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07-22-2023, 03:21 PM | #1 |
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Low compression in one cylinder
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 |
07-22-2023, 04:13 PM | #2 |
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Re: Low compression in one cylinder
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
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07-22-2023, 04:15 PM | #3 |
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Re: Low compression in one cylinder
The oily threads isn't necessarily cause for concern. It happens fairly often on engines that are just fine.
Terry
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07-22-2023, 04:33 PM | #4 |
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Re: Low compression in one cylinder
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.
Last edited by Flathead Fever; 07-22-2023 at 04:39 PM. |
07-22-2023, 04:44 PM | #5 | |
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Re: Low compression in one cylinder
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Thank you |
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07-22-2023, 04:45 PM | #6 |
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Re: Low compression in one cylinder
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07-22-2023, 04:48 PM | #7 | |
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Re: Low compression in one cylinder
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Thanks for the tips |
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07-22-2023, 10:52 PM | #8 |
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Re: Low compression in one cylinder
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07-23-2023, 07:38 AM | #9 |
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Re: Low compression in one cylinder
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 |
07-23-2023, 08:15 AM | #10 |
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Re: Low compression in one cylinder
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
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07-23-2023, 10:23 AM | #11 | |
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Re: Low compression in one cylinder
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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
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07-23-2023, 11:07 AM | #12 | |
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Re: Low compression in one cylinder
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07-23-2023, 11:12 AM | #13 | |
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Re: Low compression in one cylinder
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Thanks for the help |
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07-23-2023, 11:26 AM | #14 |
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Re: Low compression in one cylinder
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.
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07-23-2023, 01:15 PM | #15 | |
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Re: Low compression in one cylinder
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07-23-2023, 01:20 PM | #16 |
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Re: Low compression in one cylinder
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. |
07-23-2023, 01:58 PM | #17 |
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Re: Low compression in one cylinder
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 |
07-23-2023, 02:10 PM | #18 |
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Re: Low compression in one cylinder
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.
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07-23-2023, 04:01 PM | #19 |
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Re: Low compression in one cylinder
#2, and add some Marvelous Mystery Oil to the crankcase, and take her for a spin.
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07-23-2023, 06:18 PM | #20 | |||
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Re: Low compression in one cylinder
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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 Quote:
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