My 96 5.2 V8 4x4 Grand Cherokee runs fine then after it heats up and I am running about 50 mph It cuts out like the key was turned off the comes right back on. If I slow down to 45 it doesn't do it. It's not overheating( just replaced the radiator, water pump and hoses).
Does anyone have any ideas what could be causing this?
Mine did exactly what you describe, especially pulling my boat, sometimes I had to come to almost a complete stop and then proceed gradually, it got so bad I had to do something, it ended being the fuel filter, the guy asked me when it was last changed "never" I would have never quessed the engine was starving for gas.
But, I can't think (logically) of a reason why this would be the Crankshaft Position Sensor. Although a bad CKPS causes engine cut-offs, it makes little sense that it fails because the frequency hits a certain mark.
More likely, I would think, would be a fuel related problem - perhaps your pump. But, I'm curious first about the codes you get.
12 - Battery Power to PCM disconnected
55 - End of codes
21 could be one of several codes (it can be narrowed down with an ODBII scan tool), but all relate to the O2 sensors. If it were me, I would (a) put an ODBII scan tool on it to get the exact codes, and (b) change my upstream O2 sensor.
The O2 sensor wouldn't make it cut out and jerk like that would it?
Perhaps. If an O2 sensor is not correctly reading the exhaust, then it could cause a too rich or too lean condition as the PCM compensates. For me, the first thing I would do is get the exact code from an ODBII scanner to help with diagnosis.
I'll admit that I am perplexed by the deep mysteries that are programed into the PCM's operation. But, our Jeep PCMs are not very robust when it comes to faulty sensor readings ... if the readings are off, the PCM does the unfathomable thing of allowing our engines to sputter, jerk, and die. If you read these forums enough, you will find that there are perhaps a dozen different sensor errors that can cause a Jeep's PCM to kill the engine (it has no "limp" mode). That, in my opinion, is very poor engineering.