Low compression in one cylinder

[Abchoppers]

1956 Mercury 312, around 61,000 miles. I don’t know if engine has ever been rebuilt. I pulled a plug to see how they look and it had oil on the threads. I pulled all of them and all ended up having oil on the threads. That led me to perform a compression test. All but one cylinder had good compression :
1 155
2 150
3 150
4 145
5 150
6 135
7 90
8 145

I did the test on a cold engine. I removed the valve cover on the drivers side after getting the number 7 cylinder reading of 90. All of the rocker arms were moving and there was plenty of oil.

So what can cause one cylinder to be low? I’m sure there are a few things. Does any of this explain the oil on the plug threads?

The car runs great, plenty of power but does have a slightly rough idle sometimes. It has a 57 intake and distributor, a PCV valve, new Holley 4160 carb.
Thanks all,
Ryan

[cadillac512]

Usually a valve isn't sealing properly. I'd check the valve adjustment paying particular attention to #7. Likely an exhaust valve but not always. If it's not a too-tight valve, the valve may be leaking due to a burned face.

It's also possible the low compression is due to a ring problem, but then you should see excessive blow-by from the crankcase breather (and it would pulse with the weak cylinder firing).




Terry

[cadillac512]

The oily threads isn't necessarily cause for concern. It happens fairly often on engines that are just fine.



Terry

[Flathead Fever]

Squirt some oil in the cylinder and if the compression comes up it's the rings, the oil helps make a temporary seal. While its running pull the plug wire and if you notice an rpm change that cylinder still has enough compression to fire. Do not pull the wire if it has electronic ignition, only if it still has points. It can damage an ignition module. If it's still firing I wouldn't worry about it. You can also get the piston up on TDC and take the compression tester hose and hook it to a compressor and pressurize the cylinder. Listen for escaping air int o the exhaust pipe, carb or crankcase. Make sure the valves are opening and closing the same amount as the other cylinders. Also put a vacuum gauge on the engine and check for a steady needle.

[Abchoppers]

Originally Posted by cadillac512

Usually a valve isn't sealing properly. I'd check the valve adjustment paying particular attention to #7. Likely an exhaust valve but not always. If it's not a too-tight valve, the valve may be leaking due to a burned face.

It's also possible the low compression is due to a ring problem, but then you should see excessive blow-by from the crankcase breather (and it would pulse with the weak cylinder firing).




Terry

I wanted to check the valve adjustment anyway so I’ll do that a definitely pay attention to number 7
Thank you

[Abchoppers]

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Originally Posted by cadillac512

The oily threads isn't necessarily cause for concern. It happens fairly often on engines that are just fine.



Terry

Good to know, thank you

[Abchoppers]

Originally Posted by Flathead Fever

Squirt some oil in the cylinder and if the compression comes up it's the rings, the oil helps make a temporary seal. While its running pull the plug wire and if you notice an rpm change that cylinder still has enough compression to fire. Do not pull the wire if it has electronic ignition, only if it still has points. It can damage an ignition module. If it's still firing I wouldn't worry about it. You can also get the piston up on TDC and take the compression tester hose and hook it to a compressor and pressurize the cylinder. Listen for escaping air int o the exhaust pipe, carb or crankcase. Make sure the valves are opening and closing the same amount as the other cylinders. Also put a vacuum gauge on the engine and check for a steady needle.

I read about the oil trick and I will try that next. It does have electronic ignition. As far as I can tell it is firing on all cylinders. My vacuum reads at a steady 20 pounds.
Thanks for the tips

[dmsfrr]

Originally Posted by Abchoppers

... My vacuum reads at a steady 20 pounds. ...

That should rule out a leaky intake valve, or the vacuum would pulse when that cyl fired.

[Abchoppers]

I did the compression test again on cylinder 7 after adding oil, no change. I did it a couple of times just to be sure. Like dmsfrr mentioned, since the vacuum is pretty steady at 20 to 22 inches, I’m leaning towards an exhaust valve like Terry first suggested. My next step is to check the valve lash and hopefully #7 is too tight. So here’s my questions -
Is it bad to drive the car with an exhaust valve not fully seating? If my valve adjustments are all good, is the only way to fix this is by pulling the head?
Thanks guys

[wellcraft17]

I had a 55 ford yblock with the exact same issue as you back in my college days, late 60s, 7 cylinders with 155-160 psi and one at about 90. I had bought the car from an older gentleman who had parked the car for approximately 10 years due to he could no longer drive. It ran ok with a slight miss at idle which really bothered me . So one day talking to an old ford mechanic explaining what my issue was he says that cyl probably has something stuck between the valve and head. Hooked up an air hose to the cyl and I could hear air leaking out of the exhaust valve, rusty carbon valve seat or burnt valve? He told me the cheap fix, I had no money nor the time to remove the head, would be to remove the valve spring, hook up a drill and spin the valve with an up and down motion with the hope if something was on the valve seat It would clean it off. That’s exactly what I did, reinstalled the spring and my compression came up to approximately 150 psi cold. Cheap fix, couple hours labor drove the car until the late 80s and sold it

