2012 Yukon 1500 XL Denali 6.2L while driving truck shudders, rpms run up and down until engine dies. Update at #44

[tooleyondeck]

What tuner are you recommending and what is the cost of doing that?

If you know someone with HP tuners, it costs $100 and takes 30 seconds to unlock your ECU and permanently disable your AFM. With your mileage and having the AFM components in place, be cautious if considering a performance tune.

 

[SpareParts]

Drive up to North Idaho. I have HP Tuners and know how to disable it.

 

[Joseph Garcia]

If you know someone with HP tuners, it costs $100 and takes 30 seconds to unlock your ECU and permanently disable your AFM. With your mileage and having the AFM components in place, be cautious if considering a performance tune.

Depending on how the motor has been serviced over its life, I don't think that is it unreasonable to install a performance tune at 170,000. I supercharged and added a performance tune to my truck's motor at 160,000 miles, and it is still going strong. Agreed, there is more risk at 170,000 miles versus 20,000 miles, but.......

 

[testdepth]

Well I'm back again. I installed the AFM disabler Oct 7th and the truck has been driving fine to include yesterday afternoon when it was driven to and from a store. The disabler was installed when this happened. It sat all night and this morning it was driven 2 miles up the road to a traffic light. Once the light went green my wife turned left and the truck started to shudder, RPM gauge fluctuating and she pulled to the side of the road as the engine died. She immediately tried to start it back up and it acted like it was flooded by cranking but not starting. She let it sit a few minutes and it started, so she drove back home the 2 miles. By this time the engine is up to normal temp of 210. Before doing anything else I placed my TOPDON bidirectional scanner on the truck to pull codes. There are no codes and no check engine light. I started the truck back up and drove it for several miles without any problems. I had a friend at the local GM dealer (30yrs mechanic) look at it and scan with his equipment. He found nothing and there are no current updates needed for anything. He has never seen this problem.

Observations:
Problem does not set codes and there are currently no codes
No check engine
This happens after sitting for long periods like overnight
This happens while driving at speeds under 50
This happens before the engine is at normal temp of 210
It's as if the fuel pump is being shut off so the truck shudders, RPMs go up and down until engine dies and immediate crank is like the lines are empty of gas and they have to fill back up hence the prolonged cranking.
Once the truck is warm to 210 no further problems
No problems driving at higher speed over 50
My mechanic friend's equipment showed fuel pump pressure at 43psi with engine running at idle in park.

I'm not sure what to do now to finally kill this Gremlin!

 

[testdepth]

This morning I drove this truck a few miles and before it warmed to normal temp of 210F at 25mph the engine starts surging and tach revs up and down but this time it did not stall. Yesterday morning it did this to my wife and stalled the engine. She waited a few minutes and it started back up so she drove home. After the engine is at 210F this problem does not happen. Only happens when cold ant at low speed. No codes and no check engine light.

https://www.facebook.com/video.php?v=1132539982278521

 

[solli5pack]

it can be figured out your just going to have to do the equivalent of a EKG, connect a fuel pressure gauge and stick it up thru the hood opening, connect the scanner and start monitoring all the live data, knock retard, coils, injectors, 02 sensors, misfires, map sensor, ambient temperature, etc.
sometimes the problem is enough to feel but not enough to throw a code until it gets to a XX count or fails 3 times in a row/drive cycle.
new parts can be faulty and some aftermarket parts are well known to often fail in short order.
just like you discovered with your brand new spark plug....

Follow the advice @Doubeleive provided...otherwise you're just guessing and the more parts you throw at it the furter the real issue gets buried...

 

[Doubeleive]

This morning I drove this truck a few miles and before it warmed to normal temp of 210F at 25mph the engine starts surging and tach revs up and down but this time it did not stall. Yesterday morning it did this to my wife and stalled the engine. She waited a few minutes and it started back up so she drove home. After the engine is at 210F this problem does not happen. Only happens when cold ant at low speed. No codes and no check engine light.

https://www.facebook.com/video.php?v=1132539982278521

if you ask me I say time to take it to a transmission shop and have them look at your transmission
you could also put it in m1/m2/m3/m4/m5 and see if it does anything different/worse or nothing at all, do each gear individually and don't baby it. the goal is to exercise the transmission
normally that kind of rpm fluctuation would cause bucking with the speed and you would feel it jerking you around.

 

[mikez71]

I like Doub's angle..

maybe @NickTransmissions can look a the video 3 posts above and see if it looks familiar.

Torque converter clutch going out?

Tapping the brakes might make it unlock..
If heavy-throttle / heavy-braking drives fine..

 

[NickTransmissions]

This morning I drove this truck a few miles and before it warmed to normal temp of 210F at 25mph the engine starts surging and tach revs up and down but this time it did not stall. Yesterday morning it did this to my wife and stalled the engine. She waited a few minutes and it started back up so she drove home. After the engine is at 210F this problem does not happen. Only happens when cold ant at low speed. No codes and no check engine light.

https://www.facebook.com/video.php?v=1132539982278521

Symptoms sound like extremely worn pressure regulator valve in the pump. The PR valve is not accessible with the trans in the vehicle.

Once it has been sitting overnight, put a mechanical transmission pressure gauge on the unit, fire it up and let us know what your pressures are at idle in P, R,N,D,M.

 

[testdepth]

Just drove it again after sitting all night and no surging or tach jumping like my video.

