1990 jeep cherokee inline 6 4.0l the problem is...

hey there, artismo here! I have a similar problem with the idle fluctuating, and the trans down shifting itself at light throttle, which is the lock-up in the converter getting the wrong signal ? from the ECM.

You'll need to determine if the oil pressure is actually changing, or if the sending unit or gauge is lying to you. Be certain the oil level is up to normal, then check the actual oil pressure with a test gauge. If the pressure varies (drops) at engine idle, perhaps your pressure bypass is defective, or perhaps the oil pump is worn. Testing with a known good pressure gauge should lead you in the right direction.

check the connection on the battery but it dose sound like a alternator problem

Sounds like insufficient fuel delivery. Likely problem is the fuel filter is plugged.

any leaks in intake or exhaust upstream of the sensors, sparkplug condition and engine controls/sensors for out of range operation (including fuel pressure and condition of regulator)
IAC should be cleaned and checked as this can cause poor idle and shut down. I'm not saying not to change O2 sensors, but unless everything else if OK, it probably won't help.

it is a possibility that the cps is bad. but also check to see if your car has a safety which will shut the car off if the oil pressure drops to low some vehicles have an automatic shutoff to save the engine. have you checked for codes the cps will throw a trouble code if it is bad, usually. check all your basics as well adequate fuel, spark etc. hope this helps some

DROP DOWN YOUR OIL PAN BY UNBOLTING IT THEN TWO MACHINE BOLTS HOLD THE PUMP UP TO THE MOTOR.THIS IS A EASY FIX YOU CAN DO IT.

the firing order is 153624

the aux fan only works when the ac is on, and has come to a predetermined temp(?) i found this out while going up a long grade in the back country without the ac on and later found out the radiator was half plugged!

check the ...PVC valve and the MAF sensor.