Charlie207
Full Access Member
Thanks!
Edit: I just went out to my Yukon and I had no vacuum at the valve, so it's presumably good.
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[SOLVED] 2012 6.2L Yukon Denali XL 172000 miles? Need help and cannot figure this very random problem out.
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Edit: I just went out to my Yukon and I had no vacuum at the valve, so it's presumably good.
I installed a brand new DELPHI MAF sensor yesterday and drove 80 miles round trip yesterday with engine warm at 210F. No problems.
The truck sat all night and today until 5pm when we got in it to drive up the road. Went two blocks at 15mph engine temp about 180. Foot on gas and in drive the engine starts surging and the RPMs run up to about 1500 and down to below 500 and back up. As I cruise to a stop at the light and still in drive I watch the RPM gauge drop to zero and the engine dies. I shifted to park and restarted the engine no problem. We kept driving and no more problems after engine was at normal 210F temp.
The truck sat all night and today until 5pm when we got in it to drive up the road. Went two blocks at 15mph engine temp about 180. Foot on gas and in drive the engine starts surging and the RPMs run up to about 1500 and down to below 500 and back up. As I cruise to a stop at the light and still in drive I watch the RPM gauge drop to zero and the engine dies. I shifted to park and restarted the engine no problem. We kept driving and no more problems after engine was at normal 210F temp.
mikez71
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Maybe failing crank position sensor if no codes.. ?
I let this truck sit overnight again and just started it and sprayed the intake manifold with carb cleaner to check for vacuum leaks. No leaks found. The problem with this truck happens while driving and while the engine is cold but does not throw a check engine light or DTC code. I can start this truck up parked in my carport and it warms up and idles normally.
Personally i have only had that method work once. Carbed engine and the throttle plate rod was leaking bad.
I just can't seem to get the spray where it needs to go on the bottom side.
I just can't seem to get the spray where it needs to go on the bottom side.
Charlie207
Full Access Member
I'm wondering if it is fuel pump related, and I've been troubleshooting my issue for over a year now.
It shows a steady ~43psi via a scanner, but as soon as I kill the engine (ACC still on for OBD2 port power) I watch the pressure rise to 80+psi over 30 seconds or so. It's like the pump is running in overdrive.
Then, any subsequent startup will intermittently stutter, and not fire, or if it does fire it struggles to fine a steady idle for 5-10seconds before stalling/dying.
I have replaced a bunch of parts, and spraying around the intake with flammable stuff does nothing to affect idle speed.
I can't say we're having the same root-issur, but some of our symptoms are very similar.
My friends Yukon would die and run bad when cold until it warmed up a little. After a few months i told him to take it to the dealer as i was just out of ideas and had no clue where to look next.
Using a scanner to watch data the pump looked perfect.
He took it to the dealer and they replaced an exhaust manifold bolt and 2 pre cat O2 sensors. This did not fix anything and the dealer could not fix the problem either.
As a last ditch effort i put a new fuel pump in it. Problem fixed, truck runs perfect.
Still no idea what it changed. Data still all appears to be the same.
Anyway, there is a possibility the fuel pump is the problem.
Using a scanner to watch data the pump looked perfect.
He took it to the dealer and they replaced an exhaust manifold bolt and 2 pre cat O2 sensors. This did not fix anything and the dealer could not fix the problem either.
As a last ditch effort i put a new fuel pump in it. Problem fixed, truck runs perfect.
Still no idea what it changed. Data still all appears to be the same.
Anyway, there is a possibility the fuel pump is the problem.
My Yukon has the FLEX engine and a few months ago I replaced my old fuel pump with a brand new AC Delco one. My fuel pressure under load is 43psi while driving. I checked with the scan tool and a gauge hooked up to the fuel rail taped to the windshield so I could see it (both 43psi). I've fired the parts canon as well. Heck I even asked AI what it thought the problem was. Like Clint Eastwood said "A man has to know his limitations" so I'm tossing in the towel. My local shop owner says his diagnostic tech can fix it. I drop it off tomorrow and his guy will look at it Monday. I'll let you know what he finds. @Charlie207
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Did you use an AC Delco OEM fuel pump or an AMAZON fuel pump?
Charlie207
Full Access Member
@Charlie207 , @rdezs
I replaced
New AC Delco fuel pump
New Delphi MAF sensor
New throttle body and did the relearn
New crank sensor
New coolant temp sensor
New fuel injection sensor
New battery negative cable set
New spark plugs
New coil packs
New ground from engine block front to driver side firewall
New oil pressure sensor and filter screen
Tested the fuel pressure of this new pump on this FLEX engine as good
New EVAP purge solenoid
No DTC codes
No check engine light
These are original to this 14yr old truck and have not been changed:
All four O2 sensors
Fuel Pump Control Module
VVT solenoid and magnetic sensor
Plug wires
Intake manifold gaskets
MAP sensor
Even after changing all of these parts my problem persists!! Truck sat all night and get in to drove a few miles. Start the truck normally and it idles normal. Drive 200 feet down my driveway and turn left to drive 1 block to the stop sign. Come to a stop just fine no issues. Accelerate from the stop and engine starts surging and RPMs bouncing as truck struggles to keep running as I give it more gas to get to the light. Stop at the light and the engine stalls out while in drive. I shift to park and restart the engine and it idles fine and runs good the rest of the day.
It will be interesting to see what the professional shop diagnostic tech says is wrong and how he found it.
Charlie207
Full Access Member
The other thing I've read is random corrosion on the bottom of the fuse block.
well unless you missed something obvious my money is on the transmission or there is something electrical happening, cold solder on a module, etc.
part of the reason I say transmission is if let to sit and warm up first then the problem does not occur?
has other drivetrain data been looked at?
I sure would be looking hard at all data on a scanner. good luck
this one is brand new
I would also check the vacuum line to the power brake booster, and the vacuum check valve that plugs into the booster itself.
It would be worth getting a vacuum reading off the intake manifold when the engine is cold and pressing on the brakes.
It would be worth getting a vacuum reading off the intake manifold when the engine is cold and pressing on the brakes.
Unfortunately brand new is no longer a guarantee. Even some ACDelco genuine GM parts are now made in China. That vacuum Port that's plugged off from the factory on the passenger side of the intake manifold? I'd be tempted to hook a vacuum hose on to it with a generic vacuum gauge zip tied to the windshield wiper.... And see if you're suddenly losing vacuum when it starts to stumble. You seem to have ruled out fuel pressure.
In these newer vehicles the idle air control has been replaced by the throttle position sensor on the side of the throttle body. If your vacuum starts going down when it's stumbling, that pretty much points to the throttle position sensor, or a vacuum leak through the purge solenoid, for the intake manifold gasket. But I would expect that to be more consistent.
When you replace the throttle body did you use a Tech 2 or bidirectional scanner to set it to relearn the idle? Often times they are literally plug and Play, but sometimes they need a little help.
