| mercury39 | -- 09-25-2016 @ 2:51 PM |
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engine began missing a few days ago, thought it could be an issue with points but did a compression test today and all cylinders are in the 80 to 95 range except for number 7 that will not go above 30. Any tips on how to trouble shoot before I go into a full engine repair? Also any idea where I can get a truck emblem. thanks
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| cliftford | -- 09-25-2016 @ 8:10 PM |
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From the info. you give, my first guess would be a valve sticking and /or not closing all the way. You might try pouring some marvel mystery oil in the crankcase and run it a while. It might be a head gasket leak has developed and is leaking into the water jacket. If this were the case, considering the amount of pressure loss, it would probably be building pressure in the cooling system, and you would be able to tell it when you take off the radiator cap.
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| TomO | -- 09-26-2016 @ 6:58 AM |
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Do you hear a clicking noise? A stuck valve can usuall be heard. Tom
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| mercury39 | -- 09-29-2016 @ 4:03 PM |
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No Tom, no clicking noise heard.
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| mercury39 | -- 09-29-2016 @ 4:04 PM |
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Thanks, on my way to get some mystery oil....hopefully that will cure this mystery!
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| TomO | -- 10-03-2016 @ 8:02 AM |
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MMO may resolve the problem, but I would still check the compression after adding some heavy oil to cyl 7. That will tell you whether the problem is rings or some other cause for the loss of compression. You should also post the year of the engine as the earlier engines had different issue than the later engines. Tom
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| shogun1940 | -- 10-09-2016 @ 6:06 PM |
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You will have to put the engine on top dead center on # 7 cylinder ,and then put shop air in to the cylinder. You could make an adapter out of an old spark plug or take the valve out of a compression tester. If the piston moves the crank then you have to keep adjusting get until the piston does not move the crank when introducing the air. If air leaks out of the exhaust its the valve .carb/intake valve/oil breather its rings/ bubbles in the radiator head gasket or crack in the head or block. Its called a leak down test. Hope this helps
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| 56MarkII | -- 10-17-2016 @ 9:13 PM |
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Is this truck driven on a regular basis or has it been sitting for a very long time? My valves sometimes stick in my 255 when sitting for over a year without being started or turned over. Usually I don't bother with thinning the oil etc. I let it roar on fast idle for a half hour or much less and that usually does it. Also sometimes it keeps missing until I slow it down (kick it off of fast idle) and shut it down, Then you will hear a snap or clack as the valves snap shut as the engine temperature changes or cools down. I used to have more problems with this using a heavy oil. I only use a good quality 5W30 and haven't had problems since even when sitting in storage. If it is driven on a regular basis you probably have a burnt valve or something holding the valve open. If you had a cracked or leaking head gasket where the compression or combustion leaked into the coolant you would have an engine boiling over hot or cold due to extreme exhaust or gas pockets in the coolant or white exhaust from coolant burning. 30 PSI is so bad that if it was a leak into the coolant it would be a very bad leak. Over the years I have had to deal with so called over heating issues on customers equipment and so many would say it's the radiator, thermostat etc. even though they were replaced, cleaned etc. with no results. A dead give away is while the engine is cold, pull the rad cap off, leave it off, start the engine and bang hit it with a heavy load and boom you will see coolant puke out of the rad and the temperature skyrockets like right now. This is a tiny leak with the engine running fine. This stops almost right away when the load is removed and the throttle closed. Think of what a leak with 30 PSI only being reached would do??
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