thermostat question

so the jeep (2001) has been overheating lately so me and the dad replaced the thermostat...the old thermostat was 195 degree...and the new one we got is 210. the jeep has always run hot, so the fan usually stays on while im driving....can someone enlighten me on how the fan kicks in...now that i have a 210 degree thermostat does that mean although the jeep will be running hotter, the fan shouldn't kick on until it reaches that temp...with the 210 tstat in the fan still stays on all the time. maybe it wasn't even a bad tstat causing the overheating...but thats what i want to figure out before i spend more money on a waterpump or something. any info on the topic you guys can give?

gijosiahh said:

so the jeep (2001) has been overheating lately so me and the dad replaced the thermostat...the old thermostat was 195 degree...and the new one we got is 210. the jeep has always run hot, so the fan usually stays on while im driving....can someone enlighten me on how the fan kicks in...now that i have a 210 degree thermostat does that mean although the jeep will be running hotter, the fan shouldn't kick on until it reaches that temp...with the 210 tstat in the fan still stays on all the time. maybe it wasn't even a bad tstat causing the overheating...but thats what i want to figure out before i spend more money on a waterpump or something. any info on the topic you guys can give?

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I did not know that a 210* unit was available for the AMC six...

Anyhow, the thermostat has nothing to do with the electric fan turning on. That is a separate sensor - in your case, it is probably the sensor in the thermostat housing (which reports to the ECU, which then drives both the IP gage and cooling fan relay.)

The "temperature" of the thermostat refers to the temperature at which it opens to allow coolant to flow - therefore, lower temperatures open earlier (common units are set for 160*, 180*, and 192-195*. Some racing units are higher, but those are typically for SBChevvy, SBFord, SBMopar, and Second-Generation Hemi.) There is a little wax pill in the thermostat, under the poppet valve (the chamber for it is surrounded by a spring) that opens and closes the thing. This pill is wrecked the first time that engine temperature crosses 225* or so - so it wants changing if you ever overheat.

Putting in a "hotter" thermostat will usually mean that the engine will warm up quicker, as it delays the full flow of coolant through the system. Putting in a "cooler" one will take longer to warm up.

N. B. Do not be surprised if OBD-II throws you a code relating to engine warmup. I'm not sure if it will or not (if you'd put in a cooler one, I'd bet money that it would!) but don't be surprised if it does. OBD-II (1996-present) monitors both engine temperature and warmup time - I know there's a code for it taking too long to warm up (like when you install a cooler thermostat,) but I don't know if there's one for taking not enough time (having the hotter thermostat should decrease warmup time slightly.)

thanks for the info. that helped a lot.
the electric fan constantly stays on...i know its a vague question, but what can be done about that...or does that simply mean that the jeep is running too hot... the jeep is running hotter than i'd like...its usually past the middle 210 mark (on my 01 gauge) up to the next line...whatever temperature that would be. it didn't used to run this hot, but now i don't know if its running that hot because of the new tstat or what

gijosiahh said:

thanks for the info. that helped a lot.
the electric fan constantly stays on...i know its a vague question, but what can be done about that...or does that simply mean that the jeep is running too hot... the jeep is running hotter than i'd like...its usually past the middle 210 mark (on my 01 gauge) up to the next line...whatever temperature that would be. it didn't used to run this hot, but now i don't know if its running that hot because of the new tstat or what

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Could be the relay stuck shut. Could be the sensor is going silly (but you'd get a CEL/MIL for that - is your "Check Engine" lamp on?) Could be the relay coil has been shorted to power and it's staying shut that way. There are a few possible causes here... However, the OBD-II coolant temperature sensor (CTS) is a thermistor - the resistance of the unit varies with temperature sensed. It's not like the earlier thermal fan switch (TFS) that uses a bimetal element to "open" or "close" a circuit, but doesn't tell you "how hot" or "how cold" it is (the thermistors will do that, and the switching logic is programmed into the PCM.

I'm fairly sure it's not a bad sensor, since you don't mention a CEL/MIL - and I'm almost certain it's a "hard" code (would illuminate the CEL/MIL.) Could be any of a number of other causes - but I'd check the relay first. A simple "test by substitution" should serve - get another relay of the same type, and swap. If it works normally, you can toss the old relay. If it does the same thing, the problem is elsewhere. Put the original relay back and put the new one in the glove box (you should always have a spare or two anyhow, and ChryCo used standard relays. Either DIN or mini-DIN relays, but DIN units are easier to find. Keep the spares with your extra fuses.)

well the check engine light is on...but for an o2 sensor...so i wouldnt know if it would have caused it to come on. ive been meaning to take it to figure out what codes its now throwin.
what is your suggestion about the new thermostat? are there any advantages of running a 210 vs the 195 that used to be in there. as you can tell i don't know much about the subject, but hey..im trying to learn.
heh, thanks for the help.