I did notice a few things while shifting the transmission back and forth from D to M. In M this bypasses the AFM so no switching back and forth from 8 to 4 cylinder.
Doing 45mph in D my RPM is 1100 and if I shift to M5 RPM jumps up to 1400, If I then shift back to D RPM drops back to 1100. Not sure if RPM stays the same whether in V4 or V8?

I did go buy fuel pressure gauge and transmission/oil pressure kit so I can test and provide those numbers.

Thank you for all the help with this PIA Yukon

 

[testdepth]

I just installed the fuel pressure gauge to my fuel rail. Vehicle in park idling. Initial key on pressure is 55psi. Start the engine and the pressure jumps to 60psi but as the engine drops rpms to 600 the fuel pressure gauge shows 42 to 43 psi and stays there.

 

[Marky Dissod]

Not sure if RpM stays the same whether in V4 or V8?

RpM stays the same, the fulltime V4 does all the work, the part time V4 stops pushing, but still goes along for the ride.
ANYWAY
You really ought to consider the VERY STRONG POSSIBILITY that this has NOTHING to do with Engine Half@$$.

Symptoms sound like extremely worn pressure regulator valve in the pump. The PR valve is not accessible with the trans in the vehicle.
Once it has been sitting overnight, put a mechanical transmission pressure gauge on the unit, fire it up and let us know what your pressures are at idle in P, R,N,D,M.

 

[testdepth]

Fuel pressure gauge connected to fuel rail test port reads:
Key on 55psi
Engine start initially at 60psi
Engine start at idle 600 rpm 43 psi and stays there
I watched the gauge as I increased rpm steady up to 3000 the pressure stayed steady at 43psi

With my scanner connected
Desired fuel pressure in park idling at 600rpm shows 43psi
Fuel pressure sensor shows 43psi
Holding gas pedal at 1000rpm, 2000rpm and 3000rpm : fuel pressure stays constant 43psi and never changes.

 

[kbuskill]

Just drove it again after sitting all night and no surging or tach jumping like my video.

I did notice a few things while shifting the transmission back and forth from D to M. In M this bypasses the AFM so no switching back and forth from 8 to 4 cylinder.
Doing 45mph in D my RPM is 1100 and if I shift to M5 RPM jumps up to 1400, If I then shift back to D RPM drops back to 1100. Not sure if RPM stays the same whether in V4 or V8?

I did go buy fuel pressure gauge and transmission/oil pressure kit so I can test and provide those numbers.

Thank you for all the help with this PIA Yukon

I am not super familiar with the 6 speed transmission but I believe the difference you are seeing in RPM when shifting from D to M5 is just your torque converter unlocking which raises the rpm slightly and then relocking when shifted back to D.

Also 43 psi is proper for these trucks under normal circumstances.

 

[Fless]

Fuel pressure gauge connected to fuel rail test port reads:
Key on 55psi
Engine start initially at 60psi
Engine start at idle 600 rpm 43 psi and stays there
I watched the gauge as I increased rpm steady up to 3000 the pressure stayed steady at 43psi

With my scanner connected
Desired fuel pressure in park idling at 600rpm shows 43psi
Fuel pressure sensor shows 43psi
Holding gas pedal at 1000rpm, 2000rpm and 3000rpm : fuel pressure stays constant 43psi and never changes.

Fuel pressure is acting good -- continue looking elsewhere.

 

[testdepth]

Symptoms sound like extremely worn pressure regulator valve in the pump. The PR valve is not accessible with the trans in the vehicle.

Once it has been sitting overnight, put a mechanical transmission pressure gauge on the unit, fire it up and let us know what your pressures are at idle in P, R,N,D,M.

I have not been able to find any video or info on how to connect the mechanical gauge and perform the test you mentioned. Do you know where I can find this info?

This truck has 170000 miles on it and I don't think the trans has ever been serviced. Fluid from the dipstick is a brownish color.

 

[testdepth]

Fuel pressure is acting good -- continue looking elsewhere.

That's a relief I did not want to drop the tank again.

 

[NickTransmissions]

I have not been able to find any video or info on how to connect the mechanical gauge and perform the test you mentioned. Do you know where I can find this info?

This truck has 170000 miles on it and I don't think the trans has ever been serviced. Fluid from the dipstick is a brownish color.

Go to my "6L80 Info Thread", page 1 post 1 and scroll down to the link to the ATSG manual. There are some instructions covering pressure testing on the 6L80/90.

Make sure you are using a transmission pressure gauge, not an engine oil gauge.

 

[Doubeleive]

I have not been able to find any video or info on how to connect the mechanical gauge and perform the test you mentioned. Do you know where I can find this info?

This truck has 170000 miles on it and I don't think the trans has ever been serviced. Fluid from the dipstick is a brownish color.

Proper color is pink to slightly red, brown indicates burnt......usually from clutch material but not totally unexpected if it has not been flushed in a good long time, some people never change the fluid

 

[Marky Dissod]

This truck has 170000 miles on it and I don't think the trans has ever been serviced. Fluid from the dipstick is a brownish color.

As in ... doo doo brown?

Proper color is pink to slightly red, brown indicates burnt ... usually from clutch material,
but not totally unexpected if it has not been flushed in a good long time, some people never change the fluid

If you've never serviced your ATF it's very likely that the symptoms your vehicle is experiencing are prelude to your transmission schidt-ting itself (huh huh).

IFF you are considering servicing the ATF, may I strongly recommend using this ATF:

COMPLETE Full Synthetic ATF - Lubegard

lubegard.com

It works for Dexron6 (AND Dexron ULV aka Dexron8, by the way)