now as it is only the first start of the day that it happens on. every other start of that day is normal which is why it is hard to find the problem. The dealer said they checked the injector leak down and all of them held, i did check the fuel pressure before i turned the key on and it was at 0 then when the key on it jumped up to 60 psi.

[mars636]

Ok so i tried to do some stuff with the fuel pressure to try and figure this out. once again this morning there was no fuel pressure and i started the truck and the stumble was there. so i left the gauge on and wanted to watch the fuel pressure to see if it would drop quickly after i shut it down, when i shut it down the pressure was at 45psi which is where it was when the truck was idling, so i put a timer of for 10 minutes and waited to see if it would drop, well the pressure went up 10 psi, so i waited another 10minutes and it went up another 10 psi up to 65psi. so then i drained the fuel pressure to see if it would stumble and it didn't. So what i am thinking is if the fuel pressure is pressuring up enough then leaking through one or more injectors it could be somewhat flooding the truck for the first start up, which is why it might be stumbling at startup being too rich in which ever cylinder has the leaky injector. Does this make sense? I just don't know how with it pressuring up that it is at no pressure the next day. and why would it be pressuring up?

I always have run shell premium in the truck

[brandonleg]

2011 6.2 Silverado. All stock, never been to the dealer. Same symptoms as yours, first start in the morning, or after working all day (parked outside at work, inside at home).

Seems to be a commom occurance on E38 6.2's, from the google results at least.


cold start stumbe 3July.hpl

[brandonleg]

I got a chance to tweak mine a little over the weekend. I've had good luck (so far) modifying the Open Loop EQ Ratio Gas Table. If you look in 3D you'll see a little "valley" in the ~ 80 deg IVT and Engine coolant area. I smoothed that out and it's almost undetectable now.

EQ.jpg

[GHuggins]

Sounds like someone needs to tell one of GM's techs so they can pass along the info to GM? I'm sure there will be some sort of recal for it. Odd that these weren't doing this when newer? Wonder what a good cleaning might do if anything? Of course I've also always wondered why the OL modifiers will take sudden dips like that in the slightly warm areas then just in the corresponding temp cells go back richer? Just more and more questions I guess...

[mars636]

So i filled up the truck at a petro-canada gas station with there ultra 94 and have started the truck twice now where it would have normally stumbled but it started like normal.

[10_SS]

you could try some injector cleaner... could be leaking fuel when off.. even though GM "Checked" them once... doesnt mean they dont leak randomly. Leaky injectors was a huge problem with the older LS truck motors... they leaked so bad that it would mess up idle LTFT's... it would pull 20% fuel from left or right bank, only because 1 injector was leaking, it would make the other 3 cyl's on that bank extremely lean, missfire...

[mars636]

The last time the truck was in they replaced the fuel rail and all the injectors, and that didn't change anything. And to try and prove that it wasn't the fuel injectors i bled all the pressure off of the rail one night so that if it was a leaky injector it wouldn't be able to leak into the cylinder with no fuel in the rail, and it still had the stumble at the initial start of the day.