I find one hole in your therory about the thermostat staying open full after the engine comes to temp. It has to open and close somewhat because otherwise in our cold WI winters when its 20 degrees out and you are driving 60 mph the windchill is like -10 so technically the radiator is also getting cooled with that same cold temp. I was taught in tech school that the
t-stat opens a little or alot depending on the outside temp. It also helps slow down the water in the radiator so the air can cool it. Otherwise coolant would pass through so fast it wouldn't have time to dissipate the heat to its full potential. I guess that was why it was invented in the first place. Also so that you could have heat in the winter.
My temp even on the coldest days stays a tick under the 210 mark on the gauge. Since hooking up my scanner is a pain (because the data connection is under the hood) I cannot verify exactly what the outlet temp really is but I'd say 195 is a pretty good guess. It would be neat to do an experiment to compare the inlet temp and see how much of a mix is really going on. ( IE inlet temp at waterpump is 32 outlet is 210 ect) I guess if someone had some 2 inch clear plastic pipe and had an extra rad hose you could section it in and simply rev the engine and watch how much coolant flows and how often.
In my stockcar I have a 5/8 inch hole restrictor instead of a thermostat which does a pretty good job of providing enough restriction to help the engine cool.
Even my boat which is freshwater cooled has a thermostat. One year I noticed it was not getting past 130 in temp. When I removed the thermostat housing I found the spring had broken. Now when I put in a new one my temp came back to 160 like it is supposed to be so your therory again about it staying open at temp is incorrect because mine was broken(hence staying open at all times) and it over cooled.

A bit more on this .....

My original request was for some direction to someone capable of making changes to factory computer settings.

The ongoing thermostat operation discussion certainly has some varied opinions ... For my part a 180'F thermostat provides a slightly cooler op temp to normal but is presumed to be fully open at the normal engine temps of 190' and higher ... which suits my conditions.

One of my concerns was increased fuel use ... This is regularly referred to by some on various forums due to their understanding of the open / close loop system.

Having recently found in the ...  97+ update model manual the following;

1/. a code is thrown if engine at too low an operating temp,
2/. That code is P0125 - Closed Loop Temp Not Reached - Engine does not reach 20°F within 5 minutes with a vehicle speed signal.

It would appear that as long as the vehicle is moving and operating temps are above 20'F ... the system is in closed loop and all is working OK ... ???

Am now guessing that previous higher fuel use on this vehicle was due to speed / load and maybe a bit of poor servicing.
Its not like I keep a day to day logbook or anything ..

Anybody got any more ideas ???? ... I'm not one to totally trust the factory manual so all input appreciated.

-- Edited by Comet #2 on Wednesday 20th of May 2009 10:56:59 PM

-- Edited by Comet #2 on Wednesday 20th of May 2009 11:00:40 PM

-- Edited by Comet #2 on Wednesday 20th of May 2009 11:01:15 PM

-- Edited by Comet #2 on Thursday 21st of May 2009 10:59:45 AM

xjryder wrote:

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I have done the 180 thermo on my 98 XJ and it doesnt do anything to help with regulating temp, you have to flush the radiator/cooling system to see better performance, I have been learning about the open/closed loop system, you have to be careful with changing things, you can lose MPG, screw up sensors, your fuel to air mixture(injection) can get really messed up creating problems, (which I am now experiencing) My cold air, cutting hood vents, running 180 thermostat are now causing me to get 218 miles on a tank not good,because I am running too cold. These are only things i have experienced with my XJ. At no time am I telling anyone what they should and shouldnt do with their rigs.

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More than happy to be corrected on any of this .....

1). I think you would need to be running temps well below 180' to affect the fuel that much ... The 180' t'stat shouldnt really drop your engine temp by much more than about 5' on a stock cooling system ... depending on your climate and vehicle use ( my experience )

2). Have you changed your diff ratios to make up for the 33" tyres ???

3). Your lift will have affected airflow as well - changing the vehicle drag characteristics.

 OK ... just checked out Ft Wayne .... and you only get 85" over there in summer ??? ... Im running a 180' t'stat because of summer temps of .... 100' to 115' ...


And I agree completely ... changing too much can be a likely cause of problems ...

Pulled the scangauge out of the toolbox the other day - was originally bought just to do code fault reading.

Hooked it up .....

Set the gauge info readouts to what I recently found was available .....


And watched the Open Loop setting change to Closed Loop ..... at 17'C ... thats about 65'F for you fellas.

Will be doing a bit of watching over the next few days and see if there are any variations.