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Flathead Fever · 07-23-2023, 01:20 PM

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, 01:20 PM	#16
Flathead Fever Senior Member Join Date: May 2010 Location: Yucaipa, CA Posts: 1,097	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.