If your gauge is going above 210 constantly you have a problem, clogged radiator, bad fan clutch, bad water pump, bad sender on the Tstat housing. The fan clutch quick diagnosis from my experience. When it's cold from sitting over nite and you start it up the mechanical fan should 'roar' for about 30 seconds or so as the fluid in it gets redistributed. Then it should quiet down. If it makes no noise I'd be suspicious, they do go. Now if you start it cold and the electric fan comes on right away you have either stuck relay or bad temp sensor turning the fan on early.
As for temp range, 105F for the stat. 210F is I was in alaska maybe but I'd only run that in the winter.

would running a lower degree thermostat help reduce the engine temps at all?

Also anytime that your AC compressor is on I.E. AC/defrost settings, that will cause the electric fan to come on. Try running it with the heater controlls on the vent and with the blower on low. Hope that helps. I would also stay with the factory 195* t-stat. That is what I run and as soon as the temp hits just below 210 that is where it stays. That area is average for your jeep. Also you may want to try some nice clean coolant and a new radiator cap also. Remember that if the cap isn't sealing then the pressure will excape and cause it to run hottttt!:lecture:

gijosiahh said:

thanks for the info. that helped a lot.
the electric fan constantly stays on...i know its a vague question, but what can be done about that...

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could be a faulty relay, or maybe you have the defrost on (defrost uses AC compressor and the elec fans comes on with the AC to cool the compressor)

RichP said:

If your gauge is going above 210 constantly you have a problem, clogged radiator, bad fan clutch, bad water pump, bad sender on the Tstat housing. The fan clutch quick diagnosis from my experience. When it's cold from sitting over nite and you start it up the mechanical fan should 'roar' for about 30 seconds or so as the fluid in it gets redistributed. Then it should quiet down. If it makes no noise I'd be suspicious, they do go. Now if you start it cold and the electric fan comes on right away you have either stuck relay or bad temp sensor turning the fan on early.
As for temp range, 105F for the stat. 210F is I was in alaska maybe but I'd only run that in the winter.

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105*? Where the Hell did you find those?:rof:

Seriously, tho, are you sure you're just getting a code for the oxygen sensor? Have you pulled codes yet, to see if there are any other problems lurking for you?

Your cooling system should be flushed every two years - if you haven't, that could contribute. Your fan clutch is good for four to five years - I usually replace them out of hand around four.

DO NOT use tap water in your cooling system! Distilled or reverse osmosis (R/O) filtered water ONLY! Yes, I know it's in all caps and italics, but it's important - if you flush your system and refill with tap water, you shouldn't have bothered flushing...

If you have access to a "non-contact" thermometer, idle up to operating temperature and take readings on various parts of the radiator. Apart from a gradual drop from the passenger side (hot side) to the driver's side (cold side,) you shouldn't see any significant variation. A zone of significantly lower temperature means a blocked core.

My first guess is, usually, the fan clutch.

gijosiahh said:

would running a lower degree thermostat help reduce the engine temps at all?

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yes

you want a good 195* thermostat. that's what they designed the cooling system around.

5-90 said:

105*? Where the Hell did you find those?:rof:

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Blame it on fat fingers and not proof reading, company arrived, house phone and cell phone went off at the same time.

RichP said:

Blame it on fat fingers and not proof reading, company arrived, house phone and cell phone went off at the same time.

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Yeah, I figured - just giving you hard time!

You know, I never did hear from you when you were out this way. Did I catch the pox and not know about it? Or were you just crammed this last trip?:eyes:

5-90 said:

Yeah, I figured - just giving you hard time!

You know, I never did hear from you when you were out this way. Did I catch the pox and not know about it? Or were you just crammed this last trip?:eyes:

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Trip never happened, still in the air, moved out of the NYC office and they plan on opening an office out here in the pokes. Got a stack of servers in two racks in one end of the cellar I moved here last week waiting for the owner to sign a lease on a new office. Meanwhile I'm busy building networks at the raceways new buildings to keep me busy..