Here is the story I am newbie to XJs but I like to think I am mechanically inclined. I got a 1988 4.0 L XJ a couple of windows where down on it for a few years (interior wasted) so i decided to use it as a parts rig. I purchased a 1990 4.0L XJ with decent interior it had a horrible knock to it. Did some research and assumed 1987-1990 where the same. I proceeded to swap the engines replacing all of the sensors, distributor components with the swap and carefully documenting the EGR system etc. The engine will run at a very low idle but will not rev up. Questions do I need to swap out computers too or look for something internally within the engine. I know this is alot but anything will help at this point as I am stumped.

 

The pcm is fine,it maybe a bit out of time,also make sure egr is all the way closed,I have had them stick open just a bit and cause what you are describing.

 

I checked the timing it was on time at TDC, one odd thing though the harmonic balancer was off at TDC on the compression stroke looks like round about 24 degrees clockwise. Can the harmonic balancer slip away from the timing marks? EGR seems to be OK it springs back what seems to be closed when i actuate it with a screwdriver. EGR is one of the things i have not replaced though.

 

no take the distributor out and turn the motor till you have compression on number one cylender(top dead center) then check the manual for firing order on your motor.your distributor should be on number one when you put it in. the harmonic balancer should be on the zero mark

 

The harmonic balancer can slip,they get oil on them and the outer ring can move,not uncommon. Just make sure #1 is at tdc and check where your rotor is pointing in regards to #1 wire,if it is not right on it will idle but not pick-up rpm without spitting and sputtering.

 

Sorry I may have worded that wrong, that is what i did puzzling huh?

 

I will try to swap out the EGR with the one i know works.

 

if you are 100% sure that it is at tdc and the rotor is pointing at #1,then the mark on the balancer should be pointing at 0.
If not then the balancer has slipped.
If it has slipped just take the valve cover off and when you have it close to tdc just rotate it until both rockers are slack and a little loose,you can also look in the spark plug hole and see when you are at tdc.

 

Yeah, I used the spark plug hole method to ensure that it was on the compression stroke and at TDC.

 

If it is off just a 1/4" it will not run right,or you 100% sure it is timing,it sounds from what your are saying you have it timed right.