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Low compression in cylinders 4 and 6

Tiny
DAHANSEN
  • MEMBER
  • TOYOTA 4RUNNER
I have a 1993 Toyota 4runner with 145,000 miles. It started running rough at idle and lost power. I ran a compression test and found that cylinders 1, 3, 5 and 2 were between 120 and 130 psi. While cylinder 4 was at 50 psi. And cylinder 6 was at 55 psi. I added a little oil to the low clylinders and they each came up about 20 psi. I haven't noticed any water in the oil and both plugs had good coloration. Am I looking at a head gasket, valves, or cylinder/ piston damage?
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Sunday, May 6th, 2007 AT 12:00 AM

1 Reply

Tiny
RASMATAZ
  • MECHANIC
  • 75,992 POSTS
If compression is low in one or more cylinders, you can isolate the problem to the valves or rings by squirting a little 30 weight motor oil into the cylinder through the spark plug hole and repeating the compression test. The oil temporarily seals the rings.

If the compression readings are higher the second time around, it means the rings and/or cylinder is worn. No change in the compression readings would tell you the cylinder has a bad valve.

You can also do a leakdown test to really get at it-cause it will only come out on 3 different location-tail pipe-dipstick tube-out of the I/manifold.
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Sunday, May 6th, 2007 AT 3:55 AM

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