Overheating: sometimes it _is_ the thermostat (1 Viewer)

Mar 20, 2007
'94 Land Cruiser with 226K. Started running hot--near red line with RavenTai temp gauge mod. Mostly on the freeway but also around town.

I had already modded the fan clutch, but that was 50K miles ago so I got a new blue fan clutch and modded it with 15K (I think) fluid. No change at all.

After much worrying about a clogged radiator or head gasket problems I changed the thermostat (new Toyota thermostat).

Now: runs perfect, right in the middle of the temp gauge for normal driving.

Just posting this as very few threads pointed to bad thermostats. By the way, the old one looks perfect (not corroded or nasty)...
Dec 10, 2007
Also had this happen, at first was concerned that the head gasket had popped but found the Tstat would only open about 1/8" when put in a pot of boiling water. Added to a rusty partially clogged system it was enough to overheat on the highway.

Seems like the Tstat should be a routine PM (after initial PM when buying an 80 of course and no service history). Maybe replace every ?10 years/100,000 miles whether the Tstat appears to be working well or not, schedule to do it in conjunction with flushing the cooling system??
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Irish Reiver

Oct 24, 2015
Orlando, Florida
I replaced the thermosat a few weeks back as part of some other cooling system maintenance. I didn't have an issues beforehand and had replaced the thermosat about 30k miles earlier but since i was in there .... I had an OEM thermosat on the shelf and like everyone should, i tested it before installing it. It should start to open at 84c. It was 96c before it budged. I ordered 2 more 1 each from different vendors and when they arrived i tested them all. No two opened at the same temp and none started opening before 88c except the original which opened at 86c. I was reluctant to use the old one so i installed the one that opened at 88c. Been solid so far and i doubt if it will be a problem in the summer but it does prove that there are significant tolerance issues with these units. Moral of the story is to make sure you test before you install.


Waiting for the Great Pumpkin
Mar 29, 2003
Hunh. That’s a new one by me & I’ve seen plenty of fail t-stats — but they all fail open (or has been my experience)

I was given the Shaman lesson — (way back when he was a parts pimp & we paid to keep him in Keystones & dog food :cool: ) — to always run real ‘Yota t-stats early on, so all 3) 1FZ‘s I had/have all got that familiar red box one.

Frankly, I’d go 1 more O/C-D than @Kernal & say 50K / 5yrs, whichever you hit first.

But that’s just me - a ~$30 OE t-stat is cheap compared to if these can get sticky/fail in a closed style position.

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