LS 6.2L tech

Baconator

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I have an 08 Yukon with a 6.2 in it, about a year ago while driving up I70 into the mountains it shut off while coasting down a hill, it started back up after pulling off to the side and hadnt had an issue since.
Today while pulling up I70 to the Eisenhower tunnel it died again while coasting to a stop, went to push on the gas and nothing, just dead. This time it would not restart, so CDOT pulled me off to the side where I pulled relays fuses and pounded on the fuel tank thinking it was the fuel pump, and popped the gas cap on and off just out of habit.
It would crank and ever so slightly kick, but not take off. I then pushed in on one of the shrader valves on the fuel rail and had plenty of fuel coming out, making me believe the fuel pump was ok.
After sitting for about 45 minutes it started to try a little harder to start and after 10 or so attempts finally started and ran fine.
I drove it 100 miles home with no issues until I got of the highways, whenever you would slow down and come to a stop for a light it would pull down below 500rpm and nearly stall, then have very poor throttle response until it got above 1000rpm. I started popping it into N at lights and keeping the revs above 1500rpm which seemed to keep it alive.
When I got home I came to a stop and it about died when getting into the garage so I popped it in N and just let it try to idle thinking something in the trans was the issue, but it just surged around 4-500rpm, when I gave it some throttle it stumlbed really hard and struggled to climb about 1000rpm, but once it did it ran fine.
There were no active or stored codes on it. I was thinking fuel pump or pressure regulator, but I am not very convinced thats my issue. After sitting for a few hours at home it runs fine now and I cant repeat any of it.
 
Everything you say sounds like fuel pressure. Put a gauge on it and run it out the hood so you can watch it while driving under load.
 
larboc said:
Everything you say sounds like fuel pressure. Put a gauge on it and run it out the hood so you can watch it while driving under load.
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I was thinking fuel pump, but it runs fine under load. Only has issues at idle. Would that be a sticking pressure regulator?
Its a very annoying issue, I went out and drove it for 20 minutes with HPtuners hooked up and couldnt get it to trip up at all
 
Is the oil pressure ok? A buddy just did a swap with a 6.2 and had a oil pressure problem. Research showed low oil pressure can cause a "limp mode" issue.

Apparently the pickup screen can get clogged or the o-ring on the pickup tube can get damaged and cause issues.
 
The big underhood fuse/relay box goes bad on GM vehicles and cause similar symptoms. Next time it happens, smack the relay box or put ice on it to see if it changes.

Also, that thing needs 60 psi to start. If it didn’t get in your eyes or at least on your shirt when you pushed The schrader valve, it may have been a fuel pressure issue.
 
Clean the throttle body. I know it sounds stupid but I had the same issue on two different trucks. The problem won’t throw a code but what was happening:

oil deposits built up around the butterfly and the plate couldn’t calibrate properly so when I let off the throttle the TB would operate at say 10% but because of the gunk inside the TB it would completely close and stall the engine only when coming to a stop or coasting. I had to do the same thing put it in neutral and keep the revs up. First I thought it was an alternator because I was seeing the voltage drop coming to a stop.
 
Baconator said:
I was thinking fuel pump, but it runs fine under load. Only has issues at idle. Would that be a sticking pressure regulator?
Its a very annoying issue, I went out and drove it for 20 minutes with HPtuners hooked up and couldnt get it to trip up at all
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Maf signal look plausible? K N airfilter?
 
I second the throttle body or IAC. Similar problem on a car I have was caused by junk build up in the idle air control. I'm sure it's a different set-up in your engine than mine but it probably has something to perform the same function.
 
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WHATAHEEP said:
I second the throttle body or IAC. Similar problem on a car I have was caused by junk build up in the idle air control. I'm sure it's a different set-up in your engine than mine but it probably has something to perform the same function.
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No iac, but the TB issue is a tsb for those engines so it sounds likely. I would have thought that would throw a code though.
 
Does it have a factory Intake? I had a 6.2L Silverado that had a AEM intake, the filter kept falling off the Intake tube in the filter box and it wouldn't throw a code, but I had all sorts of drivability issues.
 
Sounds to me like intake gaskets. Perhaps the ECU is reading something different than what is actually happening.
 