[cadillac512]

Originally Posted by Abchoppers

I did the compression test again on cylinder 7 after adding oil, no change. I did it a couple of times just to be sure. Like dmsfrr mentioned, since the vacuum is pretty steady at 20 to 22 inches, I’m leaning towards an exhaust valve like Terry first suggested. My next step is to check the valve lash and hopefully #7 is too tight. So here’s my questions -
Is it bad to drive the car with an exhaust valve not fully seating? If my valve adjustments are all good, is the only way to fix this is by pulling the head?
Thanks guys

If adjustment doesn't cure it, the best way to be sure it's a valve is to pressurize the cyl with air and see where it comes out (piston at exactly tdc or valve adjusters on that cyl backed all the way off so valves can't open). Tail pipe, carburetor, or crankcase. ( though you'll always get some from the crankcase). If it's a valve there is a possibility it's as wellcraft said above, although the adjustment on that valve should have been loose when checked if something was on the seat. If it is coming from a valve and all easy efforts don't fix it, then yes the head must come off.
As for driving it, if a valve is beginning to burn it will continue to get worse until you notice a dead miss on that cyl. Usually it only damages the valve but the seat can suffer as well sometimes.



Terry



Terry

[Abchoppers]

Originally Posted by wellcraft17

I had a 55 ford yblock with the exact same issue as you back in my college days, late 60s, 7 cylinders with 155-160 psi and one at about 90. I had bought the car from an older gentleman who had parked the car for approximately 10 years due to he could no longer drive. It ran ok with a slight miss at idle which really bothered me . So one day talking to an old ford mechanic explaining what my issue was he says that cyl probably has something stuck between the valve and head. Hooked up an air hose to the cyl and I could hear air leaking out of the exhaust valve, rusty carbon valve seat or burnt valve? He told me the cheap fix, I had no money nor the time to remove the head, would be to remove the valve spring, hook up a drill and spin the valve with an up and down motion with the hope if something was on the valve seat It would clean it off. That’s exactly what I did, reinstalled the spring and my compression came up to approximately 150 psi cold. Cheap fix, couple hours labor drove the car until the late 80s and sold it

I love hearing stories like this! If it’s not a valve adjustment issue I hope that trick will work on mine. Thanks for sharing

[Abchoppers]

Originally Posted by cadillac512

If adjustment doesn't cure it, the best way to be sure it's a valve is to pressurize the cyl with air and see where it comes out (piston at exactly tdc or valve adjusters on that cyl backed all the way off so valves can't open). Tail pipe, carburetor, or crankcase. ( though you'll always get some from the crankcase). If it's a valve there is a possibility it's as wellcraft said above, although the adjustment on that valve should have been loose when checked if something was on the seat. If it is coming from a valve and all easy efforts don't fix it, then yes the head must come off.
As for driving it, if a valve is beginning to burn it will continue to get worse until you notice a dead miss on that cyl. Usually it only damages the valve but the seat can suffer as well sometimes.



Terry



Terry

I’ll try a pressure test after I check valve adjustment. Getting a remote starter delivered tomorrow to make the job easier. My 11 year old daughter is great at cranking for compression tests, but I think she’s getting tired of it haha
Thanks for the help

[miker98038]

Reminds me of trickling water down the carb at high idle to let the resulting steam clean the carbon out. Which might also be worth a try. Great story.

[Abchoppers]

Originally Posted by miker98038

Reminds me of trickling water down the carb at high idle to let the resulting steam clean the carbon out. Which might also be worth a try. Great story.

I actually just read about that for the first time yesterday when searching different threads, I thought it was a joke at first. I’ve also heard about using sea foam treatment as well. Hopefully I don’t need it but you’re right, might be worth a try
Thanks

[Flathead Fever]

I enjoy challenges like this, especially on someone else's engine. This is war, you never let the vehicle win no matter how much it costs and sleep you lose. That's good that the oil trick did not raise the compression, better a valve issue than a ring one. I bought a Case walk behind trencher that would not run. I squirted oil in the cylinder, and it fired right up and ran great. It ran until it stopped smoking. The oil was making a seal until it burned off. Bought a new piston and rings and that solved it.

A small head gasket leak could cause it, it usually causes white smoke when you first start it, and the exhaust would have a sweet smell to it. The plug would be shiny clean, the water steam cleans plug. It might run just fine and not overheat with a tiny leak. It would be losing coolant over time. The white smoke, sweet smell at fist startup and shiny plug would be the clue. A very small leak runs into the cylinder while itis sitting and then burns off when you first start it.

