So i changed to a 180 degree t-stat yesterday before i left the shop and didnt get off the lot before check engine light came on. p0138 code downstream o2 sensor. My question is could 12 degrees of temp difference be enough to trip this code? is it running to cold for the plugs now? or could it just be coincidence?

 

nobody knows anything huh? guess i will have to figure it out.

 

They are designed for a 195* tstat. Put the stock spec back in.
My guess would be its running too cold and staying in "warmup" mode, making it run rich. Therefore, causing your oxygen sensor to become fouled.

 

it needs the 195 t-stat put back in,its not letting it get to proper temp for everything to function right

 

Thats what i was afraid you were gonna say. Then tell me how to make it run cooler. please!

 

What temp was it running at with the 195?

 

195 to 210 is normal running temp,whats yours running at?

 

Isn't it funny that a 4.0L has not problem running 15 degrees over normal operating temp but as soon as you try and make it run a little cooler oh no we cant have that. Conspiracy i tell you.

 

188 degrees just cruising with the 180 t-stat. with the stock one it was about 208 just cruising even at 40 degree ambient temp.

 

200-210 is normal. Put the 195 back in.

 

X2

 

oh ok. good bigger aftermarket radiator ideas?

 

I beg to differ......180F t-stat in our '98 XJ and it runs like a top - mid teens mpg in town and low 20's mpg highway. The ScanGauge shows the system enters "closed loop" within a few seconds of cold engine start-up, which is the way it should be with a properly functioning heated O2 sensor(s). ScanGauge shows coolant temp ranges 180F - 188F. I think the downstream O2 code is purely coincidental.

 

dj you were right. i monitored the pids this morning and the o2 is switching fine. plus the code was for a downstream short to voltage and i could nto get it to make any sense that the t-stat change could cause this. by the way the light was not on this morning and i have a scan gauge and it reads exatcly like you said. thanks for all the help everybody.

 

ScanGauge rocks, don't leave home without it.