AMC 150 compression test specs

[1987Comanche]

Here's the story. Earlier today I came across a '90 Sportruck, 4.0L, 5 speed, 4WD with 118K for sale. The truck definitely needs some TLC but seemed to run ok (couldn't drive it as it had no tags). Other than high oil consumption the '87 2.5L runs pretty well and doesn't need anything but the oil consumption is kind of worrisome. Also she's about 25 miles from 198K.

If I bought the '90 I'd have to sell the '87 so it seems a thorough checkout is in order. The FSM lists adequate compression as 155-185psi/cyl. A few of the books I have say +-10% is the max variation, other say up to +-20% is acceptable. I want to use a compression test of the '87 to help me decide whether or not to go after the '90. What is the min acceptable compression on the AMC 150? Based on the FSM it seems 140 psi is the min. Any thoughts?

[Dave Stark]

A piston engine will actually run on very low pressure (70-80PSI), if with very low power. The most important thing to look for is difference in compression between cylinders, if you have a difference of more than 10-15PSI on any of the cylinders then you have a problem & it may be time for a rebuild.

 

You'll be doing good if you in the neighborhood of 140 - 150 PSI on all cylinders.

 

 

~Dave :banana:

[Eagle]

What is the min acceptable compression on the AMC 150? Based on the FSM it seems 140 psi is the min. Any thoughts?

What factory service manual are you looking at? I have the 1986 MJ FSM open in front of me. The acceptable compression range is 155 to 185 psi, with a maximum variation between cylinders of 30 psi. That doesn't mean you can take the lowest pressure they cite and then subtract another 20 percent to get the minimum. That means your highest doesn't need to be over 185, and your lowest should not be less than 155. Ideally, they'll all be in between those numbers and fairly close to each other.

 

It'll run with less than 155 psi compression. It'll run with a LOT less compression ... but it'll be a weak sister engine.

[1987Comanche]

That makes sense. I'll have to disregard the numbers they quote in the Jeep Bible as they list 120-130psi min for the AMC 150. I was trying to figure out the discrepancy between that and the FSM. I'll do the compression test keeping 155psi in mind as the min.

[Eagle]

Are you sure the Jeep Bible is talking about the same engine? "Our" 4-cylinder MJ engine has NEVER been referred to as an "AMC 150." It was called a 2.5 liter when it was introduced in the 1984 Cherokee, and it has always been known as that. Jeep has had other, older 4-cylinder engines, and one of those might have been 150 cubic inches.

 

The Jeep Bible is a decent book, but Moses has never been much into XJs and MJs. That book was really written for and around the older CJs and such.

[wallisek]

Some of the CJ-5's and early model CJ-7's had the GM 151 4cyl. They later introduced the AMC 150 4cyl in late model CJ-7's. :thumbsup: Ahh memories...I want my damn CJ-7 back. :( :D