Temp guage/sending unit Problem

I Have a '94 XJ with about 50k on the stock 4.0 engine, 210k on the rest of the Jeep, the other day, my temp guage started running cooler than it has been. Replaced the T-stat about 8k ago with a fail-safe unit, and have check engine codes as follows:

12, 17, 22, and the end code 55.

17 states engine running too cool-fix: check T-stat for proper operation, check coolant sensor and circuit, Replace sensor.
12 is a battery disconnect which is from when I changed the headlight switch about 2 weeks ago.
22 states Coolant Temperature sensor or circuit-fix: check coolant temperature sensor circuit, check sensor resistance, Replace sensor.

I pulled the wire off the temp sensor on the T-stat housing and the electric fan kicked on. I don't know how to test the temp guage sender at the rear of the head on the engine, but I would need to know how to test the guage and or temp guage sender for resistance or voltage-whichever it reads, so I can figger out what the heck is going on. My Haynes book wants to lean towards the T-stat, I would think so, but surely after a 20 mile drive on the Interstate at over 65mph, the water temp should get high enough to make the guage rise right? If the T-stat was stuck open, know it would take some time for the engine to warm up, but why not eventually would the guage read a normal temp? So here is the delema, do I change the T-stat, or do I test all and see what is messed up? All the wiring is in great shape, no corrosion or damage of any kind. All the hoses feel hot like they should, but am I missing something? What kind of T-stat should I be using? Thoughts or suggestions??

Jeff

Don't bother with the gauge themperature sensor. The one in the thermostat housing is the one reporting to the computer that is throwing the codes, etc.

If the gauge in the dash is showing lower coolant temperatures as well as the low temp codes from the computer, you have two temperature sensors that are agreeing with each other. Unlikely they are both bad and agreeing with each other!

Sounds like the fail safe thermostat failed in the open position!

Why did you already replace the thermostat on a 50K engine? I suggest getting a dealer thermostat based on what others have posted here already!

I replaced the T-stat because it was running waaayy too :flame: . Didn't want to burn up the engine. So does the general consensus think I should just go ahead and go with a OE(stealership) Thermostat? I don't want to keep replacing this godforsaken thing. It was bad enough changing it the first time.

Jeff

ihscoutlover said:

I replaced the T-stat because it was running waaayy too :flame: . Didn't want to burn up the engine. So does the general consensus think I should just go ahead and go with a OE(stealership) Thermostat? I don't want to keep replacing this godforsaken thing. It was bad enough changing it the first time.

Jeff

Click to expand...

There are a number of posts here about various thermostat brands and different (even wierd) problems. Some have suggested that the safest thing to use is the stealership oem thermostat.

Personally I have never had the sometimes regular thermostat problems others have complained about here. Mine has always been leaking radiators, shot fan clutches, and electronic issues (engine running to rich/cold or lean/hot.) and that includes jeeps, fords, dodge and chrysler vehicles since 1973.

Go with a Stant SuperStat thermostat, I like mine.....lifetime warranty, and more durable and thicker than the dealer thermostats.

I went ahead and put in an OEM T-stat this morning, seems to be working okay, drilled the holes in it to burp out the air. The temp guage seems to go up and down, I guess according to how the water is flowing past the sending unit for the guage. Thanks for the help as when I pulled the Motorad Failsafe unit out, it was stuck open, and was a little cockeyed in the open position at that, must have failed, and got shot crooked in its mount or something, I will post a pic of it the next chance I get. Thanks for the replies.

Jeff

I heard that once your temp spikes, the failsafe will stick open. I didn't think that was how it designed though. I only thought that if the thermal spring failed it would stay opened, I didn't think it had anything to do with coolant temperature and sticking open.

If the coolent get to hot it locks open.

If you live up north that sounds like a good reason not to buy a failsafe stat, especially if your engine/coolant system already runs hot.

Any idea what the differential is, meaning the difference between the temp it opens at an the temp it fails open at?

Down here south of Dallas we just yank out the thermostat and use them for door stops and paper weights! LOL.:laugh3:

langer1 said:

If the coolent get to hot it locks open.

Click to expand...

That sucks, I thought it was engineered differently.......aka it didn't rely on temp to "fail". I figured the spring was different so that if it broke or whatever it would open, like the spring was there to keep it opened vs. closed like on usual thermostats.

I like my SuperStat though, by Stant.

I think when mine failed, somehow the part that moves got crooked on the shaft, and got locked open accidentally. I will try to post a pic tommorrow and you can see what I found. Thanks

Jeff

Can you reset a failsafe?

Might be able to, if I can, I will keep it as a spare.

Jeff