2008 Yukon Denali vey rough misfire out of nowhere

Drew1987gn · Oct 19, 2024

Hi to all. New to this forum and looking for some guidance. I have a 2008 Yukon Denali. Running fine then within a minute while drivIng it went from a little rough to an extremely bad misfire. Really popping through the exhaust. I had to pull into a parking lot and shut her off. Barely made it into the parking lot! The check engine light did not come on which is throwing me off as to where to start troubleshooting. I would think the check engine light would have came on if it was a bad coil, clogged cat, maf but nothing. Any ideas? Should I put my scanner on it anyway? It would be greatly appreciated. Thank you!

Miami-Dade · Oct 19, 2024

@Drew1987gn Moved you here.
Welcome from Miami Beach!

wjburken · Oct 19, 2024

Your 2008 shouldn’t have AFM so it shouldn’t be a collapsed lifter.

I have had that happen exactly twice in our Yukon Denali’s. Both times, it was a broken valve spring. It takes about 45 minutes to pull your valve covers to see if you have a broken valve spring.

Just had to replace one on our 2013. Also had to do it on our 2007 when we had it.

Joseph Garcia · Oct 20, 2024

Welcome to the Forum from NH.

Lots of knowledgeable folks here who freely share their knowledge, experiences, and perspectives. Knowledge is power.

I hope that you will become a participating member in the Forum's discussions.

Pics of the truck, please.

You are already receiving sage advice from the knowledgeable folks on this Forum.

opfor2 · Oct 20, 2024

Are you using E85 fuel?

Drew1987gn · Oct 21, 2024

Good morning! Even though my Yukon was misfiring and no check engine light on, I did drive home the next morning (about 2 miles). When I got home and backing up, the check engine light finally came on and was blinking. I shut her off and put my scanner on. 3 codes showed up. P0300 (misfire), P0151 (O2 sensor bank 2, sensor 1) and P0174 (Too lean bank 2). I'm thinking a bad injector. I'm going to start troubleshooting there. See if the connections are good. I'll check the fuel pressure. Pull the plugs and take a look at them to see if the plugs to tell me which cylinder is misfiring. I use regular 85 octane.

B-train · Oct 21, 2024

85 octane is also an issue. These motors can run fine on 87, but definitely like premium or better due to the compression ratio.

If you have E85 available, you can make a blend of 25 to 30 percent of total capacity (I go by the fuel used counter on the DIC), and that will give you 93-97 octane. My 2008 Denali LOVES that blend. I get the same, or better, than 87 fuel mpg. It's less than premium non-ethanol, but the cost per mile is WAY less, so I don't really base it off mpg anymore. I just use the mpg to see where I'm at per tank of fuel.

mikez71 · Oct 21, 2024

Sounds like you need that bank 2 sensor 1. That might he enough to cause all those codes.?

I would replace that sensor first before getting into pulling plugs and fuel injectors.

If it was just 1 cylinder misfiring, you would have probably gotten a cylinder specific code I think...

Drew1987gn · Oct 21, 2024

My thought was that if I was having a misfire, it would cause the O2 sensor to be off. I do agree with you that it would be an easy step just to replace the O2 sensor and see what happens.

Fless · Oct 21, 2024

Before you do any work on it, know how much ethanol is in your fuel and compare that to what a scanner reads for alcohol content. If they don't match within a few percentage points, you may want to consider resetting the alcohol content (unless you're actually running E85).

2008 Yukon Denali vey rough misfire out of nowhere

Drew1987gn

TYF Newbie

Miami-Dade

wjburken

Joseph Garcia

opfor2

Full Access Member

Drew1987gn

TYF Newbie

B-train

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mikez71

Full Access Member

Drew1987gn

TYF Newbie

Fless

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