It could have a flat intake cam lobe not opening the valve as much as it should. Also, a bore scope down the intake to check for carbon deposits on the back of the valve would be a fun test. If the valve was leaking the vacuum gauge needle should flutter but you said its steady. Can you here any popping in the tailpipe or carb?

Are there any noisy lifters? a flat cam lobe would increase the clearance a lot and it would be making noise with a solid lifter. With hydraulic lifter engines the lifter takes up the clearance and you don't hear the noise. Yoo have to watch the rockers moving to see if one does not move as much. Something to watch if you are going adjust the valves.

Every time a piston creates vacuum it moves the vacuum gauge needle, together they are taking turns creating vacuum so fast that it shows up as a steady reading on the needle. If one cylinder fails to do its job the needle drops for a split second. Your cylinder is still creating enough vacuum to keep that needle steady. It's a lot harder to diagnose stuff like this that have not totally failed yet. I would get engine misfire codes on the Dodge 5.2L and 5.9L engine on the phone trucks and vans. They ran fine, I could not find any problem. Nothing showed up on a scope. compression was great. Everything looked and tested fine. You would never know there was a problem driving it except for that stored misfire code. I pulled the valve cover and rocker arms to the cylinder with the misfire code and put a straight edge across the valve tips and there was one valve sticking up a lot higher than the others, the straight edge rocked back and forth on it. The valve had sunk into the head so far it cracked the head, but it still ran fine. They did not have harden seats. If one cylinder is not producing the same power the rpm slows for a split second and the computer can see this. It knows where #1 is, between the crank and cam sensors it can calculate which cylinder caused the rpm to slow down. That speeds up the diagnsing time. We sent the Dodge heads out to have the cracks welded and hardened seats installed and they last longer than they did originally. At first I did not trust welding heads, but we never had a problem with any of them, better than they were new. We had so many of those engines and it started happening to all of them. If we got a misfire code, we pulled the valve cover first and that was almost always the problem. Even though the engines ran fine we had to fix them if we had a light on and code when we did the CA State Smog tests every two-year's. We were a licensed fleet test station ( I couldn't do my own vehicles which sucked, only the fleet vehicles) We kept a pair of heads and gaskets ready to go so we could swap them out at night and not have the vehicle out of service the next day. They were really big on not having more than 1% out of service at any one time in a yard with 400 vehicles.

Either the air is leaking out of the cylinder or there is not enough flowing into the cylinder to compress it to 150 lbs.

[Flathead Fever]

Cylinder Leak tests


Pull the plugs on either side of the cylinder you are testing. If you don't have a leak down gauge, you can take your compression tester hose and remove the shrader valve from it. Hook shop air to the hose and listen to where it goes This good way to check an engine out of a car your thinling of buying.

https://www.youtube.com/watch?v=og9nXfXSaoY&t=27s

https://www.youtube.com/watch?v=yhHv-AwbFiY

https://www.youtube.com/watch?v=NbxW6HRYHAo

[Flathead Fever]

Originally Posted by Abchoppers

I actually just read about that for the first time yesterday when searching different threads, I thought it was a joke at first. I’ve also heard about using sea foam treatment as well. Hopefully I don’t need it but you’re right, might be worth a try
Thanks

Carbon buildup on moder engines will raise the compression and cause a vehicle to fail a smog test for excessive NOX readings. Excessive pressure and heat combine the oxygen and nitrogen to create a NOX molecule, the brown haze in photochemical smog. A non-working EGR valve is usually the problem but when it was not, we poured water down the running engine to the point of almost drowning the engine from running (phone company trucks who cares). You don't want to pour so much you hydraulic the engine and break something. That steam cleaned off the carbon and would bring the reading down enough to pass. This engine we are working on here has a cylinder with low compression, excessive carbon on the piston is not the problem, it could use more carbon.

[Gene F]

#2, and add some Marvelous Mystery Oil to the crankcase, and take her for a spin.

[Abchoppers]

Originally Posted by Flathead Fever

I enjoy challenges like this, especially on someone else's engine. This is war, you never let the vehicle win no matter how much it costs and sleep you lose. That's good that the oil trick did not raise the compression, better a valve issue than a ring one. I bought a Case walk behind trencher that would not run. I squirted oil in the cylinder, and it fired right up and ran great. It ran until it stopped smoking. The oil was making a seal until it burned off. Bought a new piston and rings and that solved it.

A small head gasket leak could cause it, it usually causes white smoke when you first start it, and the exhaust would have a sweet smell to it. The plug would be shiny clean, the water steam cleans plug. It might run just fine and not overheat with a tiny leak. It would be losing coolant over time. The white smoke, sweet smell at fist startup and shiny plug would be the clue. A very small leak runs into the cylinder while itis sitting and then burns off when you first start it.

