CTS, thermostat, or something else? Also, replacement for gauge coolant temp sensor?

Background time: 1990 4.0-changed over to the open system about 4 months ago...new radiator, new hoses, new thermostat. Everything has been very good since then as far as the cooling system has gone. Well, today for the first time it was 65 degrees here, I let my Jeep run for about 10 minutes at idle. I came back out to it and it was almost to the red zone on the temp gauge... I started freaking out. I drove for about 30 seconds and it went down to 1/2 and then dropped to 1/4 within about 10 more seconds, usually if it was really at 240 or whatever I think it would take longer. I had also been noticing that there had been days where when I was driving on the freeway the gauge would jump a lot between 1/2 to BELOW 1/4 which I don't think is normal after it has already been warmed up. Do you guys think this is my coolant temp sender, the thermostat or something else? The engine wasn't acting strange when I came back out to it and rain that was falling on the hood didn't make it steam. Even on days when it’s been like 60 I've let it sit for long periods of time(upwards of 30 minutes) idling and it stayed at 210. Also, is there an aftermarket replacement for the actual gauge sending unit(the one that is in the back of the head) Any good ideas?
Thanks a lot for your help,
Collin

Re: CTS, thermostat, or something else? Also, replacement for gauge coolant temp sensor?

So you never ran it much in warm weather? Why did you change to the open system?

Re: CTS, thermostat, or something else? Also, replacement for gauge coolant temp sensor?

I would suggest that the fan clutch has seen some better days and the electric fan should have kept things a bit cooler if it is in fact still working.

Re: CTS, thermostat, or something else? Also, replacement for gauge coolant temp sensor?

langer1 said:

So you never ran it much in warm weather? Why did you change to the open system?

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Well no, it hasn't been very warm here for very long. Although it had hit 60 a couple times and I'd let it just idle and it was fine, that's why I'm confused that 5 degrees ambient temp. would make a difference. I changed to the open system NOT because I think the closed system is a poor one but for the following reasons...
1. No more pressure bottles(I had 2 blow on me in 2 years)
2. Cleaner hose routing(not as many joints to worry about)
3. Self bleeding(although 5-90 had helped me enough that my closed system was more or less self bleeding itself)

BillyBob said:

I would suggest that the fan clutch has seen some better days and the electric fan should have kept things a bit cooler if it is in fact still working.

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5-90 suggested maybe the fan clutch as well, I'm just not sure why it would have gone out all of a sudden, it's not making any abnormal noise. I have had the e-fan off all winter changing to the 97+ motor on it, I guess it's time to put it back on now.

Does anyone else have any suggestions to add? I'm still stumped by this whole thing. And is there an aftermarket sender for the gauges? Thanks for the help so far fellas.

Re: CTS, thermostat, or something else? Also, replacement for gauge coolant temp sensor?

The electric fan should be left connected all year round, it will do its' job when called upon. The fan clutch is like any other mechanical device it can/will fail in an instant whether it is making noise or not. I've never heard a fan belt make any noise before it broke apart.......

The Thermal Fan Clutch won't make noise when it fails (normally,) it just slips more and more. In fact, careful listening will actually show that it makes a little LESS noise - because the fan isn't running as fast. The TFS usually fails by allowing more slip until it is only coupled by inertia, and that doesn't work very well.

Like I may have mentioned in my PMs, the TFS in the XJ just doesn't last very long - typically, a TFS will last abour five years - and somewhat less in the XJ. I'm getting to the point where I've been changing mine every two years, whether it needs it or not (although it usually does.) I'm a little pickier about maintenance than most - but that's a good policy. If my truck doesn't work, neither do I.

I redid my heater hoses in copper tube for the reason of the "coupling" issues, also the right-angle moulded heater hoses get spendy, and I just got tired of doing it. If anyone would like a pic, I've got to fix that page at WiP - so email me at dragonland2001 AT yahoo DOT com, and use the subject line "XJ Heater Pic Request." (Dammit, I get a lot of useless message traffic there - it helps if I get useful messages with a subject that sticks out...)

Do bear in mind that the TFS is a fluid coupling, the fluid is supposed to get thicker as it warms up. You'd not think a fluid could "wear out," but when it's a silicone base with all sorts of viscosity additives, it can and do. Oops.

That's why I'm designing and testing a direct, 1:1 mechanical coupling to replace the fan clutch (OK, and because the fan clutch also tends to mill a 6" round hole in the radiator when engine mounts fail...)

5-90

BigBlackHeep said:

is there an aftermarket replacement for the actual gauge sending unit(the one that is in the back of the head) Any good ideas?
Thanks a lot for your help,
Collin

Click to expand...

I did the sender/gauge thing like this..............





3/4 inch brass "T", 2 hosebarbs from the plumbing dept at Home Depot and the brass adapter that came with the gauge.
When I put it on, I made sure that the end with the sender in it was pointed down to avoid an air pocket.

Re: CTS, thermostat, or something else? Also, replacement for gauge coolant temp sensor?

Figured out the problem, my serpentine belt had lost its tension somehow like right after I replaced it. Wasn't gripping any of the pulleys, hmm....I guess thats why my power steering was acting funny too. Anyway thanks for the help guys.