Low Compression - keep trying or rebuild?

[pokey]

I'm finally getting started on the 34 pickup I inherited from my Grandfather. I had hopes of getting the 59A engine that is currently in the truck running without major engine work, but hope is fading. The initial compression test was pretty dismal, but compression has improved somewhat by squirting oil into each cylinder, cranking it over and letting it sit for a few days. Currently compression (in psi) on cylinders 1 to 8 is 70, 65, 40, 35, 40, 60, 42, 25. I know it ran when it was parked, but that was 1972, and I can't vouch for how well it ran.


My question: should I throw in the towel and rebuild it, or try additional steps to bring up the compression. Suggestions for other things to try are welcome.

[Ggmac]

Could be stuck rings / valves . I'd stick new oil in it , add water and fire it up . Run it for 1-2 minutes, shut it down , check oil water let cool and repeat . Do it a few times , see if the compression comes up . Heat and movement should give you the answer you need .
If you hear any thunks shut it down .
If you don't want to do that. At least remove the intake and heads , examine and check condition. If it all looks good reinstall and go back to first suggestion of running for a minute or two .

[Lawrie]

My 34 has a stock engine,that has low compression, it would not pull a maggot off a pork chop, but it runs very nice,is a smooth as ,and will run a 60 mph all day on the hiway,
Lawrie

[AnthonyG]

Did u use the procedures outlined in posts here and other places for "steps to start an engine that's been sitting a long time" before u turned it over the first time?

[JSeery]

Like posted, could just be rings or valves not sealing due to long storage. Try something like MMO in the cylinders and let it set a bit. Run it a bit to see if it continues to improve.

[petehoovie]

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Go for the gusto! Fire that puppy up!....

[alanwoodieman]

we will assume that the engine was warm, the throttle was wide open, all plugs removed and you let it spin for 4/6 revolutions. if this resulted in that low a compression then you can try a couple of things--what was the vacuum when the engine was running? 20 inches? if not do you hear a popping sound out of the carb or tailpipe? indicates a valve problem. I would soak the engine with a mixture of acetone/ATF after a few days then I would squirt some oil down each spark plug hole, put the plugs back in and fire it up. and see what happens

[AnthonyG]

Probably should have done what alanwoodieman said before turning it over the first time. Now that that horse left the barn u should do it now. I recently used a combo of wd40 penetrating oil, kroil, atf & diesel fuel in all cylinders & let it marinate 4 a week. Engine turned with starter motor easily after not being started & stuck for 20yrs. Drained all put new Plugs, oil & filter & she fired up. ran a few times 4 5 minutes. did compression check & all were 118 to 123, still runs fine. Good Luck!

[slowforty]

I drove my 40 with 65 lbs of compression on 7 cylinders and 35 lbs on one for ten years. Get it running and then decide what your goals are. A poor compression driver is a lot better than a car you cant drive.

[pokey]

Thank you everyone for your words of encouragement. I am following the Van Pelt instructions for starting an engine for the first time, and have not tried to fire it up yet. I still have to get some ignition parts (new condenser) and get some gas to the carb (rusted out gas tank). The engine turned by hand easily, so I have just squirted oil into the cylinders through the spark plug holes and turned it over with the starter. Based on what you all said, I'll get the parts I need and give it a go. The radiator is disconnected currently and I am trying to decide if I should replace the water pumps before I put on new hoses and reconnect the radiator. One of the pumps turns easily by hand, the other is a little stiff but can be turned. There is also a lot of flaky rust visible in the pumps.

Thanks again everyone for your suggestions.

Regards, Jim

[pokey]

What do you mean by "soak the engine with a mixture of …" ? Could you be more specific? Any additional info is much appreciated.

Regards, Jim

[JSeery]

MMO is one agent you can use, but a mixture of acetone and automatic transmission fluid is also good. These two and several other suggestions were provided in the previous post. The idea is something that will penetrate into tight spaces such as between the rings and the piston and around the valve stems. The method you used to check the compression is only good for a rough measurement. To get a better reading need to follow the recommendations posted above. You can fire up the engine without any water in it, but can't run it for very long.

[Kruzn40]

if you have sticky rings and valves then Marvel Mystery Oil is your friend. Put a good dose in each cylinder and let that marinate for several days while turning the motor over by hand a couple of times in the interim.


I did this with a modern OT motor that had been sitting idle for several years that I bought for an old truck project. When fire up day finally arrived I found that the compression was so low that it wouldn't start. I used the MMO process and after a few days i removed the spark plugs and spun it over with the starter to blow out the excess oil, put the plugs back in and she started!! on about 5 cylinders, but the longer it ran the smoother it got and after about 15 minutes it was purring like a kitten, and after 25K miles it still does.
Good luck......

[alanwoodieman]

50% acetone
50% automatic trans fluid of your choice

[dumb person]

Not all ATF is equal. Some brands mix more readily than others. You may want to test some first.

[Red98]

Originally Posted by pokey

... Currently compression (in psi) on cylinders 1 to 8 is 70, 65, 40, 35, 40, 60, 42, 25.

Compression ratio is calculated as 15 divided into compression psi pressure. An 8:1 cylinder would be 120/15. An 6:1 cylinder would read 90psi. You highest reading of 75psi means CR of 5 ??? If I recall correctly it takes a CR of 4 to ignite the gas. So either you have a not really working engine or a bad pressure gauge. More likely the latter.

Borrow a friends gauge and make certain you have a fully charged battery and turn the starter over for 4-5 revolutions and get new readings.

[mfirth]

A friend used a product called Penetrol on a Model A and it worked GR8. Anyone else use it or know if it is still available?

[tubman]

After reading this, I don' believe that you have gotten the true compression readings from your engine. I suggest that you try to beg, borrow, or steal another (hopefully good) compression gauge. Then follow "alanwoodieman"s procedure exactly. Do it twice, once with each gauge. Then come back and tell us what you got. You might be pleasantly surprised

[pokey]

I'll see if I can track down another compression gauge. This one is a brand new Bosch brand, so I would hope it's accurate, but you never know.

[tubman]

Make sure you block the throttle open when making the test.