It could have a flat intake cam lobe not opening the valve as much as it should. Also, a bore scope down the intake to check for carbon deposits on the back of the valve would be a fun test. If the valve was leaking the vacuum gauge needle should flutter but you said its steady. Can you here any popping in the tailpipe or carb?

Are there any noisy lifters? a flat cam lobe would increase the clearance a lot and it would be making noise with a solid lifter. With hydraulic lifter engines the lifter takes up the clearance and you don't hear the noise. Yoo have to watch the rockers moving to see if one does not move as much. Something to watch if you are going adjust the valves.

Every time a piston creates vacuum it moves the vacuum gauge needle, together they are taking turns creating vacuum so fast that it shows up as a steady reading on the needle. If one cylinder fails to do its job the needle drops for a split second. Your cylinder is still creating enough vacuum to keep that needle steady. It's a lot harder to diagnose stuff like this that have not totally failed yet. I would get engine misfire codes on the Dodge 5.2L and 5.9L engine on the phone trucks and vans. They ran fine, I could not find any problem. Nothing showed up on a scope. compression was great. Everything looked and tested fine. You would never know there was a problem driving it except for that stored misfire code. I pulled the valve cover and rocker arms to the cylinder with the misfire code and put a straight edge across the valve tips and there was one valve sticking up a lot higher than the others, the straight edge rocked back and forth on it. The valve had sunk into the head so far it cracked the head, but it still ran fine. They did not have harden seats. If one cylinder is not producing the same power the rpm slows for a split second and the computer can see this. It knows where #1 is, between the crank and cam sensors it can calculate which cylinder caused the rpm to slow down. That speeds up the diagnsing time. We sent the Dodge heads out to have the cracks welded and hardened seats installed and they last longer than they did originally. At first I did not trust welding heads, but we never had a problem with any of them, better than they were new. We had so many of those engines and it started happening to all of them. If we got a misfire code, we pulled the valve cover first and that was almost always the problem. Even though the engines ran fine we had to fix them if we had a light on and code when we did the CA State Smog tests every two-year's. We were a licensed fleet test station ( I couldn't do my own vehicles which sucked, only the fleet vehicles) We kept a pair of heads and gaskets ready to go so we could swap them out at night and not have the vehicle out of service the next day. They were really big on not having more than 1% out of service at any one time in a yard with 400 vehicles.

Either the air is leaking out of the cylinder or there is not enough flowing into the cylinder to compress it to 150 lbs.

You’ve given me so much good info here, much appreciated the time you put in! I’m glad it’s pointing to a valve rather than rings. I don’t see any of the signs that you said about a head gasket either. Definitely no smoke at all, no sweet smell and no super clean plugs. I tested the vacuum again and it holds steady at 20 inches still. I do have what I think is a slight miss in the engine, very slight. My father was the one who noticed it actually. I’m leaning towards an exhaust valve but need to run the test to be sure. I think I do have noisy lifters, another reason why I wanted to adjust them. It doesn’t seem to be just one, but there is an overall light ticking noise while running.

I like the vehicle never wins. With 2 jobs, 3 preteen girls and a big house, time and money are both short. I was worrying about this, hoping it wouldn’t snowball into something big. I have to look at it as a challenge, one that I’ll win! Thank you guys for the help

Originally Posted by Flathead Fever

Cylinder Leak tests


Pull the plugs on either side of the cylinder you are testing. If you don't have a leak down gauge, you can take your compression tester hose and remove the shrader valve from it. Hook shop air to the hose and listen to where it goes This good way to check an engine out of a car your thinling of buying.

https://www.youtube.com/watch?v=og9nXfXSaoY&t=27s

https://www.youtube.com/watch?v=yhHv-AwbFiY

https://www.youtube.com/watch?v=NbxW6HRYHAo

I watched all 3 videos, thank you!

Originally Posted by Flathead Fever

Carbon buildup on moder engines will raise the compression and cause a vehicle to fail a smog test for excessive NOX readings. Excessive pressure and heat combine the oxygen and nitrogen to create a NOX molecule, the brown haze in photochemical smog. A non-working EGR valve is usually the problem but when it was not, we poured water down the running engine to the point of almost drowning the engine from running (phone company trucks who cares). You don't want to pour so much you hydraulic the engine and break something. That steam cleaned off the carbon and would bring the reading down enough to pass. This engine we are working on here has a cylinder with low compression, excessive carbon on the piston is not the problem, it could use more carbon.

The thread I was reading was exactly this, in fact after it was steamed the compression actually lowered in the cylinders that were high. Interesting stuff for sure