Any advice or input is greatly appreciated

Jeep is a 99 4.0 wj

The other day I was driving and all of a sudden she just died on me and wouldn't restart, good fuel pressure and has spark. Now when cranking it sounds like it has dead where it will freespin with no compression. Took a compression test and lowest reading was around 80 and highest was like 120. Im thinking dropped valve but wouldn't that cause no compression? Possible timming chain skipped a tooth ? Cracked head gasket ? Motor is a reman with about 50k on it. Cam/crank sensors are new

 

I can imagine that if the timing chain skipped a tooth that it would indeed cause the symptoms you have. Also a worn distributor drive gear could skip a tooth. I once had a reman Ford engine that last somewhere in the upper 30's before the gear on the cam wore out. I also had a Chev truck that the bottom of the distributor broke on (plastic) and let the gear move enough that it jumped time.

I first thought about suggesting a cracked valve as opposed to a dropped valve, but I don't know if that would make it die. With that much of a difference in your cylinder pressures, it sure sounds like a mechanical failure of some sort.

 

Try this. Take out all the spark plugs, bring the crank to bring #1 to top dead center on the compression stroke. Make sure it exactly on TDC and on compression. Apply air pressure into the spark plug hole with a fitting. If it's not on exactly TDC, it will spin. Listen in the intake and exhaust for air leaking. If it jumped timing, you should hear air leaking past a partially open valve. To be sure it has jumped timing, remove the valve cover and measure the height of the two valve springs. Both should be the same if it hasn't.

You can check all 6 cylinders this way but it's tricky because you have to turn the crank exactly 120* to check each cylinder in the firing order. Remember that each cylinder fires every 2 revolutions. This can be used for any engine to check for burnt valves or bad rings. Of course with a V8, you have to go 90* each step. (180* with a 4 cyl.) For bad rings, you listen in the oil filler cap and look for bubbles in the coolant for a bad head gasket.

 

Thanks Dave im gonna try applying air and see what I can hear... the second way sounds easier tho, which two springs do I measure tho? The intake and exhaust valve springs ?

 

Yes. The intake and exhaust on the same cylinder. With #1, that should be the first 2. You may have trouble getting a scale (ruler) down to the bottom of the spring pocket. I've never had a 4.0 apart so I don't know if the spring seats are recessed or not. Most of my experience is with small block Chevys. You can also try prying on the rocker arms. If the valve is fully closed, you should be able to get a feeler gauge under the spring end. If it's tight, that means the lifter is coming up and has sealed the oil port or still moving down and hasn't cleared it yet. Those valves should both be closed for about 180* of rotation for compression and firing.

IDK if the 4.0 is a zero-clearance engine or not. By that I mean the crankshaft will not turn with a valve open without striking it. I've seen engines where all the valves were bent because the pistons hit them when it went out of time while running. This is critical at the top of the exhaust stoke because BOTH valve are open slightly at that time. This may be the cause of your lost compression, but I'm hoping not. Of course, the only REAL way of knowing is a tear-down, which we're trying to avoid.

 

Thanks for the advice. I want to say the 4.0 is a non interference engine but not a 100%. Yes I was trying to avoid pulling the head if I didn't have to since its 10 degrees out and I don't have a garage but it seems like I may have to after all. Ill see if I can measure the springs anyway and see what happens

 

Oh, shades of my youth! I can remember pitching a canvas tent over a car and using a kerosene heater. I'm too old for that crap now!