2009 GMX Yukon XL Denali 6.2L P0305 - What else to do?

notnomis · July 17, 2016

Hi All. I could really use the forum's help.

My (wife's) 2009 GMC Yukon XL Denali, with 162,000 miles on its 6.2L engine, started running erratic yesterday morning and had the default 'Service Stabilitrac' message on the dash. I connected my scanner and found a P0305 code, which of course means misfire on #5. I've been around GM V8s all my life, so I know #5 is the third one back on the driver's side. I swapped out components with #3, one at a time, clearing the code each time. I swapped the plug wire, spark plug, coil pack, and the injector, but the result remained the same. With each test, if I JUST let the engine idle, I get a P0300, but if I clear that code, then rev it to 1500-2000, the code pops up as P0305.

I used a timing light to see if the coil is getting a signal, and the timing light does trigger, but only intermittently. I will test using an actual spark plug tester tomorrow.

I have read that these engines can have valve trouble, eliminating compression to the cylinder, causing the misfire, and I will be doing a compression test tomorrow to see if that (or perhaps the lifter) could be the issue.

Is there anything else I should be checking? Should I be looking at doing a buzz test on the injector (and how would i do that on this engine)? What other tests can/should I do without throwing parts at it, like MAF, TPS, EGR, Cam sensor, Crank sensor, etc?

We have owned this truck for about 3 weeks, and I really don't want to have to make payments for such a pretty decoration on the driveway. We bought it with no CEL indicated, no blue smoke in the exhaust, and the engine running smoothly.

Thanks for any insights you all can provide!

notnomis · July 19, 2016

Before sending it to a dealership for an official diagnostic, I decided to open up the drivers side valve cover. I thought maybe a broken spring or rocker arm might be evident, and the valve cover is pretty easy to remove. So there's no broken spring, all looks right - no debris. Even the oil looks clean. I went ahead and turned the engine manually, using a breaker bar on the crankshaft. I thought if there was a broken lifter, the push rods won't move the valves. As I turned, I saw both the intake and the exhaust valves move evenly, and the same distance as the other valves.

Does that mean that there's no issue with the lifter either?

Duelin I just saw your post. That's a good idea. I will test the other cylinders for compression, because the issue might be with the inept shade-tree mechanic trying to do what the pros do. If the other cylinders test at 30 also, then the issue is with the way I tested it, not with the compression. If the other cylinders are higher, then off to the dealership it goes, along with my right arm and leg.

My old crappy boat had between 90 and 100 on each cylinder. I am sure this Yukon will have that or more.

Daniel Ghormley · July 19, 2016

I had a similar problem, the brand new spark plugs I put in had cracked at the insulator transition to the thread area and wasn't getting good signal out always. If you can get your hands on a computer u can see the misfire cycles as well. I know you did a through systematic parts swap, just sharing my expierence.

notnomis · July 26, 2016

I thought I would log back in and wrap this thing up. The other cylinders were up over 100psi, but #5 was at 30. So it has been towed to the dealership to get a new long block 6.2. In the end, I will have paid an extra $6,000 for this truck, but it will have 0 miles on the engine. I will probably have to start ubering with it to save up enough for the transmission that will inevitably fail.

I am really going to try to take it easy on once I get it back. That transmission has to last a while, given how poor I am now.

Duelin · July 17, 2016

Check the spark with a spark tester. Have you done a compression test? Also is the computer triggering that injector. Also check the intake for vac leak.

notnomis · July 17, 2016

I have an update on the additional tests. I checked the spark with a real spark tester, and that's fine. Then came the compression test. If I did it right, it peaked at about 30 PSI. I can't seem to find what 10.4:1 translates to in PSI, but I am sure that it isn't near 30. I didn't check the others.

Based on this, it seems like I will need to open up the head to see if it is lifters or valves, correct? I believe I saw a bulletin that required technicians to replace the entire block rather than fixing the lifter due to the debris that scatters inside the motor.

I have a labor-only warranty on this thing for up to $2500 if the issue is with internally lubricated parts. It looks like there's a good chance that I will be using that.

Big ol fail, GM.