I cleaned the TB, made no difference. I replaced it just to rule it out and made no difference either.
It died on me today in an intersection, I bled the fuel from the rail and it tried starting a few times then finally took off and ran fine the rest of my trip. I pulled it into the garage an it started to choke a bit, I let it sit there in Drive to see what it would do and just stumbled around 500rpm, I gave it a little throttle and it recovered and idled fine.
 
Sounds like a fuel pressure problem. I've got that engine in my buggy and it requirsr really high pressure to run properly.
 
At this point, you need to have a scanner that can read real time values plugged in so it can record wtf is going on the next time it happens.
 
HillbillyDeluxe said:
At this point, you need to have a scanner that can read real time values plugged in so it can record wtf is going on the next time it happens.
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He said he has hptuners which can be used as a really good scanner.
It doesn't sound like he has measured fuel pressure yet.
My first bet is fuel pressure, second bet is MAF.
 
larboc said:
He said he has hptuners which can be used as a really good scanner.
It doesn't sound like he has measured fuel pressure yet.
My first bet is fuel pressure, second bet is MAF.
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Fuel pressure on HPTuners never changes, made me think it wasnt reading right. As I recall it stays at 399kPa engine running, WOT, idle, or engine off key on.
Edit- Did a little searching and there is no fuel pressure sensor on this, so the reading is junk, Ill have to setup a manual gauge and rig it to the wiper or something. I drove it a bit today with HPtuners hooked up and never had an issue.
 
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Well this fucking **** of a car keeps giving me issues.
Ive replaced the fuel pump again, and the throttle body. We rarely drive it on shorter trips, mostly longer ones. Ive made my wife drive it around town for the last few weeks with no issues, been running great.
Took it up to the mountains today and it started acting up the first time we hit traffic, went from 50-5-0 and it stalled out. Pulled off to the side of the road and tried to start it a few times without sucess. So I popped the hood and put my fuel pressure gauge on it, I just bled it down to 0 to get a new reading and had my wife kick the key on, rather than just turning it to on she went to start and it started immediately and had 55-60psi.
Ran it in traffic with no issues for about 30 minutes.
Once traffic broke it ran great, but as soon as we hit traffic again and had to come to a stop it started stalling out again, popped it into N and revved the engine above 1000rpm and it ran fine, if I let the throttle go below 600rpm it would try to die.
Really seems like vapor lock, every time it has died it will restart after sitting a short while, or if you bleed the fuel rail.
I popped the gas cap off and that didnt seem to make any difference, however I think there is a check valve in the filler neck, that would only make sense if the tank was being pressurized though.
Could the tank be pressurizing to a point that it cant return fuel, which would kill it at idle?
Scan tool doesnt show anything out of the ordinary while cranking, just seems like no fuel is coming from the injectors.
 
What kind of pump are you replacing it with? How does the plug on top of the tank look? No melted pins or burnt wires?
 
Bend a coat hanger in a hook shape so it cant fall in, fish it down the filler neck, leave the cap off and continue driving. This would rule out vapor lock
 
I am pretty certain its vapor lock, there is something with the 07-08 Yukons with the 6.2 that have this issue from my searching. Looks like I need to wrap the fuel lines to insulate them.
I have had my 2007 Denali 6.2L since new and never had a problem until last summer. It developed the same problems as others are having. When coming to a stop after towing in hot conditions, it would die and not restart for about 30 minutes. It was very hard to diagnose because it was impossible to duplicate the problem at a shop. My mechanic hooked up his Snap On Scan tool with live data, a fuel pressure gauge and I hooked up my trailer on a hot day and headed up the pass. When I pulled to the side it failed and would crank but not restart. I had good fuel pressure, crank signal, coil signal, injector pulse. Basically everything it needs to run so I was stumped. I got out to disconnect the fuel pressure gauge and this is when I discovered the problem almost by accident, boiling hot aerated gas came shooting out of the bleed off hose. I turned the key to on and bled fuel a few seconds until no bubbles appeared and the engine fired immediately. I then headed back up the pass and duplicated the failure 3 more times, each time starting up after bleeding fuel. I cured the problem by wrapping the fuel rail with insulation and also wrapped the fuel lines where they get close to the transmission and up the back of the engine leadin to the fuel rail. it has never failed since even towing on days over 102 degrees.
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Baconator said:
Well this fucking **** of a car keeps giving me issues.
Ive replaced the fuel pump again, and the throttle body. We rarely drive it on shorter trips, mostly longer ones. Ive made my wife drive it around town for the last few weeks with no issues, been running great.
Took it up to the mountains today and it started acting up the first time we hit traffic, went from 50-5-0 and it stalled out. Pulled off to the side of the road and tried to start it a few times without sucess. So I popped the hood and put my fuel pressure gauge on it, I just bled it down to 0 to get a new reading and had my wife kick the key on, rather than just turning it to on she went to start and it started immediately and had 55-60psi.
Ran it in traffic with no issues for about 30 minutes.
Once traffic broke it ran great, but as soon as we hit traffic again and had to come to a stop it started stalling out again, popped it into N and revved the engine above 1000rpm and it ran fine, if I let the throttle go below 600rpm it would try to die.
Really seems like vapor lock, every time it has died it will restart after sitting a short while, or if you bleed the fuel rail.
I popped the gas cap off and that didnt seem to make any difference, however I think there is a check valve in the filler neck, that would only make sense if the tank was being pressurized though.
Could the tank be pressurizing to a point that it cant return fuel, which would kill it at idle?
Scan tool doesnt show anything out of the ordinary while cranking, just seems like no fuel is coming from the injectors.
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It has a High pressure terminal fuel system so don’t Worry about vapor lock or fuel return issues.
 
