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Discussion Starter · #1 ·
I posted a while back about having a dead cylinder in my 1999 cobra. The heads are 03 cobra heads. I had fuel, spark and air and checked compression to find 0psi on the third cylinder back on the driver side bank. I pulled the engine and found all valves to look in decent shape just looking it over quickly. The lifters look ok and the rockers are fine. Cam lobes are not flattened out anything either. The head gasket looks fine and the cylinder wall looks like new. When running the engine burned a good amount of oil too. I cannot figure this out. Any suggestions on what to look for? Thank You!
 

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King Trashmouth
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Let's think through it logically. There's three spots you can lose compression.

1.) Head. This is usually in the form of a damaged valve. You can check further by dribbling gas on the combustion chamber of that cylinder and seeing if it seeps down to the runners.

2.) Head gasket. Very difficult to test at this point. Generally if it was losing it, it would lose compression to the water jacket or crankcase. If it wasn't puking coolant or pushing out the dipstick it's probably not that. Since it was on one cylinder that also rules out a cylinder to cylinder failure.

3.) Bottom end/piston. This is another common failure point. In mild cases you'll see a scuffed down bore indicating the rings aren't sealing. In a moderate failure expect to see broken rings or ringlands. In a total failure, you'd likely see a big 'ol hole in the piston. Any of these can also result in burning a fair amount of oil as the rings no longer control it.
 

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Discussion Starter · #4 ·
I haven't pulled any pistons yet to check rings but the cylinder walls look like new, no scuffing or scratches to indicate ring or piston failure. It's possible the rings are the issue bit I'm leaning away from it. I was getting a lot of oil vapor blowing from my crankcase breather on the passenger valve covet which I corrected by reconnecting the pcv to the intake which I had previously bypassed (suggested by my tuner). The exhaust valves are black on that cylinder too, which I'm thinking was from the oil burning and or not firing correctly. What do you think the chances of it beginning a failed valve guide causing the valve to noir track straight?
 

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Discussion Starter · #5 ·
I should mention I built this motor about 20k miles and two years ago.
 

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King Trashmouth
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It's all a crap shoot until you test it. There were some valve guide issues, but seemed to be more prominent on earlier revisions, and only on cylinders 7+8.

I'm surprised though, because usually when you have 0psi the cause of it jumps out at you, whether it's a hole in a valve, bent valves, or a hole in a piston.
 

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Discussion Starter · #7 ·
They are early revision heads and it's the number 7 cylinder I'm having issues with.
 

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Discussion Starter · #8 ·
I took my head apart today. The cam lobes look fine and the roller followers look great. All the bearing surfaces look great too. One thing I did find was that I had some side to side movement on the valves in the head. I took the springs off on the bad cylinder and lowered the valve about a half inch, I could wiggle the valve back and forth by about 3/32". That is the #7 cylinder and had 0 compression before I pulled the engine. I should mention that the engine ticked loudly before the misfire started, to me it sounded like every lifter was tapping. The tapping was loudest when cold and went mostly away when revved and got better when out warmed up. I had great oil pressure at all times as well. I noticed when I pulled the lash adjusters that they had circular wear patterns on the bottoms that I could feel with my finger nail. I cannot collapse any of them either even though the engine hasn't ran in more than a week. I can confirm I was getting oil to the Heads and I'm very confident the timing set is fine and was installed properly.
 

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King Trashmouth
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The lash adjusters can be pretty tough to collapse.

That said, sounds like you found an issue, if not the issue. I'd count yourself lucky you found it out this soon, because if you let them go too long like that they tend to beat the crap out of the seats and either drop seats or break valves, and that would be really expensive.

Just out of curiosity, were you running a head cooling mod?
 

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Discussion Starter · #10 ·
I was not running the Head cooling mod. But I plan to when it goes back together. Is it possible the lifters being tight could be what was ticking? I'm not really that familiar with dohc motors. Is the valve movement abnormal?
 

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Discussion Starter · #11 ·
On the #7 cylinder I found that one of the lash adjusters looked like the ring crimped on the top started to lift up.
 

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King Trashmouth
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Well the valve guide wear and ticking is pretty infamous as the "Cobra head tick." You can get dozens if not hunreds of hits on that, though a lot of the good pictures and vids are lost by now.

For the head cooling mod I recommend just putting in a second "crack pipe" on the back of the driver's side head. Just drill a new bolt hole to get the orientation you want. Tee that in with the other supply line and you're good to go.
 

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Discussion Starter · #13 ·
I just flipped the head over with the cams out and poured alcohol into each combustion chamber. Every one had no leakage at all. I even poured alcohol into the cylinder and had none leak past the rings. I'm at a loss. Maybe my lash adjusters were stuck extended and were keeping the valves from closing? I'm to the point now where I'm going to just rebuild the Heads and put it back together and hope for the best. Any more ideas?
 

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King Trashmouth
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I'm surprised, usually if the guide is worn enough it starts to chew up the seat and cause the leakage. You may have caught it early enough.

If the guides are as sloppy as you say they are, they definitely need to be replaced, and that would cause the ticking. Typical tolerances are on the order of a couple thousandths, and you're saying it's way larger than that. Rebuilding the heads would be a good idea. New guides, inspect the valves, check for clearance, runout, and throw a new set of lash adjusters in them while you're at it.

And if you're as paranoid as me, you might either pull the #7 piston to inspect it, and/or give it a leakdown check before you put the motor back in.
 
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