Unusual misfire situation...

[jarring]

Hello guys, I am having a misfire on cylinder 5 that I can't seem to get away from at all.

The P0305 code is not making a light on the gauge (it's pending) but I can still scan for it and slightly feel it at idle.

Vehicle: 2014 Chevy Suburban (Vin "0").

What I tried:

• Moving the spark plug to a different cylinder (didn't do anything)
• Moving coil and boot to a different cylinder and replacing said coil/boot (didn't do anything)
• Replacing fuel injector (didn't do anything)
• Compression test (good compression)
• Head gasket leak test (no leak)

Other notes:
I replaced the head gasket some months ago, and one of the top small head bolts snapped due to a faulty torque wrench... I have yet to deal with it as my compression/head gasket are seemingly fine.

I pulled the spark plugs from cylinder 1 (left) and cylinder 5 (right). I did notice a small amount of oil on the last couple of threads of the cylinder 5 plug, could that mean something?

Took the valve cover off and inspected the push rods, something is definitely weird with the cylinder 5 intake push rod. It looks like it has this orange scratchy appearance on the end that is closest to the camshaft. I haven't been able to find anything about this online at all, no idea what this is. Anyone ever seen something like this?

Images push rod order (left to right): Intake 1, exhaust 1, intake 3, exhaust 3, intake 5, exhaust 5.

Another angle of the same ends:

 

[Number_35]

@jarring, welcome!

It sounds like you've looked at all the obvious stuff, and done some good troubleshooting. This is a long shot, but I would roll the pushrods on a piece of glass. Clicking would indicate a bent pushrod.

What engine? LS 5.3 or 6.0?

I presume the intake and exhaust pushrods are the same length, unlike some of the older Chevy V6 engines.

 

[jarring]

@jarring, welcome!

It sounds like you've looked at all the obvious stuff, and done some good troubleshooting. This is a long shot, but I would roll the pushrods on a piece of glass. Clicking would indicate a bent pushrod.

What engine? LS 5.3 or 6.0?

I presume the intake and exhaust pushrods are the same length, unlike some of the older Chevy V6 engines.

Hello, thanks for having me. Yeah a very slight bend may well be problematic, if you're right about that and it's not a worn cam lobe or lifter, that'd save me a lot of trouble. Will try the glass rolling.

It's off a 5.3, they're all supposed to be the same length yeah.

 

[mattd]

Check your dynamic compression I’ll bet you’ll find your answer.

 

[clinebarger]

I second a dynamic compression test.

Is there any tapping noise anywhere in the RPM range?

Gen IV engines aren't known to bend pushrods like Gen V's are, Missing one of the M8 bolts on the cylinder head will at most cause a oil leak.

Why was the cylinder head removed?

 

[jarring]

Is there any tapping noise anywhere in the RPM range?

There is no noise to indicate anything wrong. The only way I could know it was misfiring is from closely watching the RPM gauge and feeling the vibration when sitting in the car, and of course the pending code. It's a slight misfire for sure.

I can't feel/notice it at all outside of idle situation.

Why was the cylinder head removed?

Long story short, the intake broke and sent stuff into the cylinders. I had to take the heads off to clean out the junk from the cylinders and valves. New OEM MLS head gaskets used and the head surface was professionally resurfaced at the machine shop.

Check your dynamic compression I’ll bet you’ll find your answer.

I second a dynamic compression test.

I will try that and report back. Thank you for the guidance.

 

[jarring]

Dynamic compression test resulted in:

Cylinder 1: ~85 PSI
Cylinder 2: ~90 PSI
Cylinder 3: ~88 PSI
Cylinder 4: ~90 PSI
Cylinder 5: ~90 PSI
Cylinder 6: ~88 PSI

Didn't bother with 7 or 8.

Normal, right? Cylinder 1 being at 85 confused me at first but I didn't think much of it as it's not having any issues.

 

[mattd]

Did you run it with the valve core removed from the tool?

 

[Trav]

Dynamic compression test resulted in:

Cylinder 1: ~85 PSI
Cylinder 2: ~90 PSI
Cylinder 3: ~88 PSI
Cylinder 4: ~90 PSI
Cylinder 5: ~90 PSI
Cylinder 6: ~88 PSI

Didn't bother with 7 or 8.

Normal, right? Cylinder 1 being at 85 confused me at first but I didn't think much of it as it's not having any issues.

Did you have the throttle wide open?

 

[zzyzzx]

Is that rust on the push rod? I'm wondering if there is a coolant issue as well.

 

[mattd]

Did you have the throttle wide open?

Can’t with a dynamic compression test, engine is running.

 

[jarring]

Did you run it with the valve core removed from the tool?

No, is that the proper way to do it?

All I did was unplug the coil & injector, take the plug out, put the compression gauge in, fire it up, clear the gauge a couple of times then recorded the PSI number.

Did you have the throttle wide open?

It was at idle.

Is that rust on the push rod? I'm wondering if there is a coolant issue as well.

I do not think it is rust as it wipes off easily, but you may be right as I have noticed a small coolant leak. Could that snapped head bolt be letting water in there?

I'll take a closer look today and probably deal with that small snapped head bolt already.

 

[mattd]

No, is that the proper way to do it?

Yes may times you’ll get skewed readings if you have the core in. I would re do the test

You have to watch the gauge closely.

 

[zzyzzx]

Could that snapped head bolt be letting water in there?

YES.

 

[willbur]

Dynamic compression test resulted in:

Cylinder 1: ~85 PSI
Cylinder 2: ~90 PSI
Cylinder 3: ~88 PSI
Cylinder 4: ~90 PSI
Cylinder 5: ~90 PSI
Cylinder 6: ~88 PSI

Didn't bother with 7 or 8.

Normal, right? Cylinder 1 being at 85 confused me at first but I didn't think much of it as it's not having any issues.

They look fine to me. Dynamic vs static is typically much lower- I'd guess dynamic is maybe 40-50% of static. Ignition and fuel appear normal from your trouble shooting so it's probably something mechanical in the engine. You did not mention what the pending code was. I'd start there

 

[willbur]

Sorry about the pending code. You did start there...

If you're doing dynamic compression already, just do a snap test, too

 

[jarring]

YES.

Well, I tried to take it out today with an extractor with the head/intake still on.

I'm not sure if my technique is wrong or what, but that bolt ain't going nowhere. Might have to just bite the bullet and take the head off to see what I can do with it. Just sucks because I just put brand new TTY bolts and gasket in that I'll have to replace I guess.

 

[Trav]

Can’t with a dynamic compression test, engine is running.

Yes, I confused dynamic and static.

 

[clinebarger]

Well, I tried to take it out today with an extractor with the head/intake still on.

I'm not sure if my technique is wrong or what, but that bolt ain't going nowhere. Might have to just bite the bullet and take the head off to see what I can do with it. Just sucks because I just put brand new TTY bolts and gasket in that I'll have to replace I guess.

A missing/broke M8 bolt will not cause a issue other than a oil leak worse case scenario......I even doubt that as Gen V engines don't have the top row of M8 bolts.

The 10 M11 bolts are what clamps the head down!

 

[Starving Electrician]

What I have noticed is you have done very little electrical troubleshooting. Weak spark can be caused by a high resistance connection in your wiring. I dont know the operating principles exactly for your coils but it at minimum must have power and a ground connection. I would check if you are getting good power under a load. Get a 12volt light that runs at a wattage similar to your coil. Hook it to a good cylinder see how bright it looks hook it to the problem cylinder and see if it's noticeably dimmer. That would verify the integrity of your wiring.