We drove it back down the mountain today and had several issues, I was able to keep it running by popping it in N whenever we came to a stop and keeping the engine revved above 1000rpm.
Pulled in the garage and got it to act up, while it was trying to idle and surging all over I hooked up the fuel pressure gauge and had 55psi, I bled a little fuel out and it seemed really full of air, but it was tough to be certain about. After about 30 seconds of it really sputtering it died out and would not restart. I bled the rail a few times down to 0psi and let it charge back up to 55psi and it would not restart.
Grabbed HP Tuners and hooked up to it and I noticed that the voltage was getting pretty low, like 9v while cranking. After sitting for 45 minutes it finally restarted, I went and drove it around and noticed that the voltage fluctuates between 14-12.9v, pulled up to a stop sign and it started to act up a bit and the voltage was down around 12.9-13v, revved it up to keep it alive and the voltage stayed around 13 volts.
I am suspecting the alternator being bad now, but I am not sure how that would effect idle only.
 
Baconator said:
We drove it back down the mountain today and had several issues, I was able to keep it running by popping it in N whenever we came to a stop and keeping the engine revved above 1000rpm.
Pulled in the garage and got it to act up, while it was trying to idle and surging all over I hooked up the fuel pressure gauge and had 55psi, I bled a little fuel out and it seemed really full of air, but it was tough to be certain about. After about 30 seconds of it really sputtering it died out and would not restart. I bled the rail a few times down to 0psi and let it charge back up to 55psi and it would not restart.
Grabbed HP Tuners and hooked up to it and I noticed that the voltage was getting pretty low, like 9v while cranking. After sitting for 45 minutes it finally restarted, I went and drove it around and noticed that the voltage fluctuates between 14-12.9v, pulled up to a stop sign and it started to act up a bit and the voltage was down around 12.9-13v, revved it up to keep it alive and the voltage stayed around 13 volts.
I am suspecting the alternator being bad now, but I am not sure how that would effect idle only.
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My alternator in my semi died last week. Truck literally ran perfect up till the minute a bunch of random lights came on, on the dash. When I shut it off, it was completely dead. When the mechanic took it off, it couldn't be turned. It had been putting a drag on the engine for a few weeks, as I noticed the gauge was reading lower than normal, but it was charging enough to keep the batteries up enough to power the electronics.

Point is, a faulty charging system can cause all kinds of issues, some that dont make sense.
 
I replaced the alternator, and battery ground cable and loosened and cleaned the grounds that I could find on the engine.
Still does it, only when hot, voltage drops to 12.9-13.1V and stumbles and dies at idle.
No idea where to go from here, this damn thing has left us stranded so many times now Ive lost count.
 
Baconator said:
I replaced the alternator, and battery ground cable and loosened and cleaned the grounds that I could find on the engine.
Still does it, only when hot, voltage drops to 12.9-13.1V and stumbles and dies at idle.
No idea where to go from here, this damn thing has left us stranded so many times now Ive lost count.
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My dad had similar stumbling issues long term that eventually ended up getting solved by the starter finally giving up the ghost. Once the starter was replaced, all issues were gone, stumbling and randomly not starting at times. Our only guess is the starter had some sort of weird short.

This was an 08 Ford V10 though so ymmv.
 
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