phantomfixer

The vehicle is a 2002 Grand Cherokee 4.0. Does the thermostat require a small bypass hole in it? My cherokee has a big rpm flux and at times shuts down when or near when the thermostat opens according to the temp gauge.

was wondering if the hole allowed for resistance to build up in the sensor thereby not "shocking" the pcm when the thermo opened???or could it be the thermo sensor?
it only does this once a day below 70 degrees////outside temp...was fine all summer now cooler temps are around it started acting up again

I changed the pcm....no joy

dave1123

The hole in the thermostat is just there to help bleed air out if the system. It really isn't necessary. The thermostat housing has a large bypass passage that allows coolant to flow thru the heater so no pressure is built up. There is no "shock" to the system when it opens, however the coolant temp will drop as fresh coolant enters the system from the radiator.

Your problem seems to be at the transition point between closed-loop mode and open-loop when the PCM begins to read the sensors. It may be the IAT (intake air temperature) sensor. Just a guess.

phantomfixer

Dave,

Thanks for that...changed the water temp sensor on the housing..just a guess...will check it out tomorrow once the temps drop...if still a problem will check the IAT sensor....thanks much for a starting point...
JZ

phantomfixer

Well so far have changed PCM, AIT sensor, water temp sensor, TPS,
was a conflict with the manual...manual said on TPS 1,3 for 5vlts...for me it was 2,3 bl/bl wire on 2 and red or orange on 3...as per schematic..
has not shut down with the new TPS but will see.....only drove it once since the TPS change
Could it be the cam sensor...or CS wiring seems it is right there getting knocked around every oil change


cleaned up the CS wire harness...
Whatever it is it doesn't shut down when the engine is warm, only that brief period between thermo opening....
Dave
can you explain more about the open/close loop... I assume the engine kinda runs on preset parameters when cold and goes into open loop whenpcm senses warm up?
My thinking based on your open/closed loop, is one of the sensors is stuck/malfunctioning when cold???

John

dave1123

What I was saying is the PCM uses preset parameters for mixture until the IAT and 02 sensors start to change output because of engine heat. The idle air control actually raises the idle speed to about 1000 rpms until the PCM is convinced the engine is running. Part of that "sensing" is the MAP sensor.

I've found with my jeep that sitting, waiting to get my newspapers when it's cold outside, I'll shut the engine off until it gets cold inside and then start it up to get heat. The engine idles a little rough for about a minute or so and then smooths out. I've come to the conclusion that the 02 sensor has cooled off enough for the PCM to think it's a cold start when the rest of the engine is still warm.

On the CS wiring, if you change the filter from the top, you shouldn't get in trouble with the wiring. With the older 4.0s, oil pressure sensors used to get broken all the time.

phantomfixer

Ok thanks for that info Dave....times like this, I appreciate my CJ2A...no sensors!

edit

OTW home it shut down at 72degrees F outside temp,,,,just as the engine was warmed up...at times, after the engine stalls, I get a 1391 and 340 code...crank and cam sensors... I have assumed these were by products of the engine stalling, as the rpm's would flux very erratically then engine shuts down....it starts right up and press on never having another problem ....So I am still thinking the cam and crank sensors are ok or they would be spitting out codes all day long...or am I wrong in this ???

Will try the MAP sensor next...
Thanks Dave for the help..

John