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post #1 of (permalink) Old 09-28-1999, 11:43 PM
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35 PSI in #4 CYL

I have an 87 Wrangler 6 cylinder 258. After recent tune-up it was discovered that there is only 35 PSI in the #4 cylinder. There is no smoke, which would indicate the valve (so I'm told). How long can I run in this condition and can this be fixed or adjusted without an engine rebuild??? What are the approximate costs involved????

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post #2 of (permalink) Old 09-28-1999, 11:50 PM
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Re: 35 PSI in #4 CYL

remove the plug on #4 and squirt in some oil then test compression
an increase means bad piston rings
SAME indicates valves or head gasket
Are you losing any coolent?
If underhead cam (I dont know) pull the valve cover to see if the rocker arm or push rod are damaged or loose

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post #3 of (permalink) Old 09-28-1999, 11:53 PM
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Re: 35 PSI in #4 CYL

There would only be smoke if the valve seal was bad, not the valve. And it would only really smoke on startup but thats not the problem. You need to find the problem. If #3 or 5 cylenders are also low then ou might have a blown head gasket between the cylenders. Other things to do is get a mechanics stethoscope and an air chuck that threads into the plug hole. Pump air into the cylener and listen with the scope in the carberator throat(if you hear air then bad valve) and listen in the dipstic tube(f you hear ar then it's bad rings) also listen around the outsideof the head b that cyl to se if the gasket is blown to the outside.

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post #4 of (permalink) Old 09-29-1999, 08:03 AM
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Re: 35 PSI in #4 CYL

You can run indefinitely if you have a cracked or burned valve(s) but perfomance will suffer greatly, and as a disclaimer it is not prudent to run a engine in this condition as it could potentially cause addtional problems over time. Listen to the exhaust at idle you may be able to detect a hiss that would indicate an exhaust valve, listen to the carb for a hiss as well to indicate an intake valve. If it is a valve problem and the cam hasn't been compromised a valve job will run you 3-6 hundred dollars, do it yourself and save and learn, a machine shop will fix the head for about 1-2 hundred dollars depending on the extent of work needed - maybe it is time to check into the 4.0L head and MPI conversion.

post #5 of (permalink) Old 09-29-1999, 09:08 AM
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Re: 35 PSI in #4 CYL

Since you didn't mention excessive blow-by ..... you may be in luck (but let's not get too happy yet) and rings may not be the case. Although it happens ......in my experience ....in normal applications .... rings usually don't go bad on just one cylinder ..... it tends to be a "whole engine" condition. It's usually the same with valves ....but they tend to have more noticable effects as they degrade. All the other posts are great ..... be careful if you squirt the oil in the cylinder ... if you do it with too much zeal you may splash it on the valves and defeat your intended purpose. When you put air in the cylinder (if you do) in addition to listening at the carb and the valve cover/dip stick .... try the exhaust pipe ......and remove the radiator cap just in case the head gasket has failed to a coolant passage (which is unlikely since you didn't express any overheating/lost coolant conditions)

Side question: Bigman ...... why do you say the valve guides/seals will only smoke on start up? If it's on a intake valve the vacuum of the intake will suck oil into the intake stream. Now granted it may take a REALLY BAD seal/guide to see SUBSTANTAIL smoke at the pipe ..... but I've seen cars that put out a puff of blue EVERY time it's the bad valve guides cylinder's turn to fire.

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