Throwing parts at an ’09 denali xl 6.2 gas

[Bob S]

Vehicle has 125k miles. Purchased new and has been a great vehicle for 10 years. Engine starting missing a couple of weeks ago. No related valve train noise so I figure it’s spark or fuel related and since the plugs have never been changed it’s going to get plugs and wires first. OEM parts from rock auto…still missing. Personal friend shop foreman for a large GMC dealership (where I purchased from) scans it and tells me it’s cyl7. Rockauto OEM coil…still missing. Dealership OEM injector…still missing. Confirmed #7 spark with a timing light and with an old plug and wire connected to the coil and grounded, engine running, light and plug both show good spark. Injector circuit tests good. Can I feel the injector pulsing? Not sure but the signal wire shows good on a test light. Compression test, non-running, 165psi. Compression test running, 165 initially then 0psi after engine starts missing and I release initial pressure. I should say that the engine usually starts and runs well at least for a few seconds before it begins missing. At this point I think it has to be DOD/AFM. Pull the valve cover and do an engine running valve check…ALL VALVES APPEAR TO BE WORKING! Valve train, even without the cover on is quiet as can be. Albeit, there is a strange noise which I think may be unburned F/A mixtures lighting off in the exhaust system and making a popping sound?

Do I throw an ECM at it??????

Regards,
Bob S

[KamiWolf93]

Your compression looks healthy at 165psi. I’m not sure I’ve understood what you mean when you say it’s dropped to 0psi, is this measured with a sensor? Usually a mechanical guage is used to check compression. If the cylinder that’s misfiring has had a new injector and coil it’s safe to assume it isn’t related to those. Could it be there’s a fault in thr wiring for the coil on cyl no 7? It would probably be a good idea to do a compression test again dry and wet. (Wet is with some oil in the cylinder). If the compression increases then you don’t have an issue with your valves. Another thing is have you checked the condition of the spark plug or plugs? At 125k theyre probably due a replacement.

[Schrodinger’s Box]

If you move the plug to a different cylinder does the trouble ‘follow the plug’ or stay with the same #7 cylinder???

If the trouble stays with #7 regardless of which plug you swap – and I’m assuming its Coil on Plug – what is the pull down current on the #7 COP, is it enough to fire the coil/plug for #7 ?????

[Bob S]

This morning I repeated the running compression test on #7 with the same results. 165psi on startup, disconnect then reconnect the gauge… 0 psi. Moved the test to the #3 cyl (non DOD cyl), good compression at all times.

I’m ordering a DOD delete from Scoggins Dickey.

Someone tell me I’m doing the right thing!! …or not!

Bob S

[KamiWolf93]

So from what I’ve read about the DOD system it shuts cylinders down automatically and will switch to v8 mode after 10 minutes. What do the other DOD cylinders show for running compression? Just to determine if this is normal behaviour for the engine. If it’s the same then your issue likely isn’t a mechanical one. Another thought, you could try swapping fuel injectors with another cylinder. It may be those DOD cylinders don’t actually have compression when they’re deactivated.

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