2007 Denali Rough Idle and Stall After Running Long

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General Stalin

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Have done a search and found a handful of threads for this similar issue but nothing all too conclusive on a solution, but I have a hunch and wondering if anyone else has had this issue and fixed it.

I have a high mileage 07 Denali (195k) and about 3 months ago at the tail end of a long day of driving it bogged down and stalled while on on the highway. I was able to pull it off the road and tried to restart but it would not stay running. Would fire up, idle rough for about 5 seconds, then stall. I thought it might be an oil pressure issue as the piston rings are worn and it burns a lot of oil. Called a buddy to bring me a couple quarts to see if that would get her running. After waiting for about 45 minutes my buddy showed up, I dumped some oil in it, and it it fired it up and ran fine. Drove it back home and hadn't experienced the problem again until a couple months later. Was driving back from an event and same thing - this time it stalled while in stop and go heavy traffic on the highway. Was able to limp it off the road and into a gas station. I put a bunch of oil in it again right away but that did not fix it this time. Still rough idle when starting then stalls soon after. Had it towed home and replaced the battery as it was old. Next day it started fine and had been running fine since the other day.

Had the same issue just a few days ago sitting in a parking lot letting it idle for a while. Was waiting for someone in a grocery store and wanted to have the AC on because it was hot so I was sitting with it running for a while. Started to bog down just sitting in the parking lot so I shut it off. Had the exact same issue... rough idle/stalling at low RPM didn't want to stay running. Let it sit overnight and next day drove it home fine.

Now here's the nuts and bolts:

It has two relevant codes - Fuel pump relay, and Catalytic Converter. The fuel relay I'm actually pretty sure is NOT the issue as this popped up only because I removed the relay when doing a wet and dry compression test on the motor. I bought a new fuel pump but have NOT installed it yet as I don't think that is the issue - as the problem is not consistent with a bad pump. Wouldn't make sense that the pump would only have issue when running for a long time, and would magically work after it sits for a while.

Here's my big question: could the cats be causing this?

As I mentioned my truck burns a LOT of oil. This has likely ruined the catalytic converters (which is why they are throwing codes) and since this rough idle/stalling issue only happens when the truck has been running for an extended period of time, I'm wondering if the cats are clogging up the exhaust when they get hot. Thoughts? Anyone else experience these same symptoms?
 

Jedig33k

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Have done a search and found a handful of threads for this similar issue but nothing all too conclusive on a solution, but I have a hunch and wondering if anyone else has had this issue and fixed it.

I have a high mileage 07 Denali (195k) and about 3 months ago at the tail end of a long day of driving it bogged down and stalled while on on the highway. I was able to pull it off the road and tried to restart but it would not stay running. Would fire up, idle rough for about 5 seconds, then stall. I thought it might be an oil pressure issue as the piston rings are worn and it burns a lot of oil. Called a buddy to bring me a couple quarts to see if that would get her running. After waiting for about 45 minutes my buddy showed up, I dumped some oil in it, and it it fired it up and ran fine. Drove it back home and hadn't experienced the problem again until a couple months later. Was driving back from an event and same thing - this time it stalled while in stop and go heavy traffic on the highway. Was able to limp it off the road and into a gas station. I put a bunch of oil in it again right away but that did not fix it this time. Still rough idle when starting then stalls soon after. Had it towed home and replaced the battery as it was old. Next day it started fine and had been running fine since the other day.

Had the same issue just a few days ago sitting in a parking lot letting it idle for a while. Was waiting for someone in a grocery store and wanted to have the AC on because it was hot so I was sitting with it running for a while. Started to bog down just sitting in the parking lot so I shut it off. Had the exact same issue... rough idle/stalling at low RPM didn't want to stay running. Let it sit overnight and next day drove it home fine.

Now here's the nuts and bolts:

It has two relevant codes - Fuel pump relay, and Catalytic Converter. The fuel relay I'm actually pretty sure is NOT the issue as this popped up only because I removed the relay when doing a wet and dry compression test on the motor. I bought a new fuel pump but have NOT installed it yet as I don't think that is the issue - as the problem is not consistent with a bad pump. Wouldn't make sense that the pump would only have issue when running for a long time, and would magically work after it sits for a while.

Here's my big question: could the cats be causing this?

As I mentioned my truck burns a LOT of oil. This has likely ruined the catalytic converters (which is why they are throwing codes) and since this rough idle/stalling issue only happens when the truck has been running for an extended period of time, I'm wondering if the cats are clogging up the exhaust when they get hot. Thoughts? Anyone else experience these same symptoms?

Nothing like that, but they should be able to test you CAT for free at a muffler shop. I had similar codes, but my stalls were when hauling a trailer in hot weather. Also, besides adding oil, have you changed the oil plus all the other fluids that need to be changed due to mileage?
 

Rocket Man

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At 195 k ifthose are the original cats I'm sure they're gone. And yes if they're plugged it will cause those issues and get worse the hotter they get. And if you shut it off and let it cool down they can open up again until they get hot. Have a shop check them.
 
