Shippwreckk

So, I have a 96 XJ 4.0 that has been overheating recently. The coolant looks good. I replaced the thermostat and fan clutch. But I'm almost positive it is due to the cooling fan. Iv replaced both the coolant temp sensor and the sender twice. Iv tested the temp of the motor and it was consistent with the reading on the gauge. The fan is not kicking on until the gauge is reading about 235°-240°. I replaced the relay and still not working properly. When I unplug the temp sensor the fan comes on. So I know the fan is working properly. If I leave it unplugged the fan does bring the temp down to where it needs to be. So I think the rest of my cooling system is fine. I'm kinda stumped. It seems like the computer is to blame. Seems like it is not telling the fan to kick on at the right temp. Any help would be greatly appreciated. Thanks.

BlueRidgeMark

Hmmm. There are two temp senders on some years, but I don't remember which years have two, and which years have one. I'm thinking that the earlier ones had two (which is a dumb design), and it might have changed to one in 97. BUT! I'm not sure of the year it changed.

On the years with two temp senders, one is the gauge, and the other informs the computer, which would be the one that takes care of your fan.

Anyway, if you have the two-sensor setup, you have probably changed the wrong one - the one that drives the gauge.

CurrySoSpicy

I think 96 was the last year for that. There is one on the thermostat housing and one on the back of the head IIRC. Not sure which one does the PCM info and which one feeds the dash gauge.

jordan96xj

The 96 has 2 sensors. One in the thermostat housing, and one on the rear of the intake manifold by the firewall. The one in the thermostat housing is the Engine Coolant Temperature sensor and is used extensively by the computer for a variety of startup and fuel delivery decisions. The other sensor I believe is the Intake Air Temperature sensor.

I would have to review the FSM, and unfortunately the FSM for the 96 year is not always spot on. But I believe the electric fan is being driven only by the readings from the ECT (engine coolant temperature) sensor in the thermostat housing. The fact that the fan is commanded on when the sensor is disconnected is some indication that is true. But from where I'm sitting I can't be certain.

And for reference the fan is typically commanded on in the following circumstances on a 96:

1. The AC compressor is engaged
2. The ECT temp has reached approx 219F (not based on what the temp gauge is reading - but based on what resistance values the computer is receiving from the ECT sensor)
3. The ECT temp sensor is not available (disconnected/open circuit)

When you tested the temp of the motor (you said it was consistent with the gauge) how did you perform that test? Do you have an OBD2 scan tool you can use to get your actual ECT temperature reading from the computer? (I'm assuming you have a late model 96 with OBD2 support - but perhaps you do not).

Shippwreckk

Okay so here's an update. I went out and bought a better OBD ll reader that gives live data today and did another test. (This time not with an infrared laser thermometer) and have come to find out that the gauge is not reading correctly. During the test the gauge is reading 235° when the fan switched on and the OBD reader is saying 223°. When the fan flipped off the reader said 214° and the gauge was still reading around 225-230°. All this time replacing both the coolant temp sensor and sender for the gauge(twice) and other coolant system parts and all along I should have just bought a better OBD reader and not trusted a thermometer...Anyway thanks for the input guys. I didn't even think to try checking with the OBD ll reader until I read through the comments. The motor is still running a little hotter either way but it has been in the 90s the past few days. It should be cooler as soon as the weather changes.

Knucklebuster

I bypassed a leaky heater core and my motor ran cooler. Apparently the core was sucking air back into the system. Got all the air out but still over 210 on hot days with AC on. I found out my original Bosch electric fan was worn out. It would turn, but not fast enough to cool the engine with AC on. Replaced it with a noisy Dorman aftermarket fan and I now run below 210 all the time, even in 95 deg with ac on in a traffic jam. This is my 98 Cherokee in my profile with now 278,000 miles and a Jasper engine with only 10.000 miles on it.

nujeepguy

air got sucked into a hot,pressurized system??

jordan96xj

Yes, if you have an air leak somewhere. As the system heats up and expands, some amount of coolant will be pushed to the reservoir, but as the cooling system cools down, it will pull coolant from the bottom of the reservoir back into the system (from the bottom as to avoid air entering the system). But if you have an air leak somewhere, not only will hot coolant seep out of it as things get hotter, but when things are cooling, that same leak can suck in air. Not a lot, but enough to cause air pockets, that once they coalesce can begin to disrupt the smooth flow of coolant through the pump and whatnot.

Knucklebuster

As the motor cools down, the coolant volume decreases, and air is sucked in through the heater core. Screw the cap tight on a small empty plastic water container and stick it in the fridge. It will collapse from the reduction in pressure in the bottle. This is the same way your overflow bottle works on your jeep, but in this case coolant is sucked back into the engine.