4.0 OHV #4 cylinder low compression

[Elutheros]

My 1991 Ranger with 4.0 OHV 4x4 has a rough idle and very low compression on #4 cylinder. Put oil in cylinder and compression was the same. Rotated the crank to measure movement on the exhaust valve and it compares to other cylinders. The rocker arm had a 2mm depression worn in it so I replaced it which stopped the clatter at idle, but I still get some valve clatter at lower RPM. Feels strong on acceleration above 2000 RPM. I tried Seafoam in the oil for a little while and no change. I'm thinking the lifter is not pumping up but not sure. I'm trying all the low cost stuff first, so my next step is to replace the oil with ATF and run it for an hour or so. Any thoughts before I spend a $1000 on valve jobs, etc. ?

 

[Uncle Gump]

How low is low?

Do a leak down test... that will tell you if you have a burned valve.

 

[Elutheros]

40 psi

 

[RonD]

You have a burnt exhaust valve

Yes, put air pressure in the spark plug hole at #4 TDC, "leak down", and listen at the tail pipe, should hear lots of air

 

[Elutheros]

You have a burnt exhaust valve

Yes, put air pressure in the spark plug hole at #4 TDC, "leak down", and listen at the tail pipe, should hear lots of air

That is what I hoped it wouldn't be, but knew it probably is. I guess it's going to be a labor intensive July/August as I ear into this thing.

 

[adsm08]

Have fun. Might as well pull the other head while you are there and do a full valve job.

 

[Uncle Gump]

I see it's up for sale here...

Ever think of selling it "as is"?

Might be a tougher sell if you put the money into it and raise the price to get back further investment.