I did a test with a used and new thermostat after seeing my coolant temp increased with the new thermostat. I heated up water to around 180 degrees in a pot on my stove and placed both thermostats into the water. I then took them out and measured how far each one opened. The old one opened further than the new one, which is understandable because the old thermostat spring having cycled thousands of times had fatigued somewhat and the spring wasn't as strong. So, what did I do? I put the old thermostat back in and kept the new one as a spare. The old thermostat is allowing a larger flow of coolant past it and thus is keeping my engine cooler.

Unlike you, I've seen pretty high temps, but my rig is WAYYYY heavier than stock with a full belly of skids, aftermarket bumpers, roof rack, winch, etc. The engine has to work harder and when I'm climbing grades and/or driving in hot ambient temps, my coolant temp will rise much higher than 193. There's nothing cooling system wise that I haven't addressed. With the Koyo aluminum radiator, I finally feel I have my cooling system under control. The aluminum radiator does make a difference in the cooling efficiency. I've personally witnessed it on my rig.

I'll just add these before and after shots because I have nowhere else to put them. I was noticing temps creeping up over a month (194+ in town) and replaced the fan clutch. Didn't help a bit and things got worse. The Before shot is creeping up to a red light at midnight (58 degrees and foggy). The system was heating the ATF (in-series cooler) too! The After shot is a couple of days later flogging it on I-80 from SF to Davis and back in nearly 90 degree ambient temps. The the only thing I did was I parked it on a very steep SF hill and burped it. It took what seemed like 10 minutes but it finally ejected a flurry of bubbles. The 185 shown in the After pic was a bit of an anomaly but I couldn't believe it (I was climbing a bit of a grade). It now runs at 186-188 around town and ATF is right on spec at 150-175. Crazy. Still going to get a Koyorad one day. I still have one eye on the gauge because it's too good to be true.....

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Just curious how come a OEM radiator cap is so important? I have one of those caps with the pressure relief lever on mine I got off rock auto. Seems to work fine. Although this summer now I have a scangauge I see my coolant temps now. Early this year during spring when it was cooler outside it would run 188 solid never change based on driving. Now in summer it sits 190 normally and 191 on the freeway cruising 75 with the ac. I see it spike up sometimes though and it has been concerning me. It goes up to 193 and even 195 sometimes. I've seen 197 when I was going up a hill. I have replaced everything possible related to the coolant system, fan clutch, new raditaor & coolant, water pump, temp sensor, etc. It usually only gets that hot when I run the ac. lol my engine gets worked when I run the ac and have to go up a hill. but anyways. Also a second question. I bought a koyo rad full alu. radiator a while back and planning on installing that but my 1998 4runner has green coolant and i was wondering if i should ever get that flushed out for red / also if i should before i install the koyo aluminum radiator. Not sure. I know toyota switched to red sometime around 1998 right. But green is just as good right? thanks

First the easy one - with green coolant, you need to be VERY diligent about coolant changes (like, every 2 years). Red can go much longer before causing problems. Both cool equally, but the Toyota Red is MUCH better for anti-corrosion in the long term. The official interval for the Toyota red is 2 years / 36k miles but it does last longer than that...

Many/most aftermarket caps leak or don't hold pressure correctly. No pressure = localized boiling of the coolant (around the cylinders) and poor cooling system performance and overheating.

"Spiking" to 197*F is a non-issue. Don't worry about it. The factory gauge doesn't even move from the middle (by design of Toyota) until something like 215*F, which means temps below that are fine. Knowing your exact temperature is helpful for keeping an eye on your cooling system for small issues, but you are still VERY safe.

-Charlie

Good to know. See this is why I come to you for info like this. You know so much haha. Sounds like I will be ordering a oem radiator cap then. Can't hurt. Also good to know about the coolant. I am fine with green and i am meticulous with my 4runner anyways so every 2 years changing out coolant is fine haha. i did my new radiator when I first got my 4runner then had to fix a coolant leak when I did my vc gaskets and then little later after that did my timing belt. I've drained my coolant like 6 times within a year for other jobs too and always just put new green stuff in didn't re use the old stuff so I am probably good for a while I would say. When I put the new koyo aluminum radiator in ill fill it up with all new green stuff too. good to know about the temps too. makes sense why they have that range where it doesn't move. prevents it from going up and down and freaking you out lol. But that's why i got the scangauge so I could see that. Just wanted to make sure not trying to crack a head at 205k haha would also be inconvenient since i have work every day to. I rely on her haha.

Thanks as always charlie