Yeah, you read the title right.

Okay so a little background first. It's a '91 Cherokee 4.0 AW4. It has a new water pump, radiator and hoses, Mopar thermostat, coolant temp sensor(not sender), NTK O2 sensor, new MAP sensor, cleaned IAC and throttle body, and the entire cooling system has been flushed.

For the entire time I've owned this Jeep(stock until recently) it's got 10MPG with mixed driving. I've changed most of the parts I can think of related to the amount of fuel put into the engine with no change. I decided to live with it.

Now onto the problem, my Cherokee very, very rarely gets up to operating temp. It will normally top out at 140 degrees, my dash reads similar and this reading was taken with an IR thermometer at the thermostat housing, my heat is also terrible. During the summer after hours of wheeling it will climb closer to the 210 mark but still wont get there. I suspected a stuck open thermostat even though it was new so I threw a new Mopar 195 thermostat in and no change. The thermostat I pulled out wasn't even stuck open. This bothers me because I know this isn't right and it sucks during the winter.

Is it possible that my engine is running rich enough to cause my engine to underheat? I strongly believe that my fuel injectors may be sticking open as I lose fuel pressure after it is shut off, however this is inconsistent. Thank you for any help and hopefully I've provided enough information for you guys.

 

The most obvious answer would be a thermostat stuck open, but you definitely ruled that out. Air trapped in the system usually results in overheating, not underheating, so it's not that. Only thing I can think of would be your clutch fan is bad, making it pull all the time. Does your electric fan run ever? it should only kick on after 210*. Also is your radiator stock?

You're engine not getting up to operating temp will give you the bad mpg, but a fuel issue will make it worse too. Does the engine run normal? If there wasn't enough fuel getting shot into the cylinder it should run rough and possibly misfire.

 

Is your clutch fan locked up ? Should be able to turn it with some resistance (with engine off )

 

My fan clutch isn't locked up, it spins with resistance and passes the fan clutch test. The engine itself runs well. It take a little while to start even with the poor man's prime, when it does start very rarely it will have a bit of a stutter but it clears within a second.

 

The thermostat is the only culprit I would blame, because the system is closed until that thermostat opens. Trust me, regardless of rich fuel, clutch fan, radiator, etc, that system will overheat if the thermostat is stuck closed which is how it should be until reaching correct temperature in any system. I can watch my temperature gauge and know the exact moment my thermostat opens by watching the slow climb and quick decline as it opens. You keep throwing new parts at it, but have you thought to actually test it? A stove, pot, cooking thermometer and the thermostat is all you need to test it. I've had bad luck with non-OEM stats, but never had a bad Mopar one. I suppose it's possible though.

 

I never said I replaced those parts in an effort to solve this issue so no I haven't been "throwing" new parts at this problem. I don't have the money to blindly throw at a rig. All the parts replaced were replaced because they had failed, not to try and fix this issue. That being said, I did test the thermostat that I took out and the new thermostat before I put it in. I put them in a pot and slowly brought the water up in temp, and both opened as they should.

 

Guess you should have mentioned testing in your OP in regards to replacing the thermostat. There's nothing magical about cooling systems. The thermostat regulates temperature by opening and closing and either routing fluid through the radiator or not depending upon the temperature. Sounds like you've got it all covered, so good luck with your problem.

 

Even if the thermostat was stuck wide open it would get to temp eventually. Something else is wrong somewhere else.

 

I don't agree with this at all. Maybe if you were at idle, on a slow moving trail, stuck in traffic, (no to little air moving through the radiator) or if the weather outside was really hot, but on a cool to normal temperature day moving down the road at normal highway or so speeds you will not get to normal operating temperature with a wide open thermostat if the cooling system is up to OEM specification and cooling as it should. Otherwise, we'd all eventually overheat on the highway, and obviously we don't.

People are so perplexed by the cooling systems on these XJs, when 9/10 times it's a crappy third party thermostat causing the issue or the fact that so many of these complaints come from the trail and not normal day to day driving. Also, the cooling system is the most neglected maintenance items on our cars, because they get filthy so fast. There is simply no other malfunction to make a car run too cool except for the thermostat or lack thereof.

 

Going on your too much fuel theory, have you pulled the plugs? Are any fouled?

 

Its January in Idaho. Stick a piece of cardboard in front of your radiator. problem solved

 

Or you could use a properly working thermostat. All cardboard in front of the radiator does is not allow as much air through it, but that doesn't matter one bit if the thermostat is doing its job and now allowing coolant to flow through the radiator.

Or, use the cardboard method and drill holes into the thermostat. That seems to be a popular fix...

 

Never had an issue with an XJ not keeping heat in Canadian winters The 4.0L is a hot engine

 

OP says: so I threw a new Mopar 195 thermostat in and no change.

 

Thanks, I caught that as we've been discussing it throughout the thread.