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Author Topic: Low Compression?  (Read 1538 times)

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Offline John Turner

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Low Compression?
« on: November 19, 2012, 08:07:53 PM »
1972 Wagon, 2000cc, 58,000 mi.  Compression tested 150 lb. for 3 cylinders, and it "pumped up" to 120  lb. on the 4th cylinder.  Someone told me since the car has not run in 2 yrs., and that it was a "summer car" and put up in the winters all its life, that it may only be a stuck ring, and I should squirt oil in the offending cylinder, let is sit, then drive it and hope that the ring frees up. 

Of course it might be a valve.  If it is a valve, and I have the valves done, do I need to have "updated" valve seats installed in the head since gas is now all unleaded?

Can I hurt it by driving it with the low compression in one cylinder?  Thanks, jt
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Offline cabecho

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Re: Low Compression?
« Reply #1 on: November 19, 2012, 08:37:51 PM »
you can put oil on that one cylinder just dont put to much. if it is a stuck ring it will free up. but it could also be the valves or one of the valve springs, the 30 psi difference to me is not a big deal but im sure other people wont agree with me.
what i normaly do when i take a motor out or park a car and i dont know how long is not going to run for i loosen the cam, or rocker arms so that all the valves are close and none of the valve springs are compress, and i put after run oil that they sale so that the rings wont get stuck. and i had engine with out running for over 3 years and they will start right up after adjusting the valves.
if the you want to check if the valves are sealing good there are ways to check them too with out taking the head out. 
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Aerodynamics is for those who can't build engines

If ford pintos are not fast then why chevy's have to use there parts to make them fast?

Offline D.R.Ball

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Re: Low Compression?
« Reply #2 on: November 20, 2012, 01:03:59 AM »
150+150+150+120=570 divided by 4 = 142.7 well with in the average...Is the car doing anything else, weird noise on that cylinder etc....As far as the unleaded valve seats I'm not sure...Cars in the early 1960's had that problem but unleaded gas hit the gas stations during the late 1960's so you should be fine....
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Offline jeremysdad

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Re: Low Compression?
« Reply #3 on: November 20, 2012, 07:46:29 AM »
Yes, if you have them redone, have hardened seats installed. Ask me how I know. lol. The motors were speced for regular or low-lead, unleaded kills the exhaust seats. My #3 & 4 exhaust valves' faces were flush with the chamber, and 1&2 were getting there. It's cheap insurance, I think it was around 60 bucks worth of machine work to install them, and now I drive worry free. :)

My compression readings were 120, 120, 0, 30. Still not sure how #4 was able to make any compression. lol
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Offline HOSS429

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Re: Low Compression?
« Reply #4 on: November 20, 2012, 12:57:59 PM »
if it is not missing on # 4 piston then live with it ..
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Offline John Turner

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Re: Low Compression?
« Reply #5 on: November 27, 2012, 03:14:00 PM »
Put a mix of mystery oil and wd 40 in cylinders for 24 hours.  Fired it up, warmed up and got up to 130 lbs. on the offending cylinder.  I am guessing it was an oil ring seized up.  Originally it took 8 revolutions of the cylinder (#3) to "pump-up" to 120.  Other cylinders pumped up to 140# in 2 revolutions.  So now I have 3 cylinders at #140 and one at #130.  I'm driving it.  jt
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