'96 XJ running too cold - even with Robert shaw tstat

[Gojeep]

Getting back to the original question, your engine may be runing cooler simply becasue it is running rich, which would indicate a bad O2 sensor. Did you get any antifreeze on the O2 sensor? I hear it can kill an O2 sensor. It would also explain your drop is gas mileage.

When I got my O2 sensor rewired, my operating temperature went back up from about 165 F to about 190 F (I have a 165 F thermostat), and the gas mileage went back up.

Interesting. Not long since I changed the o2 sensor ( 2 years maybe ) and I have also leaned out the A/F with a adjustable map but that would not be effecting it anyway as only has an effect at WOT. This was all before the fluctuating temperature though which has only happened since the radiator change. I could run a wide band A/F tester up the tail pipe again to make sure the mixture is right while at the right temperature. Did yours swing up until the themostat opens and then down in temperature every few minutes or so like mine?

 

[Ecomike]

Interesting. Not long since I changed the o2 sensor ( 2 years maybe ) and I have also leaned out the A/F with a adjustable map but that would not be effecting it anyway as only has an effect at WOT. This was all before the fluctuating temperature though which has only happened since the radiator change. I could run a wide band A/F tester up the tail pipe again to make sure the mixture is right while at the right temperature. Did yours swing up until the themostat opens and then down in temperature every few minutes or so like mine?

No, mine takes a good long while to move up or down. The fastest move is when it gets hot enough to turn on the electric fan which cools it back down about 20 F in about 3 minutes before the electric fan cuts off.

I really suspect that you guys (there have been a few others) are either having a loose sensor electrical connection problem, causing the signal voltage to move around due to changes in the connections electrical resistance which gets read as a temperature change, OR in your case I think you are getting too much water flow too fast through the cooling system which can cause the wide temperature swings. In your case I think you already figured it out, you had too much of a good thing, too much flow, to fast, to much pressure, too little Tstat flow restriction, and too much radiator causing a slug of cold water to rush into the engine all at once when the Tstat opened which of course causes engine computer issues as it sees the combustion temperature oscilating rapidly due to the coolant temperature oscilating. Better to have more precise steady control of the temperature with less flow.

It's all about balance. With an on/off restriction like a Tstat in the middle of the flow circuit, if it allows too much flow when it opens it causes cycling because when it is closed you get a large temperature diferential between the hot and cold sides of the system, then when it opens if there is too much flow the hot fluid ends up all in the radiator and the cold fluid all in the engine and bam the Tstat closes again. With a more limited flow rate this does not happen. That is why it worked better with the OEM Tstat, better more balance slower flow rate as it has a smaller equivalent orifice opening.

I would still look at the O2 sensor and test it for the MPG problem, maybe even check the exhaust system for any restrictions. See my recent other posts on Cat Converter (& muffler ) causing MPG problems and O2 sensor testing. I have been chasing power and MPG problems with my XJ for 2 years. The single largest improvements I made that improved power and MPG's were new injectors, and a new O2 sensor, and now I think the Muffler was the third, just changed it, still not enough miles to post data on it yet, but had a noticable effect on low end power and pick up. Oh yes the Cat plugged up (broke loose) right after I bought it two years ago, talk about a power loss choke!

 

[Gojeep]

I agree on your thoughts of the sudden rush of cold water once the thermostat opens is causing the fluctuations from having a efficient radiator. The standard thermostat also is assisted by the flow to close so would speed up the re-heating over a balanced thermostat I had. The cure though. It has been suggested to me that maybe drilling some holes in the thermostat to allow some flow at all times will stop the radiator coolant being as cold when the thermostat does open. Down side is that will take longer to warm the engine in the first place. Normally too much flow is also a problem with this technique but have the restrictor in there will stop it having the opposite problem of running too hot! All a balance that I am going to have to find. Maybe just removing the check ball from the stock thermostat maybe enough? Easier to try and can revert afterwards too if needed.

 

[Gojeep]

I just took it for a longer run just now keeping a close eye on what was happening. Once it reached full temp ( 195 ) the first time it would only hold it for around 7 seconds before the thermostat opened and it fell down to about 160. It would then rise up again and the pattern repeated twice every minute. But did noticed it didn't drop as far each time and would hold 195 longer until after about 15 minutes of driving it was stable. So looks like the restrictor with the standard thermostat finally worked as before I could drive for 2 hours and it would just not stabilize. Might still think about making a spacer so I can fit the restrictor before the thermostat and run the R/Shaw thermostat again.
Thanks for all the imput I have been given.