fluctuating engine temps…

So I bought an 01 a couple weeks ago and had to replace the leaking stock radiator. Went with a new CSF 3 core, new OEM thermostat, and new rad cap. Backflushed the whole system and properly burped the system. Now when running down the road regardless the speed my temps fluctuate from 195-205 almost on cue. You can watch it rise and then drop. Never gets above 205-210 while idling. Is the new 3 core creating like a flash cooling effect and causing the therm to constantly open and close or is there another issue at hand. The coolant level is the same and doesn't use any. Been paranoid as this has the 0331 head but the oil, oil cap, and coolant look fine. Just can't figure out the 10* constant fluctuation. Don't recall if it was doing the same thing with the old rad or not(didn't drive it long before replacing). Any input would be great. TIA,Sean

I'd suspect the thermostat may be the culprit.

What brand was it and did it have a small hole in it and did you install the stat with that hole oriented to the top so that it "bleeds" coolant properly?

I will say that I have had this same symptom with certain aftermarket stats but I've never experienced it with a stat purchased directly from Jeep.

OHHH!! I can answer this one.... LOL I had a 94 Sport 4.0l... same thing I changed the radiator last april, burped it etc..... before i bought it the previous owner chnaged the thermostat, and it was fine. I had parked it so that the driverside wheel was up when i bruped it. (thus making the cap the true highest point). Well january rolled around and the new radiator leaked (autozone) I changed ti filled it. the new one leaked. So i got another one from autozone put it in etc... filled burped didnt make the cap the highest point... I started to see the same issue in the middle of winter in MA it would rise up and down.. Its a combined effect of a sub par thermostat and some air in the system. I changed the engine in a 00 xj and got the thermostat and gasket right from a dealer and no problems at all. Change the thermostat and gasket, get a ramp or find a hill make the driverside wheel (front) go up higher then the rest and take off the cap and run it the air will come out.

The thermostat is in fact a mopar OEM 195* thermostat. The hole in the thermostat is at the 12'o clock position. I did bleed it by running it at idle with the front up in the air, waited for the therm to open, filled, repeated, gave it some throttle poured more coolant in and capped it. I was also under the assumption that this was a self burping system?

My '88 (with the closed cooling system) has always run like you describe mrtosh. I've also got a 3 core heavy duty radiator in it as well. Just got done with a radiator repalcement and new thermostat - Mopar OEM. It runs exactly like it did with the Moto-rad thermostat before it, and the Stant before that.

When it warms up some more (like over 60) it runs steadier, but particularly below freezing it cycles a bit.

mrtosh said:

The thermostat is in fact a mopar OEM 195* thermostat. The hole in the thermostat is at the 12'o clock position. I did bleed it by running it at idle with the front up in the air, waited for the therm to open, filled, repeated, gave it some throttle poured more coolant in and capped it. I was also under the assumption that this was a self burping system?

Click to expand...

Your vintage generally does "self burp", sounds like you did everything right. With a high quality Mopar stat in there, I'm not sure what is going on but the temp needle on my 99 stays rock solid and I've seen others in our vintage stay solid too. I don't see the radiator as being the cause.

Let us know if you find out what is going on or if you just decide to live with it. I don't see anything particularly troublesome with it as temps are in good range, just one of those things that bothers a guy I guess.....

New thermostat, mopar thermostat, whatever, does not mean GOOD thermostat. Clearly, the thermostat is not opening and closing smoothly. At any rate, Mopar does not make thermostats. They buy them, probably from some third world country. Currently all the radiator caps on brand new Chryslers come from India. If I was after the best thermostat I could get, I would get the Stant premium model listed on Rock Auto. To me, the fact that they have a regular and a premium makes it likely that the premium one is a step above in construction

MNgopher said:

My '88 (with the closed cooling system) has always run like you describe mrtosh. I've also got a 3 core heavy duty radiator in it as well. Just got done with a radiator repalcement and new thermostat - Mopar OEM. It runs exactly like it did with the Moto-rad thermostat before it, and the Stant before that.

When it warms up some more (like over 60) it runs steadier, but particularly below freezing it cycles a bit.

Click to expand...

my 89 does exatly the same thing when ever i replace the thermostat.

for a couple weeks of break in time, the thermostat will noticeable open and close and cycle like this for a while. after a couple of weeks it will open and close more smoothly and not as suddenly

I concour with winterbeater and 89xj. I'm thinking the new thermostat has something to do with it. I'll run it for a couple more weeks and are what it does. If anything I can try a superstant(part number anyone?). I've got a 1300 mile trip to moab coming up may 1st and would like to fix this more or less for peace of mind.

MNgopher said:

My '88 (with the closed cooling system) has always run like you describe mrtosh. I've also got a 3 core heavy duty radiator in it as well. Just got done with a radiator repalcement and new thermostat - Mopar OEM. It runs exactly like it did with the Moto-rad thermostat before it, and the Stant before that.

When it warms up some more (like over 60) it runs steadier, but particularly below freezing it cycles a bit.

Click to expand...

I have the same issue with my 90 and 3 core radiator. Even with our mild winters here in VA I run with the grill blocked off and the lower engine cover installed to keep it in closed loop. If the closed system is working properly a 3 core is just overkill. I open the grill up in the summer and it still stays cool even while towing on hot days.

Back to topic, I say your issue is all T-stat as the YM you have will self burp easily.

45359 part number is what you want. It's $5 at Rock Auto. Pretty expensive because it's twice the price of the standard one ($1.76). Why not go for broke?

http://www.stant.com/common/printthis.cfm?BROCHURE=205&printthis=yes&global=Stant

http://www.stant.com/common/part_lo...21879&global=Stant&locatorTable=LOCATOR_STANT

winterbeater said:

45359 part number is what you want. It's $5 at Rock Auto. Pretty expensive because it's twice the price of the standard one ($1.76). Why not go for broke?

http://www.stant.com/common/printthis.cfm?BROCHURE=205&printthis=yes&global=Stant

http://www.stant.com/common/part_lo...21879&global=Stant&locatorTable=LOCATOR_STANT

Click to expand...

Thanks for the part numbers Winterbeater. I'll have to order me up one of those. On another note and not to threadjack my own thread, how's the coolant issue of yours panning out?

Had the same problem on my 1996 country except it was fluctuating from 195 to 230 randomly during all types of driving, after changing the thermostat, radiator, water pump and temp sensor I finally took the temperature of the head right at the sending unit with the gauge reading 230 and it was barely 190. Lots O money to find a bad gauge.:flamemad:

LOL, I was just going through some of my old posts. I had this same issue on a '94 a couple years back. Seems the Superstant took care of it. Wow, need some more Ginko and I'm only 27... Ordered the Superstant from rockauto, will swap out and re-bleed next weekend.