compression problems/questions???

[tiger9297]

90 K5 TBI 350 (256,000 miles) just quit one day. Replaced sensors, fuel filter, removed and cleaned TBI. No difference. Figured it "jumped" time so replaced timing chain. Ran a little better but still runs very badly. Figure a valve was bent when it "jumped" time. Checked compression yesterday. All cylinders were 130-140 except #6 which was 90. I figure that is where my bent valve is. The other thing is that #2 was 180! It was also the only one where the spark plug was really oily and when I removed the compression tester is was really oily too. Questions:

Am I correct in that this cylinder is not firing?

So this means I'm basically running on 6 cylinders??

Why would this cylinder have so much more compression than the others?

 

[R77K10]

90 is bad

you should be no more than 20% difference from highest to lowest after testing all 8 cylinders

would be better to see 150 on them all

and 180 on number two thats pretty crazy
i dont know on that, never seen that one happen before, hmm



something is bad wrong it sounds like

either way it sound slike its time for another 350

find a good used one out of something, go by an unwanted or wrecked sbc car truck or van or something else somewhere sitting in town for dirt cheap and stick the engine in and go again, shouldnt take over a couple or three days to be done and back on the road again would be really surprised what you can snag up for 10 bucks or less if you ask the right person at the right time or leave a note with your name and number on it at the right place, sometimes ppl will give you something you ask about sitting in their backyard or drive or whatever, ive scored alot of good old chevies this way, nice engines, trannies, all kinds,

 

[tiger9297]

What would these numbers be on a NEW engine?

Is 130-140 bad on an engine with this many miles or should they still be 150+?

 

[zeroz400]

For a motor with that many miles 130-140 isnt bad at all. But with one cylinder at 90 and another at 180 you have a really bad imbalance in the engine. That is prolly why its running rough, does the motor shake alot?

You could do a leakdown test to see where your losing your compression, if its through the intake or exhaust then you prolly have a bent/burnt valve. I wouldnt be surprised if its your rings tho. Atleast do the leakdown test before you start tearing the motor down. Dont be surpsied if your gonna be replacing or rebuilding your motor.

 

[tiger9297]

yea it runs VERY rough. I don't know how to do a leak down test. Actually, I am kind of hoping I have to put in a new engine. I'm thinking though I can just rework the heads. that will be alot cheaper than a new engine.

 

[zeroz400]

It helps to have a leakdown tester but sometimes you can just use your compression tester. Take out the valve in the end, mine was the same type as a valve stem for a tire so i used one of those to remove it. Bring your #6 cylinder to tdc, i usually use a screwdriver in the cylinder while some turns the motor slowly. Put the compression tester hose in and hook an airline from your compressor to it. Then listen through your intake, exhuast, and valve cover to find out where you leaking from.

You can try reworking the heads, your gonna have to do it anyways when you rebuild the motor. While you have the heads off you can look at your bad cylinder, maybe you will get lucky and be able to put some new rings on it and it'll run for a bit longer.

I'd also recommend retesting your #2 cylinder, it shouldnt be 180...Throttle open, 4bumps on the guage is the way i do it.