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MIS-FIRE MYSTERY! Please Help!

by potato69 on Sep 12 2017, 3:51 PM

Ok so I have a 2012 GMC Sierra 4.8L and is missing on cylinder #7. I installed new spark plugs, new spark plug wires. Confirmed spark and also swapped coil #7 with known good coil to confirm its not the coil. Confirmed injector receiving signal/power and also swapped injector #7 with other known good injector. Checked compression and compression is good throughout that bank. Its continuing to miss and it literally has me stumped.

Any Ideas on this? If I can note one more thing is whenever you rev the engine to 2000 RPM on idle the engine makes this noise as if something is loose moving freely within the engine kind of like when a differential messes up and theres loose chunks of metal moving around the gears. Any input would be appriciated. I'm about to open the valve cover and see if it might be a rocker arm or valve.

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14 Replies

By truckman Oct 16 2018, 11:52 PM

Any word?

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By zman1992 Sep 18 2018, 1:48 PM

Pull valve cover and watch the valve and make sure then it is being open .The lifters on 7 if it has a afm will die.

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By 1986chevyman Sep 18 2018, 1:05 PM

Hook up a vacuum gauge and raise the engine to 2000 RPM, If the needle fluctuates (not steady) you have a valve train issue. Pull the valve cover and look for a broken valve spring or a flat lobe on the camshaft.

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By spark derk Sep 18 2018, 11:29 AM

Take a good look at oil pressure and monitor the AFM to see if the misfire is happening then.

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By strmdrgn Nov 05 2017, 10:16 PM

I would remove the valve cover and check the rocker arms. If they are in good shape, it is possible you might have a bad lifter or even the camshaft has deteriorated and coming apart

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By 395flash Oct 19 2017, 3:03 PM

Have you put a vacuum gauge on it? Easy way to eliminate a valve sealing issue.

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By dillonthemechanic Oct 18 2017, 8:44 PM

I would get customer info if there's something the customer may have done to cause the misfire aka redlining the engine causing obvious engine issue. If not, we've ran into problems with misfires due to carbon, I'd do what you're doing and remove valve cover, check for wear and see where that takes you. You can check fuel trims but typically a vacuum leak would affect more than one cylinder.

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By ketteler88 Sep 21 2017, 10:05 PM

If you have a viewer scope, stick it in the spark plug hole and make sure the valves open and close tight. Have someone turn the motor over by hand. Also are you sure its cly7, misfire counters can lie. I think they go off of a counter wheel on the crank on this engine. (half way point of the motor.) I have seen bad crank sensors and even a brake in the crank at the counter wheel.

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By vulctech1 Sep 20 2017, 6:33 PM

had multiple vehicles now with broken valve springs , same complaints

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By rtech-815 Sep 19 2017, 8:46 PM

I assume this is a displacement on demand engine. If so, you should review the following PIP.. I have seen instances where a failaed lifter will set both a P300 and a code for that cylinder...

#PIP4568S: TICK NOISE AND/OR MISFIRES ON AFM CYLINDERS 1 4 6 AND/OR 7 (MAR 16, 2016)

Condition/Concern

Some customers may comment on an SES light, engine misfire on cylinder 1, 4, 6, or 7 and/or tick noise.

This may be the result of an AFM lifter that unlocks as soon as the engine is started or one that is mechanically collapsed/stuck all of the time.

If an AFM lifter unlocks as soon as the engine is started, low compression will be found on that cylinder during an AFM compression test, along with a SES light, DTC P0300, and engine misfires on the related cylinder but it is unlikely that any noise will be experienced.

If an AFM lifter is mechanically collapsed/stuck, low compression will be found on that cylinder during an AFM compression test, along with a consistent valve train tick noise, SES light, DTC P0300 and engine misfires on the related cylinder.

These lifter concerns may be the result of internal locking pin damage, which may occur if the response time of an AFM lifter unlocking event is decreased due to low oil pressure, oil aeration, internal engine sludge, or an internal concern with an AFM lifter, VLOM, plastic lifter guide, lifter bore, and/or cam lobe wear.

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