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Discussion Starter · #1 ·
Hi,

I like to think I'm pretty good with engines, but this one has me scratching my head. This is an 09 V sedan. Mild mods. Just recently, Dash Boss started reporting that the engine was knocking under VERY light throttle. Not at WOT ... just at very light throttle ... enough to keep a steady speed and just cruising at 30 mph (and other slower or faster speeds). So more scanning and found that cyl 5 is misfire continuously. So I pulled the plug and it looked fine except it was dirty black and fouled looking ...

1. Put a new gapped plug in. Same problem on 5.
2. Swapped coils with another cylinder. Same problem on 5.
3. Swapped injectors with another cylinder. Same problem on 5.
4. Performed injector balance test from Tech 2 ... injector performed perfectly.
5. Pulled plug, reconnected it to the coil, set it on metal and turned the motor ... nice big heavy visible spark.
6. Pulled valve cover and turned the motor ... valves fully actuate up and down.
7. Pulled and Inspected rocker arms, push rods, and valve springs. No issues found.
8. Ran a compression test ... 150 psi in 4 valve cycles.
9. Ran a leak down test. Held 100% of the pressure ... no loss.

Everything I've checked seems to be perfect, but yet cyl 5 continuously misfires on every firing cycle.

Anyone see this kinda thing before with everything checking out like it does?

Any suggestions??

Bill
 

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is it knocking (retarding timing) or mis fireing ? can you feel the misfire or just the report from the logs?
any oil consumption or notice of smoke? valve seat/seal could cause this if enough oil was getting by
guessing you swapped wire with the coil as well ?
 

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Hi,

I like to think I'm pretty good with engines, but this one has me scratching my head. This is an 09 V sedan. Mild mods. Just recently, Dash Boss started reporting that the engine was knocking under VERY light throttle. Not at WOT ... just at very light throttle ... enough to keep a steady speed and just cruising at 30 mph (and other slower or faster speeds). So more scanning and found that cyl 5 is misfire continuously. So I pulled the plug and it looked fine except it was dirty black and fouled looking ...

1. Put a new gapped plug in. Same problem on 5.
2. Swapped coils with another cylinder. Same problem on 5.
3. Swapped injectors with another cylinder. Same problem on 5.
4. Performed injector balance test from Tech 2 ... injector performed perfectly.
5. Pulled plug, reconnected it to the coil, set it on metal and turned the motor ... nice big heavy visible spark.
6. Pulled valve cover and turned the motor ... valves fully actuate up and down.
7. Pulled and Inspected rocker arms, push rods, and valve springs. No issues found.
8. Ran a compression test ... 150 psi in 4 valve cycles.
9. Ran a leak down test. Held 100% of the pressure ... no loss.

Everything I've checked seems to be perfect, but yet cyl 5 continuously misfires on every firing cycle.

Anyone see this kinda thing before with everything checking out like it does?

Any suggestions??

Bill
Swap plug wires?
 

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If you can actually feel the misfire, use an infrared thermometer on the primaries to see if any are cold, pay attention to cylinders 6 and 4 as they're on either side of 5 in the firing order. Computers aren't always perfect.
 

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How many misfire counts is cyl5 getting up to? The fact you are not setting a p0305 for cyl5 misfire I assume its small counts which is norma. What are your mods? Do you have header?
 

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Discussion Starter · #7 ·
Thanks for all of the responses. All components got swapped. No change. It is indeed knocking on cyl 5 BUT only at very very light throttle inputs like enough to keep you moving at 30mph, or enough to lightly accelerate from a stop to city speeds gently. At steady cruise it knocks ... 1- 2 degrees. At the very gentle acceleration I'll see up to 6 degrees. At WOT there is no knock. No abnormal oil consumption that I've noticed. I have not tried the laser thermometer yet ... good idea !

BIll
 

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if it's missing or knocking u should see a noticeable spiky dip in the 02 feedback when it happens ? also what timing is used at that point as sometimes too low timing can cause those issues at low load and bump it up couple deg can fix it, when u looked at valve springs was the stem seal still in place as they can pop up and then allow oil into the cylinder ?
 

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I have had strange misfire issues with cars I have owned when the spark plug I was using was too cold for my application. The misfire was always noticeable at part throttle. The solution to this may be as simple as moving up one heat range in plugs.

I alway try the simple stuff first, although changing plugs in a CTS-V is no picnic.
 

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Discussion Starter · #11 ·
Update

So as it turns out, I made a mistake when I ran the leak down test. I had run the compression test first and left the compression test hose in the spark plug hole and just disconnected it from the compression tester and plugged it into the leak down tester. Turns out the compression tester has a back flow valve built into it so that it captures the air going in and doesn't let any out. The leak down is just a pass-thru hose.

So I reran the leak down test. Put 100 psi in, and had 72 at the cylinder so I was losing nearly 30% someplace. I could hear it coming out. Turns out it was not coming out the intake port or the exhaust port, but it was coming out the PCV on both heads and the oil fill, so the crankcase was getting pressurized.

Pulled the oil pan off and pulled the piston out. Top piston ring was missing moly coat in the middle of the ring between the edges. It was missing it like "slots" around the length of the outside edge of the ring.

Ordered new rings for that piston, new bolts and new gaskets and such to build the motor back up.

We'll see if that takes care of it.

Bill
 
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