Rebuilt motors, now temp reading seems to be wrong

[xaza]

So my brother and I both have 94 xj's. Bought rebuilt motor last year and ever since my gauge is reading hotter than actual. Top radiator hose reading 210 gauge reading 270. Have replaced both temp switch and fan switch twice but to no avail. We rebuilt the motor out of my Jeep and put it in brothers this weekend. He has a temp light instead of gauge but it is still lighting up before electric fan turns on and sometimes staying lit. Anyone else have similar problems or some advice?

[Brody]

This is a trick coming from Geno that may help:

When you replace the water pump, T stat, or hoses on that Jeep engine, the hoses that go back to the heater that run along the top of the engine have just enough of a droop in them to prevent the cooling system from bleeding out all the way. What happens is that you get an air bubble in the droop and it is hard to bleed out.

Here is what he does:

He adds a cooling system flush fitting, the one with the screw off cap that splits the line, into the heater hose. When you are trying to burp the air out of the system to make it work well, he loosens the screw cap and lets the air bleed out of it. This works very well.

Here is what I JUST did on my 98 after changing out the water pump:

Because I had already been to the parts store twice, I was simply too damn lazy to go back and get this fitting that I forgot the last trip. I simply got the hoses lose from the valve cover clamp device and moved them up and down to bleed the air out, all with the heater on. This got almost all the air out of the system, but probably not quite as effectively as Geno's sweet trick. I am going to add that fitting at some point and probably do exactly what Geno does.

FYI, if there is air in the system, the engine coolant temperature sensor will get a false reading. This is the sensor with the electrical plug in right on top of the thermostat housing. This will do a couple of different things:

Your engine cooling system will not work as well as I described above, and it will send false readings to the ECM, causing the ECM to read that the engine is either too cool and send more gas to the EFI or too hot, sending less gas. If the burping doesn't do the trick on the system and the gauges still read hot, I would be tempted to simply replace the thermostat first, then see what happens. Replace the sending unit secondly if the thermostat doesn't fix it.

My guess is that there is still air in the lines of both engines and that the coolant isn't circulating properly..

Hope that this helps.

[Hypoid]

What Pete said about burping the cooling system. I just park nose high and pull the small hose off the thermostat housing to let the air out...I've also been converted to drilling a small hole at the top of the new T-Stat, to let the air past when filling the cooling system.

I'll have to bone up on the H.O. Jeeps, but I think there is another sender. If I'm right, there is a switch for the electric fan, a sender for the ECU, and another sender for the instrument cluster.

You can download a FSM on this site: http://www.greatlakesxj.com/tech.html

Maybe that can helf you sort out what senders you have. I did replace the engine in my XJ with a junkyard engine. I never did replace the sender, the gauge reads a little warmer than before.