1999 GMC Jimmy 2dr. 2WD 4.3L Compression Problems

Yep, your going to at least have to remove the heads and find out what happened. These engines will crack the heads if overheated severely.

do a compression test all the way around. see what all the cylinders are compression wise, dry and wet. that will give hte mechanic a great idea of wha tis wrong. You may have a bad timing chain as well as that can cause compression problems . However be aware that these engines had head gasket and intake gasket problems as well.

I would be very shocked to see a timing chain problem on a 4.3 at 150K. I've seen them with 250K and still good. The heads however, are much more common to have problems.

The only thing the wet test could eliminate would be a ring problem and that is not likely at all, especially with "0" compression. That's a head problem.

Thank You So Much for the help! Here is where Im at now. I ran a (dry)compression test on all the cylinders #1 read at 30, #3 at 60 #5 at 120. Then on the Passengers side #2 read at 150, #4 at 140 and #6 at 155. Tomorrow I am going to do a Wet Compression test on just the two #1 and #3. Or do i need to them all wet as well? I have been doing them 1 at a time,is that ok? Or should i have them ALL OUT & do each one again? Before i go tearing apart the engine to check the Valves,shouldnt I run a Chemical Test and see if its a Blown Head Gasket First? I have everything i need to do it already and it seems pretty simple.?.
The man who purchased the vehicle for me keeps teling me Not to, that its not the head gasket. BUT....He was also the one who told me No, NOT To, when I told him that i was not going to Buy any vehicle without 1st doing a Compression test on it. AND LOOK WHERE WERE AT!!!! Lol. So Please tell me what you think I should do from here.. Keep trying or do you think it waste of time??? (152000miles on it)

There is really no difference to you whether it has a blown head gasket or a burnt valve, your steps will be the same. Yes, you could do a wet test to eliminate the possibility of the problem being rings but i doubt that would ever be anyway. Even if it's a blown head gasket, the head has to be sent to a machine shop for testing and they will tell you what else it needs. Don't assume you're just going to change a gasket and put it back together. At the very least, the head will have to be planed.

I went ahead and did a Wet Compression Test on #1 and #3 only, using by Squirting about a Tbs. of part tranny fluid and part Marvel mystery oil in each cylinder and #1 changed from reading at 30 to approx. 50 and #3 stayed the same at 60. Now I dont know what to do next, What does all that mean ? What do you think is the problem? and what will it take to Fix it?

That just means that you have some compression loss through the rings but not the bulk of your problem. It still has head problems.