Hello All! My sons jeep has a problem and I hope I can get your input. He drove home the other night and his 99 Cherokee Sport (6 CYL. 115k miles 2x4) overheated to the point of stalling. After putting warm water in after approx 1 hour she started up and we drove her home, I replaced the radiator, water pump and thermo and now she runs at 210 spot on. She idles rough though so after performing a compression test I found one cylinder at 60 and the 0ther 5 between 165 and 185. After checking the compression again, still low, I added some motor oil to the bad cylinder and after a 3rd test the compression was still at 60, no change. I am wondering if the exhaust valve is burnt ( did the dollar bill test and it sucks it up). I have read on the forum that a bad valve would give a reading near zero though. Does this sound like a ruined head or worse? Can I tell anything if I pull the valve cover? Again guys any input would be greatly appreciated!

 

Sounds like either a blown head gasket or cracked head. Any coolant loss?

 

No coolant loss or water/oil mix. I have heard that a blown head gasket would result in a low compression reading in 2 cylinders which is why I am wondering about the exhaust valve. I have noticed a slight bubbling in the overflow tank though, maybe exhaust?

 

Either way you will probably have to pull the head. If you can do a leak down test you could non invasively narrow it down further.

 

If it stalled out from that much heat, the cylinder bore is shot. Oiling a bore that worn wouldn't help much in the compression test as you saw.

 

AnyA blown head gasket will only give a a low reading in 2 cylinders if the break in the gasket is between 2 cylinders.

The break Could also be between a cylinder and an oil jacket or a cylinder and a water jacket. If that is the case you would only have low compression on one cylinder.

Honestly in my opinion with 15 years of wrenching under my belt you need to remove the head and check it and replace the head gasket. It is an easy straightforward job and a blown headgasket is your problem.

Feel free to p.m. me if you have any questions.

Matt

 

I would check a cracked piston but you never realy can tell without going internal

 

Well I finally got around to pulling the head. The gasket was not broken but the head was warped. I had the head checked out and luckily there were no cracks so I just had it resurfaced and will put her back together tomorrow. The pistons appeared ok from what I could tell. Thanks for the input guys. I have read on the forum that the there is no need to find TDC when replacing the rockers even though the Haynes manuel says to do so, and the manuel also says to adjust the valve lash as well. Is it enough to just torque the rockers and call it a day?

 

While you have it off, just for S&Gs check the casting number... Late 99 started the 0331 head known for cracking. If you find its an 0331, I would not bother putting it back on.
Look at bottom left of pic.

 

The head is already on but the shop confirmed it was a 98 head. I had asked because I wanted to make sure I bought the right gasket, which tuned out to be the same one anyway.