XJjennay

HELP!
Well my beloved '89 XJ reached 320,000 miles and I decided to do a complete rebuild. After I rebuilt the engine and broke it in at 2,000 rpm, I took it for a spin and it began to misfire at 2200 rpm and then would stop. I have rebuilt the engine top to bottom, I have replaced the: distributer, cap and rotor, plugs and wires, knock, temp, map sensor and vacuum lines and even replaced the ECM. The dang thing still will misfire. However once I changed the block temperature sensor on the driverside, the misfire jumped to 2,300 rpm. After i replaced the ECM the misfire went down to 2,100 rpm. I'm baffled. Any thoughts? It has turned into an obsession, the only thing i have left in my wheelhouse is to replace the wiring harness, but that is way too expensive.

dcollins3208

I'd start with timing and go from there

peep

All happens about 2000 rpm eh? Does sound like timing / ignition issue.
Look at the coil for really good clean connections. See any cracks?
I'll assume the ground connections to block are clean and secure.
What about the plugs. I've had defective plugs before right out of the box. Try some of the original plugs again. Check the new ones for cracks that could cause a misfire or an arcing from plug to block. I've had a plug boot tear/cut while installing, then misfired to block on that cylinder.
What about the CPS, see Cruisers tips, take a volt reading (set meter at AC voltage) to insure your CPS is a strong signal. A weak CPS will allow you to run, but poorly.
Are your injectors clean? With 320K mi there should be some wear and tear. At least having them clean helps tremendously. I've soaked injectors in Gunk carb cleaner with some good results.
Really hope you sort your 89.