I have a grand cherokee with 4.0. 2 cylinders aren't firing. checked voltage going to the injectors, thats good, did compression test thats good, its getting spark, but those 2 cylinders aren't firing i cant figure it out. it starts and runs. i know their not firing because when i take the plug off going to the injector nothing happens. the injectors are good i switched them around and the same cylinders are not firing. just changed cap and rotor plugs and wires and pickup coil. not sure what else to check here.

 

Sounds simple, but let get back to basics.
For an engine (cylinder) to run it must have Fuel, Spark, and Air. If the engine/cylinder has those 3 things, it is running fine.
I believe you when you say its not running. Problem is, based on what you said, it is running.
Need more info

Ron

 

its running on 4 cylinders, its real sluggish

 

any ideas?

 

What did the plugs look like on those 2 cylinders, were they cleaner than the others or wet/fouled? What 2 cylinders? are they in firing order?

 

they were somewhat wet and brownish red. cylinders 3&4
i just replaced them too

 

Yes they are in order i made sure of that before i changed the cap, rotor, pickup coil. i really cant figure it out they have fuel and spark, i dont see why it wouldnt have air.

 

did a real compression test instead of holding my finger on the hole #3 has no compression and #4 gets about 30psi then it just dies down im hoping its the head gasket.

 

What I said..................
In this case, its air. Did you do a wet test? That will pinpoint rings or valves.
Once we know which it is, we can go forward.

Ron

 

Recheck the firing order.

 

im going to take the valve cover off tomorrow and see if all the valves are working. ill do the wet check too.