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General Stalin

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Thanks for the tips and info guys, especially Rocket Man. I have an update, but first to answer questions:

Probably the cats Uncle Joe. What codes are you getting? How long does it take her to burn a quart of oil?

Tell-tale p0420 code. Banks 1 and 2. Had 6 of them sitting on my ECM. Also had a bank 1 lean code and some other irrelevant stuff (suspension compressor exhaust error).

I probably burn a quart every couple tanks of gas at this point.

Now onto the nitty gritty
---------------------------

I used an infared thermo to check the cats at operating temperature and they were about the same temperature on the inlet and outlet which was telling me they weren't doing their job so I made the decision to pursue replacing the cats. I returned the fuel pump and bought 2 new Magnaflow universal catalytic converts and planned to weld them onto my Y-pipe.

Long store short - this fixed the issue.

New cats are in the truck right now and the CEL is off and it seems to be running fine. In fact I went for a 50 mile drive today and back and it had zero issues. I did not replace any of my O2 sensors either.

----------------------------

Now the next issue is that the motor still burns oil, and if I don't address this in the near future then my new cats will surely get ruined too. Also sweats/seeps oil from someplace(s) as well that I can't seem to pinpoint. It's not the oilpan as I replaced the oilpan gasket last year and it seems fine there. I don't think it's the head gaskets or intake gasket as I have had zero heat issues or coolant loss/leakage. Maybe valve covers/cover gaskets? I'm pretty sure that the oil burning issue is due to worn out piston rings though, which will basically require an engine rebuilt/replacement. I did do dry/wet compression test and did not see an higher compression in the wet test which tells me I'm not getting a good oil seal around the pistons.
 
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General Stalin

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UPDATE::::

Good new and bad news.

Bad news is that replacing the cats apparently did NOT fix the problem despite the fact I have taken the truck on two 2+ hour-long drives since then with zero issues - today I drove about 120 miles roundtrip with no problems then parked my truck at my house for a few minutes before heading into my office. Get back in the truck and it started real ******. Long sputtery crank, but it fires up. Drove about 5 minutes and it died waiting at a red light. The exact same way it died in the past. Sputtery and bogging at low rpm then stall. Put it in park and sat until light turned green then fired it back up and drive back home. Parked in front of my house and it stalled as I was parking it.

Not sure what the problem is here anymore. No engine light came on. I haven't checked if it threw any codes yet but will run to oreilly and check in just a bit.

But now the good news:

Found a junkyard right in town that just got a truck in that has my same motor. Going to pull it tomorrow. Depending on what kind of shape it is in I might just clean it up a bit and swap it directly into my truck, but that is TBD. If it needs work I'll rebuild it then swap it in when ready. If it doesn't then I'll swap it in right away and rebuild my current motor.
 
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General Stalin

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Brought it to oreilly and code reader found zero codes. Also that junkyard motor was yanked before I could get to it. These 6.2's don't last long in pick yards.

So the only mildly good news is that after talking to a fellow gearhead buddy of mine he impressed upon me that my oil burning issue likely is not the piston rings but rather the cylinder heads. Probably losing oil through the valves. If that is the case I can just pull the top end of the motor apart and have the heads machined for WAY less work and money than replacing the motor.

This still likely doesn't explain this stalling issues though. I'm pretty stumped.

I replaced the air filter and cleaned up the TB with some CRC throttle body cleaner.
 

Rocket Man

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Brought it to oreilly and code reader found zero codes. Also that junkyard motor was yanked before I could get to it. These 6.2's don't last long in pick yards.

So the only mildly good news is that after talking to a fellow gearhead buddy of mine he impressed upon me that my oil burning issue likely is not the piston rings but rather the cylinder heads. Probably losing oil through the valves. If that is the case I can just pull the top end of the motor apart and have the heads machined for WAY less work and money than replacing the motor.

This still likely doesn't explain this stalling issues though. I'm pretty stumped.

I replaced the air filter and cleaned up the TB with some CRC throttle body cleaner.
How about cleaning the MAF? I had stalling issues before and it turned out to be a MAF that was failing so you could always try a new one.
 

kbuskill

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I would start by cleaning the MAF as @Rocket Man suggested to try and see if that helps your stalling issue... although I would investigate the possibility of your fuel rails getting super heated and causing a vapor lock situation. This seams to be a somewhat common issue with the 6.2L. The next time it stalls try bleeding some fuel off at the Schrader valve on the fuel rail and see if it makes any difference. If it does wrap your fuel rails and lines in a decent heat shield like the foil backed fiberglass wrap they use for starters.

As for the oil consumption...

I am confused about how you can lose oil through the valves. The only way oil, or anything else, could pass beyond the valves when they are closed would be if there was a leak... like a burnt valve, which would be obvious from the miss it would create.

The more likely culprit is gummed up piston rings caused by oil ingestion via the PCV system.

Once the engine starts ingesting oil through the intake from the PCV it only gets worse unless you stop the oil from getting into the intake and then do a good cleaning.

I would install a catch can and then do a good deep cleaning with water to decarbonize the combustion chambers and pistons and maybe even a good top end cleaning by soaking the pistons with Kreen.
 

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