No compression due to missing exhaust bolt?

[jrandallc]

I have a 2003 3.0 V6 with zero compression on the third cylinder. All other cylinders were fine, around 150 psi. When I tested the compression on #3, it was absolutely zero. So I started taking it apart to pull the cylinder head. While I was unbolting the exhaust manifold on that side, I noticed that the head to the top exhaust bolt next to cylinder #3 was missing. The shaft of the bolt was still there, but no head, so effectively no bolt.

My question is, is that one missing bolt enough to possibly cause zero compression on that third cylinder? I haven't unbolted the cylinder head yet. Is it worth replacing that bolt and putting everything back together to test it, or is it more likely that there is a deeper issue actually causing zero compression?

I'm done working on it for the night. I need to get a couple of nuts tomorrow to pull that broken stud off. Thanks for any advice in the mean time to point me in the right direction.

 

[ratdude747]

No. The exhaust valve seals the exhaust manifold from the combustion chamber, so an exhaust leak would be irrelevant.

You're looking for a big leak if it is dead zero. Burnt valves, lined up ring gaps, etc.

 

[adsm08]

Not trying to be rude or anything, but if this is the level of your mechanical knowledge please find someone else to work on your vehicle, preferably someone who can teach you what you are doing.

Ratdude is 100% right that the broken bolt is not the primary cause but it may be a secondary.

An exhaust leak at the manifold increases the chances to burn a valve which, as stated before, can cause your zero compression. However even if that is what happened, at this point replacing the bolt isn't going to fix your problem.

I would stop where you are and do a cylinder leak down test before going any farther. Knowing where the compression is going is going to aid greatly in making a proper repair.

 

[alwaysFlOoReD]

Everyone has to start learning somewhere. Carry on knowing that mistakes will be made, but that is part of learning. If you don't have the tools and knowledge it may be cheaper to hire someone.

 

[ratdude747]

Not trying to be rude or anything, but if this is the level of your mechanical knowledge please find someone else to work on your vehicle, preferably someone who can teach you what you are doing.

Ratdude is 100% right that the broken bolt is not the primary cause but it may be a secondary.

An exhaust leak at the manifold increases the chances to burn a valve which, as stated before, can cause your zero compression. However even if that is what happened, at this point replacing the bolt isn't going to fix your problem.

I would stop where you are and do a cylinder leak down test before going any farther. Knowing where the compression is going is going to aid greatly in making a proper repair.

Come to think of it, I actually had that kill a 72CC moped. I rebuilt the motor and neglected to fix a bad exhaust gasket. A few months later, riding it to college one day, it dropped the exhaust valve. Bye bye bore kit...

I agree, leakdown it if you dare continue yourself. Low compression is never an easy fix, I hate to say.

 

[jrandallc]

I have done two head jobs before on a Ford and a Jeep. I'm not a pro, but I'm confident enough in my skills to complete the job. I just wasn't sure about this particular detail. I was skeptical this was the actual cause, but I do appreciate the feedback to confirm. Thanks, everyone.

 

[adsm08]

Everyone has to start learning somewhere. Carry on knowing that mistakes will be made, but that is part of learning.

Very true, and I am all for everyone learning everything they can. I am a firm believer in the idea that knowledge is power, but also that some things need to be taught, rather than fumbled through and figured out on one's own.

I arrived at this position after trying to teach myself to do brakes.

 

[pjtoledo]

if it isn't too late already, stop when you get the valve covers off and turn the engine over while observing the rocker arms for #3. there is a chance something may have broken and one of the valves isn't opening.
you can pull the plug and force compressed air into the plug hole. use low pressure, 10-15 PSI to start. make sure both valves are closed. if the air bubbles up in the radiator its looking like a head gasket, out the tailpipe/exhaust is ex valve, back up the intake is intake valve. a hole in the piston will blow into the crankcase, then back up the oil drain holes in the head.

 

[adsm08]

^Do that^ with one caveat. Make sure you hold the crank still or you will push the piston down.

 

[ratdude747]

^Do that^ with one caveat. Make sure you hold the crank still or you will push the piston down.

Not if you keep the pressure low. Actually the process is more or less a poor man's leakdown test.

 

[adsm08]

I know, I have done that test many times, using my blow gun and being very careful to keep pressure low, and every time I have moved the piston before getting a useable result.

 

[jrandallc]

Very true, and I am all for everyone learning everything they can. I am a firm believer in the idea that knowledge is power, but also that some things need to be taught, rather than fumbled through and figured out on one's own.

I totally agree with this. My "teaching" has been videos, forums and articles online. I owe many thanks to many people who have shared information. I've learned a lot on some pretty big jobs, but I've also been careful and taken my time to make sure I do a job right out of fear of killing an engine. After the Jeep job, I had three separate people comment on how smooth my Jeep ran while riding with me, including my 70 year-old mother-in-law. Dang, if she noticed, I must have done something right!

 

[jrandallc]

To update the situation, I pulled the head and there was a piece broken out of the end of the exhaust valve in cylinder 3. The valve itself was functioning fine (not stuck) but obviously the hole in it prevented it from doing its job.