1989 GMC s15 4 cyl. - no compression need help diagnose
 

1989 GMC s15 automatic 4 cylinder ...

I found that there is very low compression in 2nd and 3rd cyl. like about 5 psi but 1st and 4th cyl. have 125 psi

is 125 psi ok or it is sill low?

also, truck was running ...can it run on 2 cylinders?

and is there any other reason why it shows 5 psi in 2nd and 3rd apart from either valves or lower engine problem?

Before I take it apart ...I want to make sure there is nothing minor...

First I pulled plug wires on each cylinder ...1st and 4th cylinder did not make any change but 2nd and 3rd when I pulled..it died..than I removed all spark plugs and than check compression in individual cylinder and found no compression on 2nd and 3rd... I dont see oil and coolant mixed

Well first, I would do a wet compression test. Add a little bit of oil to the cylinder and retest the compression. If the reading goes up, you know you have a piston ring issue. If the reading stays the same, you have an issue elsewhere.

Alternatively, you can do a leak down test. This will isolate where the loss of compression is coming from, and you can begin your search for the point of failure from this test.

If the reading does not go up with wet test ...where will be problems? do u think exhaust valves?

I will try to do leak down test

If the reading does not go up, the only thing it eliminates for you is that the rings aren't "washed" or faulty. That's it. The leak down test will definitely narrow down what area of the engine is leaking.