Running Lean and overheating

Ok so my 92 Cherokee 4.0 has been overheating since I bought it. I have replaced everything under the hood with no success. I finally gave up and took it to a mechanic who tells me that it is running extremely lean but can't seem to find out why. Do any of you have an idea where to start other than the O2 sensor and fuel filter? Oh yah it runs strong and the CEL is not on.

Here is what I have done for the cooling system
3 row CSF radiator from radiator barn
Hesco hi flow water pump
Hesco hi flow thermostat housing
OEM 195 tstat
upper and lower hoses
heater control valve
heater hoses
fan clutch
serpentine belt
Radiator cap 13lbs stock and 16lb just to check
new Autometer water temp gauge
B&M trans cooler I also bypassed the radiator
Plugs cap and rotor wires and air filter.

I have also checked the compression all 6 were around 145
had a block test done no gasses in the coolant.
no water in the oil and no oil in the water
electric fan is wired to a switch which is on all the time.
I have checked under hood temps with an IR gun and all temps towards the front of the engine seem ok, its only hot back where the sending unit is and around the back of the motor. I have flushed the motor several times with chemicals and with the Prestone kit.

I could really use some help on this one,

Mark

Well Mark, to say you've been thorough would be an understatement. I'm sure others would ask too, what are your temps (gauge and IR) and under what circumstances?

The gauge will get up to 250 before I turn on the heater. It usually only over heats on long hard pulls up a steep grade. It can drive in the city and on the freeway all day long as long as it is flat.

More than likely a vacuum leak. There are several possible sources, but for it to be running lean enough to cause performance problems, it has to be fairly good size. I would do a plug reading to make sure, but you have already changed the plugs so you have lost that information.

The most common problems would be a leaky intake manifold gasket or a loose/leaky throttle body gasket. I have seen a blown vacuum booster cause the same thing as well.

If it was a Renix, I would have you check the ground from the head to the firewall.

Get a can of carb cleaner and spray around the intake while at an idle. If the rpm's change, you can pinpoint the leak. Do the same with the throttle body base.

I would think a vacuum leak would cause a high idle, at least with renix. Perhaps the OBD-I acts differently.

My money is on an exhaust leak, flange donut leak blowing on the side of the block.