Compression Test... One bad cylinder

[mr_blasto]

I just got done measuring the compression in each cylinder. All cylinders were between 120 and 135 except the number 5 cylinder, which was at 70. So what the heck do I do now (BTW this is my daily driver)? I am guessing that the problem with compression can come from a bad seal between the piston ring and the cylinder, or a bad seal at the valves. Since all of the other cylinders are good, do I just live with it? Is it worth pursuing any further? Anybody have any suggestions?

Background as to why I was checking in the first place:
The guy I bought my truck from owned it for about a month or two, but the guy befroe him has owned the truck since '73. The guy I bought the truck from said that the motor had been rebuilt by the guy he bought it from (the long time owner), but he didn't really have an idea of how many miles ago. Being that the truck has about 220,000 miles on it, the rebuild could have 25,000 miles on it or 125,000 miles on it.

Before I started making any changes to the motor, I wanted to have an idea of what I was working with. Eventually, the goal is at least 300hp and 400ft/lbs--fun but still reasonable for DD reliability. I am debating whether I should be saving up to get an entirely rebuilt motor, or if it would be appropriate to build this one as I getmoney for parrts (intake and carb, then heads, then cam, etc.).

Thanks for your help.

 

[theperfectgarage]

Bummer, first thing to do is shoot some motor oil in the plug hole ( about a tablespoon ) then check the comp. again. If it improves that generally means ring/piston trouble. If your compression does not change, it's most likley a valve sealing prob. check it out and post up.

 

[sled_dog]

careful to keep the oil amount to about a tablespoon. I blew my gauge out last week when I accidentally put too much in(granted it was quite a bit)

 

[mr_blasto]

Will do, but being that a friend with a tester was helping me out, I may not be able to borrow it again for another week or so. Just how much oil should I squirt in there? Thanks for the tip.

 

[mr_blasto]

careful to keep the oil amount to about a tablespoon. I blew my gauge out last week when I accidentally put too much in(granted it was quite a bit)

Thanks for the tip, you posted that when I was writing my other response. I am not a TOTAL idiot, I promise

 

[sled_dog]

Thanks for the tip, you posted that when I was writing my other response. I am not a TOTAL idiot, I promise

neither am I, but like I said, I blew mine out. I'm a professional auto tech(granted new to it) but I knew to warn you because I made the mistake.

 

[ryoken]

a leakdown tester will pinpoint your prob...

 

[mr_blasto]

Ryoken (or anybody else)--What's the logic behing a leakdown test, and how does it tell you anything different than a compression test? Could you please go into a little more detail.

My understanding is that in a leakdown test, you measure what the compression is after turning the motor over a few times, and let it sit for a while to see how much compression it loses.

Sled_dog--I was saying that I am not an idiot because I asked a question, but once I posted it, you had already answered the question (I was not implying that anybody would be an idiot in my book for putting too much oil in).

Thanks for all the pointers.

 

[ryoken]

Well, in laymans terms, your airing up the cylinder with a compressor and checking your percentage of loss.. the nice thing is you can tell where the prob is by hearing where the air is escaping. carb = intake valve. exhaust = exh valve. crankcase = rings.

There are charts floating around for percentage loss. my kit came with one... basically, a high compression, tight race motor will lose 1% or 2% usually.. average production motors usually around 5%. if it's over 10%, there's usually a prob...

 

[diesel4me]

leakdown test

To perform a leakdown test,you need an air compressor....you take the spark plugs out,and scew in an air hose adapter ,or thread the 1/4" NPT air hose fitting into the plug hole BY HAND ONLY,it fits "close enough" to do the test,but DO NOT tighten it with a wrench,or the threads could be damaged..I've yet to use the adapter myself..

Then you turn the engine to TDC on the cylinder in question..it has to be at TDC,so both valves are closed,and to prevent the engine from turning,which it may do suddenly as soon as the air pressure is applied..putting it in gear if its a manual helps keep the motor at TDC..keep fingers away from the fan and belts,in case the motor turns..

With the presure applied (100 psi or more) ,any leakage can be HEARD thru the appropriate opening..if its hissing in the intake,its an intake valve leaking.(a hydraulic lifter could be pumped up and holding a valve open slightly too)....hissing in the tailpipe(s) denotes a leaky exhaust valve..if hissing is heard at the breather or valve covers,blowby from the rings are to blame,or scored cylinders,burnt piston....much easier than guessing,or taking it all apart!..

FWIW,I've had many engines with 1 "weak" cylinder as far as compression..and never knew it until I did a compression test!...they ran pretty good!(or so I thought until the test--one motor only had 65 lbs in one cylinder! )...as long as they didn't smoke screen the highway,or foul plugs,I just ran heavier oil,and checked it more often...

 

[mr_blasto]

so if I understand leakdown tests correctly, you need an air compressor. Is that right? I don't have access to a compressor yet, so I may just have to live without doing a leak down test. Maybe I will have to try my luck at the motor oil trick...

 

[ryoken]

well, oiling the cylinders informs you of the critical factor if your just gonna be pulling a head, or digging into the bottom end..

I rarely do leakdown tests, whereas I probably do a compression test once every week or so..

and yeah, you need a compressor..

 

[mr_blasto]

How bad is it to have a cylinder at 70. Is this something I should fix within the next 3,000 miles or less, or will it be fine if I ignore it for 10,000 miles or more. If I ignore it, what problems will I face?

Just trying to weigh my options here.

Thanks for everyone's help. I love learning.

 

[diesel4me]

hard to say..

I drove the motor that had 65 lbs compression in one cylinder for 50,000 miles,and it never skipped a beat..another one I had fouled the plug so badly I had to swap it with one from another cylinder or replace it to keep that cylinder firing every 500-1000 miles..but the motor didn't smoke that much,so I kept on driving..

I'd say watch the oil consumption,and have someone follow you and see how much smoke or smell its belching out..if its excessive,its best to fix it,or at least put some STP or Lucas in the oil to slow it down,and prevent you from being ticketed...but usually one weak cylinder wont kill an engine quickly,it might wear out a main or rod bearing sooner from the imbalance of the weaker cylinder,but I wouldn't worry too much about it..takes a long time..chevy motors are pretty tough,I've run some with many "bellyaches" and still got good service from them,despite being boneyard fugitives and mistreated badly..