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Does Low Compression means it needs a Valve job???

Old 09-12-2005, 08:00 AM
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Question Does Low Compression means it needs a Valve job???

I'm new to this site so bear with me.
My Check Engine came on a month ago but I figured it might have been a loose gas cover or something so I unplugged the battery and plugged it back it to reset it. The light turned back on a few days latter and the truck is starting to run a lot more sluggish, during idles it rpms too much so I took it too Auto Zone to see what the error might be. It gave two errors, Random Multiple Misfire, and Cylinder 2 misfire. Well I decided to take ito my mechanic that did the tune up about 6 or 7 months ago.

He checked the spark plugs, tested the ignitors, checked the coils and they were all good. Did a compression test and cylinder 2 was showing 125 max while the other 3 were maxing at 180. Now he's telling me that it need a valve job and its going to cost me about $900. That includes replacing the gaskets....

Is a valve job what I need and is $900 too much to for a valve job? Thanks

Last edited by iggy; 09-12-2005 at 08:01 AM.
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Old 09-12-2005, 09:01 AM
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He's probably right, low compression on one cylinder is most likely a burnt valve. As for the cost sorry can't help much I do everything myself.
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Old 09-12-2005, 09:16 AM
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a couple of things you can do to verify it is most likely the valves...

put some oil (small amount) in the spark plug hole of cylinder 2 and then do the compression test on #2. if the compression comes up with the oil in there, it is most likely the rings as opposed to the valves. if it does not, then there is a good chance it is the valves (though it could also be headgasket, cracked head, cracked block, etc.)

the way to tell for sure is to do a leakdown test.
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Old 09-12-2005, 09:20 AM
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the oil in the cylinder test is a good way to check if its a bad piston ring, if the number rises significantly (say more than 20 psi) its likely the piston ring instead of a valve gasket. If its still low after "wet testing" then the valve seal is the likely culprit. It still could be a bad headgasket but very often with a BHG two pistons besides each other usually show the same low compression since the pressure can leak through the gap in the HG.

I wouldnt say $900 was completely unreasonable for someone to do the valve job for you including parts. Its a pretty complex job.

Last edited by NCSU-4runner; 09-12-2005 at 09:22 AM.
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Old 09-14-2005, 12:28 PM
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Thanks a lot, I appreciate your comments. I decided to go ahead and go through with the valve job. They're also going to replace my shocks and struts which I've been putting off for too long now...I hope this will fix all my problems (at least with my truck, anyway).
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Old 09-27-2005, 12:13 PM
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Another test you can do is to hold a dollar bill next to the exhaust pipe so that the exhaust stream is blowing on the bill. If it sucks the bill against the pipe for a split second then you have a burned exhaust valve. Its not as good as the oil in the spark plug hole test, but it is quicker.
 
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