Zero compression on cylinder 6
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Mountain Pass
Joined: Sep 2020
Posts: 167
Likes: 4
Zero compression on cylinder 6
Did a compression test for first time in 3.5 years and the last cylinder has zero compression. Only sound is a constant whirring as all other completely strong cylinders have an a intervaled noise. It seems the engine is toast perhaps from running too lean but I’m curious of what exactly happened to #6. I’ve been driving it like this for a several months now and it stinks to high heaven and I’m lacking power on hills but she runs. What should I do and what the hell happened. Was perfect a little over 3 years ago.
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Mountain Pass
Joined: Sep 2020
Posts: 167
Likes: 4
Been having backfires consistently and one that blew up my muffler and sounded like a bomb while trying to reinstall the distributor. Recently did lifters and a few rockers and they moved fine so don’t think it’s the valves. Even if the piston rings are fine would this cause zero compression.
Laughing Gas
Joined: Sep 2016
Posts: 774
Likes: 104
From: Elk Grove Village, IL
It would be helpful to know the Model Year of the truck (EFI v Carb).
Sounds like you need to do a static leak-down test. That will determine if it's the valve(s) or rings. Basically, you're making sure that the valves are closed when you connect the cylinder to compressed air. If one of the valves is leaking, you'll be able to hear it thru the intake (carb, if it has one) or the exhaust. If it's the rings, you should be able to hear it with the dipstick removed (thru the crankcase).
Given that you say it stinks when you drive it and that you blew your muffler apart, I'm guessing rings. What you smell is unburned fuel and enough collected in the exhaust system to ignite and wipe out the weakest point (the muffler). You might be able to re-ring that cylinder but it may also be time for a rebuild.
Sounds like you need to do a static leak-down test. That will determine if it's the valve(s) or rings. Basically, you're making sure that the valves are closed when you connect the cylinder to compressed air. If one of the valves is leaking, you'll be able to hear it thru the intake (carb, if it has one) or the exhaust. If it's the rings, you should be able to hear it with the dipstick removed (thru the crankcase).
Given that you say it stinks when you drive it and that you blew your muffler apart, I'm guessing rings. What you smell is unburned fuel and enough collected in the exhaust system to ignite and wipe out the weakest point (the muffler). You might be able to re-ring that cylinder but it may also be time for a rebuild.
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Mountain Pass
Joined: Sep 2020
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86 bronco 300. Engine was recently swapped from PO out of a 90a model 6. Carburetor with offenhauser intake. It continues to have small backfires but the other big one was scary af. If it is my rings could that be what did it? It it is the exhaust valve how may I hear it?
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Mountain Pass
Joined: Sep 2020
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Laughing Gas
Joined: Aug 2020
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From: Pelham Alabama
Again, if it was mine I would pull valve cover to make sure the # 6 valves are not stuck open, or other valve issues
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Laughing Gas
Joined: Sep 2016
Posts: 774
Likes: 104
From: Elk Grove Village, IL
It's static leak down test time for Cylinder #6. That's about the only way we're going to know exactly what's going on. As an aside, the rockers can be working as they should but if a valve is burned or the seat is shot or if there's something stuck between the two, we're not going to know until that test. The compressed air will go somewhere. Where it goes is going to tell us what's wrong.
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Mountain Pass
Joined: Sep 2020
Posts: 167
Likes: 4
Im more curious now what caused it because I was replacing the distributor and accidentally installed it far off center and had a massive backfire that blew up my muffler and sounded genuinely like a bomb. Would a massive backfire if this magnitude destroy piston rings in a single cylinder?
Moderator
Joined: Dec 2007
Posts: 7,987
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From: Boise, Idaho
Zero compression is usually due to all of the compression going out of a hole. I wouldn't drive it until you get it figured out.
I think your best bet at this point is to simply pull the head off and take a look.
If it was from a massive backfire and you suddenly had zero compression, something got a hole punched in it.
I had a bad timing issue that damaged the rings on my pistons and I still had around 80 - 100 compression. They'll still work, just poorly.
Something like this with the valves
or this
Or a hole in the piston
I think your best bet at this point is to simply pull the head off and take a look.
If it was from a massive backfire and you suddenly had zero compression, something got a hole punched in it.
I had a bad timing issue that damaged the rings on my pistons and I still had around 80 - 100 compression. They'll still work, just poorly.
Something like this with the valves
or this
Or a hole in the piston
Thread Starter
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Mountain Pass
Joined: Sep 2020
Posts: 167
Likes: 4
I was thinking about getting a boroscope and taking a look. I’ve been driving the truck for over a month since the massive backfire including long distance trips to San Diego and San Francisco. Not sure if that helps with diagnosis.
Thread Starter
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Mountain Pass
Joined: Sep 2020
Posts: 167
Likes: 4
Zero compression is usually due to all of the compression going out of a hole. I wouldn't drive it until you get it figured out.
I think your best bet at this point is to simply pull the head off and take a look.
If it was from a massive backfire and you suddenly had zero compression, something got a hole punched in it.
I had a bad timing issue that damaged the rings on my pistons and I still had around 80 - 100 compression. They'll still work, just poorly.
Something like this with the valves
or this
Or a hole in the piston
I think your best bet at this point is to simply pull the head off and take a look.
If it was from a massive backfire and you suddenly had zero compression, something got a hole punched in it.
I had a bad timing issue that damaged the rings on my pistons and I still had around 80 - 100 compression. They'll still work, just poorly.
Something like this with the valves
or this
Or a hole in the piston