Hi guys,

I’m reaching the end of my rope with this.

1989 4.0L, just swapped out a cracked cylinder head and she wouldn’t fire up after the job.

I checked all of the sensors and tuned the TPS to the 0.83 factory voltage ratio. The plugs are firing and the injectors are squirting (verified visually and with duty cycle check to make sure it wasn’t just a leaky injector). I checked the crank angle sensor (resistance is within nominal spec). Ignition control module is getting power and squarewave from the ECU and the sparkplugs are firing so I didn’t bother checking the pickup coil. Fuel rail pressure holds a steady 31-39 psi depending on whether the vacuum reference is connected (it appears to be working as intended).

So I figured I best check compression to see if maybe I bound up a pushrod while installing the new head and received some bad news!

Cyl 1: 75 psi
Cyl 2: 90-91 psi
Cyl 3: 77-78 psi
Cyl 4: 99-100 psi
Cyl 5: 82-83 psi
Cyl 6: 114 psi

I wouldn’t rule out leaking valves as a contributing problem to the compression issue. I did not squirt oil into the cylinders to find out; didn’t think of it at the time. The engine has 178k on it and was running before I pulled it apart. But even with low compression shouldn’t it still start?

 

Spec compression for the 4.0 is 120-150 psi, with no more than a 30 psi variation between cylinders.

How many miles on this engine?

Might be time to freshen it, or search for a replacement.

 

yes it should

 

recheck the firing order