T-Stat

1989 4.0 XJ. On Sunday as I was completing a 400 mi. trip, coolant temperature went up from 190 to 220 on my mechanical gauge. The conditions, load, etc. do not explain the change. I diagnosed it as a bad t-stat, which is about the only thing I hadn't replaced (157K miles). I changed the stat and noticed that it had a 180 degree stat in it. So as not to add another variable to the problem, I went back with a Stant 180 degree. Today with temps in the 90's and the AC on max, the coolant temp stays at 180 to 185 and hits 190 at stoplights in D with AC on. The T-stat was the problem.

Should I go back in and swap out the 180 stat for the OEM 195 stat? Will running a 185 temp keep me from going into the normal (is it open loop?) mode and thereby cost me mileage?

Pretty much the only effect that putting in a hotter thermostat will have is that it will open a bit later. Remember to put the littel vent hole at the 12O'clock position and burp the sytems

Rich, thanks for the reply. I remember reading this view that the t-stat will not affect the eventual operating temp if you run the engine long enough and agreeing with it because it made sense. What I am seeing in practice however, is that actual operating temp is shy of the 195 I'm told we should operate at. If I run at 185 instead of 195 will this hurt mileage, power, etc or is this a negligible difference? If it's no big deal - I'll wait until this winter when I put in the Moroso bottle. What do you think? Anybody seen a difference when they swapped t-stats?

I did have a little trouble flushing air out of the system. It is solved now by removing top radiator hose, shoving a garden hose in, holding up the end of the radiator hose to achieve an elevation higher than the coolant bottle, and waiting until water spills out the open coolant bottle. Works great, but I would rather run distilled water.

XJTripp said:

Rich, thanks for the reply. I remember reading this view that the t-stat will not affect the eventual operating temp if you run the engine long enough and agreeing with it because it made sense. What I am seeing in practice however, is that actual operating temp is shy of the 195 I'm told we should operate at. If I run at 185 instead of 195 will this hurt mileage, power, etc or is this a negligible difference? If it's no big deal - I'll wait until this winter when I put in the Moroso bottle. What do you think? Anybody seen a difference when they swapped t-stats?

I did have a little trouble flushing air out of the system. It is solved now by removing top radiator hose, shoving a garden hose in, holding up the end of the radiator hose to achieve an elevation higher than the coolant bottle, and waiting until water spills out the open coolant bottle. Works great, but I would rather run distilled water.

Click to expand...

we're talking +-10 degrees.... from a factory gauge...

I wouldn't worry about it. your probably running at 210 like it supposed to anyway. the cooler Tstat just takes a little longer to get there.