Misfire on cylinder four?

Tiny
BHAMDOC1973
  • MEMBER
  • 2014 AUDI A5
  • 4 CYL
  • TURBO
  • 4WD
  • AUTOMATIC
  • 105,000 MILES
Did a smoke test nothing, changed plugs and coils as it was misfiring on all, now just cylinder 4 is. Compression is zero, all the rest are 120 PSI. Top of all cylinders look the same with scope, cylinder 4 a bit more wet. What are the chances a valve is bent rather than a piston ring issue? Starts from the first time, does not smoke or overheats. No other codes.
Sunday, April 2nd, 2023 AT 3:37 PM

5 Replies

Tiny
JACOBANDNICKOLAS
  • MECHANIC
  • 110,071 POSTS
Hi,

If you have 0 psi compression in that cylinder, either you have a hole in the top of the piston or a valve is stuck open. Even if the rings were bad, you would have some compression.

What I recommend is this. See if you can get your hands on a borescope. You can use this by placing the camera down the spark plug hole to see what is going on. Other than this, you will need to remove the cylinder head.

Let me know if that is something you can do or if we need to remove the cylinder head.

Take care,

Joe
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Sunday, April 2nd, 2023 AT 10:10 PM
Tiny
BHAMDOC1973
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Right, thanks. I have already looked with my camera but did not notice anything different than the other ones other than a bit more fuel, with all the gunk on these cylinders it is hard to see anything else, but is there a trick to find out if a valve is bent before having to remove the head?
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Monday, April 3rd, 2023 AT 6:16 AM
Tiny
JACOBANDNICKOLAS
  • MECHANIC
  • 110,071 POSTS
Hi,

If you remove the valve cover, check the cylinder in question and pay attention to see if any of the valves are not closing. If you have one stuck open and it's bent, the rocker lever will not stay in contact with the camshaft lobe. See pic below.

Joe

See pic below.
Image (Click to make bigger)
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Monday, April 3rd, 2023 AT 8:21 PM
Tiny
BHAMDOC1973
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Okay, thanks. Why do some bend valves if they timing is still correct, just in general?
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Tuesday, April 4th, 2023 AT 6:25 AM
Tiny
JACOBANDNICKOLAS
  • MECHANIC
  • 110,071 POSTS
Hi,

If the timing is correct, this isn't something that would normally happen unless the engine was being revved extremely high.

Let me know.

Joe
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Tuesday, April 4th, 2023 AT 7:21 PM

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