I posted a similar message in the engine section but no response so I'll try here and I have some updated info. 92 Bronco 5.0, 95K miles. The cylinder closest to the driver (I think it's #8 but other info suggests not) has basically 0 compression. Adding oil doesn't raise it. Others are about 160-175 so within 10%. How do I tell if this is a burnt valve or bad lifter? It doesn't appear to be "sucking" back through the exhaust pipe which my understanding is a burnt valve. There does appear to be a slight compression buildup, maybe about 20 psi but not much. Is there any possibility that this is just a bad lifter? How can I tell?

Thanks,
Biederboat

 

It's got absolute zero compression on one cylinder with good compression on the other seven? That is not common but it certainly happens. Possible causes are bad lifter, burnt valve, dropped valve, hole in piston, cam lobe flat-spotted, compression ring gone, broken crank/rod/wrist pin (not likely on the last couple without a lot of noise and probably a seizure). It could also be a head gasket and/or cracked head or block but those are not very likely either.

I think I touched on most of the possible causes for one completely dead cylinder. You can rule out the timing chain since the rest have compression. It's basically time to start taking things apart.

 

I worked on a buddy's 79 F250 351m. Same problem rocker arm had gotten so loose no noise or compression. We had one head completely off before pullin the other valve cover and finding loose rocker. In the end he came out ahead I think we fixed a lot of leaks and vacuum leaks.
Same problem on 68 327 chevy, broken rocker arm no noise.
If you haven't pull the valve covers and turn it over, make sure all the rockers are Rockin.
Could be any of those other thing mentioned too.

 

That's basically what is meant by a dropped valve, although a valve that is fully dropped would have disconnected from the rocker and would be getting pushed up and down by the piston. It's usually repairable, and easily too, just by removing the valve cover. As long as the valve stays in the guide, then it just gets pushed up and down by the piston, it may make a tapping noise but usually won't harm anything too badly. It just depends on the design of the engine if the valve can drop fully into the cylinder - but you would certainly know by now if that happened.

 

Hope you don't get into what I did with my daughter's Sable after she ran it into some high water over the road. After changing fluids (which I made her do herself!), and pulling plugs to get water out of engine, it started fine but had a bad miss. No knocking or bad noises, just a miss. Checked compression, no compression on 1 cylinder, others good. Pulled valve cover, rockers, etc. OK. Pulled head, cylinder full of water (not coolant). Turned engine over by hand, piston stayed @ bottom. Found rod broken off @ crank, couldn't believe it ran without making horrible noises but it did. Good luck, hope yours is something simple.

 

If he sucked water into a cylinder while the motor was running, then something is busted for sure. That water is in an airtight cylinder and as soon as the piston hits the compression stroke, bango! That air and water has to go somewhere. There is the potential to blow pistons up through the hood, or down through the pan. I've seen cars where valves went through hood. I don't think that he sucked water in though, he didn't mention that.

But you are right about internal broken parts sometimes making no noise. I once had a vehicle that dropped a valve, not my Bronco; I could not hear anything amiss but I had a dead cylinder. If you think about it, the valve would just sit on top of the piston and move up and down uselessly as long as it was in its guide, and there is no reason why it should make any noise unless it was bouncing off the piston and hitting the rocker. And I've read many stories of grenaded engines making no noises, some of them on this board. Someone here had a story of finding a beer can for a piston in a used car, something crazy like that with a beer can!

 

I just had the same thing happen, But mine was #7 it turned out that the valve seat came unseated in the head. So I got a boneyard head and all is fine now. Not a bad job but use caution on the front intake bolts on each side they like to break.