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Overheat issues . . .

2620 Views 22 Replies 7 Participants Last post by  TomLine

My 2000 Sienna has been good to me. Have driven her last 30K miles without any issue.

I drive about 30 miles a day.
On my way home from work, noticed that my temp gauge was climbing above the middle. By the time I made it up the hill, it was around 80-85% to the top.

I left the car cool off for about 2 hours. I returned back to the car and saw that the expansion tank for the coolant was empty. I concluded that I must have a small coolant leak somewhere, and that I've finally lost enough coolant to cause me trouble. I filled up the expansion tank to the full line, and drove around for 10 mins.

It seemed fine for about 10 mins, then the temperature gauge started climbing above the middle, and started overheating again.

I turned on the heat inside the cabin to max, and turned on the fans to max (while the temperature gauge was 75%), but nothing but cold air was coming through the vents. Next, I parked the car and opened the hood to expedite the cooling process. At this point, I noticed that the expansion tank was still filled to the full line.

Also, I noticed that the electric fans behind the radiators were not spinning, while the engine was overheating at 75%+.

I scanned the car with an OBD-2 scanner. Interestingly enough, right after the scan completed, the electric fans behind the radiators actually turned on for about a minute, and then turned off. The fans turning on was surprising, because it happened when the car was shut off. Car engine was off, but they key was in the turned position, right before cranking the starter.

What's going on here? Feels like it's a thermostat that's stuck closed ?


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I think I have a similar situation as you.... engine overheat....turn heat on and sometimes heat comes on and temp goes down, other times just cold air..... but if I play with temp dial from cold to heat....the cold air turns to heat and temp gauge goes down.... its weird..... but I haven't had real chance to look yet.... its work in progress
it happens worse on stop and go traffic.....better when car is frwy going...typical coolant behavior. The radiator fan works so that's not the turns on when it overheats...

Does the temp dial switch have something to do???

I did feel the radiator hoses... and top one is warm or hot.... but the other one to thermostat is cold or cool to touch without hot burning.... so I think its the thermostat going bad....or something else... might help if I burp the air out of the radiator coolant.... I do know there's a small leak going into the valley at top 2 valve cover gasket area...

My plan is to change out the thermostat, gasket and see if the water pipe to thermostat is cracked or not. and then whole coolant replace, flush......and then it will be time for the timing belt whole replacement again....its at 270k miles...last time I replaced was at 150k its due for it... still curious about the gasket coolant leak at top 2 valve cover area....called the valley....but I don't think its the cause of overheat...... wondering if water pump failure....don't think so.... either will be down for while for maintenance and repair...good thing I got spare car.....

I hope the coolant didn;t mix with transmission oil to make milky transmission oil that plagues some toyotas....
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