hi guys
i have a 2004 with manual trans,i have about 160k km on it,recently the temp gauge has been doing weird things,i also have a scanguauge that shows coolant temp,when its warming up it goes to about 170 then it will drop to maybe 140 then slowly go up to 160ish then drop again,it seems to drop about 20-30 degrees then go slowly up,on the dash gauge it almost goes to startup temp then close to operating temp then down,sometimes it will go to operating then stay like it should then next startup it will do the same bouncing temp,it never goes above operating temp always below,i think its affecting my mpg as it rarely gets to operating temp,i installed a new tstat and it does same thing,what else could cause this weird issue,?? thanx guys
dave

 

1.
ECT E.ngine C.oolant T.emp sender or it's wiring defective.

2. air in cooling system....gotta burb it...park with nose at least 2ft higher than back end. remove rad cap and run to operating temp....rev engine...look for any air bubbles coming out of rad. fill hole.
new rad. cap-old one probably has leaking torn outer seal
or overflow tube to burb bottle is leaking air...loose on rad neck nipple...hose clamp or 3/8" wire tie on tight.

or loosen one of the head coolant plugs to let the air out, messy.

ECT will not read correct engine temp if exposed in air and not coolant. Cummins location is prone to this problem

 

thanx,i will try that tip,i had a car that would air lock like that,thanx