So for a little while now I’ve been having some sporadic coolant heating issues, but not quite overheating.

Symptoms:

• Rising temps seen into the 209°-211° range.
• Worst seen temps into the 213°-215° range with coolant “boiling” over into reservoir.
• Every day driving sporadic temps between 198°-207°.
• Long idle (roughly 10 min in park) after high temps eventually goes back down to 198°-200° range
• Idle (in drive, on brakes) to acceleration, temp will drop.

Causes:

• Driving up hills: short hills temp range 204°-207° & long hills temp range 207°-211°
• Long hills with high temps towards 211° or hotter usually result in “bubbling” over flow of coolant into reservoir.
• Long stretch of graded road causing lower gear to be needed.
• Stopping at red lights temp increases but decreases at acceleration by as much as 5°

Notes:

• All temps read with Scangauge II
• TRD Trans cooler – inline (installed years ago)
• New Toyota brand radiator cap
• Toyota brand radiator around 3 years old
• Toyota brand fan clutch around 3 years old
• Valve cover gaskets done (after issue already persisted). No evidence of mixing fluids.
• Coolant idle test done with coolant funnel kit from cold start w/ heat running to final temp around 195° - no issues
• Toyota brand coolant
• Oil on dipstick looks normal (but due for an oil change)

I really don’t know what else to look for. Aside from taking it to Toyota and having them do a professional pressure test on the radiator and flush fluids. The “bubbling” or “boiling” fluid into the reservoir at around 213° is a concern as far as head gasket issues but it didn’t do any kind of burping or bubbling when idle test was done. Also, a 50/50 mixture boiling point is around 260° or so, so how could it be “boiling” over. I know the Scanguage isn’t messed up b/c my gauge on the cluster never shows it’s in any danger of overheating. Which technically 213° isn't overheating, but not necessarily normal. Should I try a leak-down or compression test for shits and giggles? I never have any troubles starting the truck. It operates normal aside from this issue. What am I missing?!?

What about the water pump and thermostat?

Notice any loss in coolant?

No noticeable loss in coolant, but occasionally I can smell coolant in the garage when I pull it in and get out to walk to the house door. Tbelt, water pump, thermostat, etc were done around 25k ago.

I also notice the coolant smell on startup. Next step IMO would be get a pressure tester from a parts store, you can rent/borrow it and do the test yourself. Would be a lot cheaper.

Sounds like a stuck thermostat, not letting the fluid flow and is building pressure. Only place it can release is through the overflow tank. Also if the cap is bad it can cause this. I know just replaced but I would look here. If you have the old cap I would do a quick test if it was not bad at the time of replacement.

Makes sense. More coolant rushes to cool on hills in lower gear I'm guessing and the thermostat gets stuck trying to keep up?

With the cap off and the engine running are bubbles present in the fluid? It sounds to me like something is clogged.

As @JayRolla said, rent or buy a pressure tester. It will take the guesswork out of the equation.

A leak down test may also be in order.

I'd check to see if your radiator cap is intact first. I helped another member recently address high coolant temps and his radiator cap was missing parts and not holding pressure.

It's possible your thermostat jiggle valve is in the wrong position, but I guess it depends when the overheating issues started. If you've been monitoring coolant temps for a while and were running much cooler before, maybe it is the thermostat sticking. BTW, the jiggle valve needs to be in the 6 0'clock position, pointing down towards the ground.

he just changed his cap to a toyota cap recently, and symptoms still subsides. I would not change that over again.

Rad and fan are also new.

Only thing remaining that relatively easy and cheap is the thermostat. Do this and update.

The fact that it cools down during light acceleration points to a underflow of coolant through the radiator for whatever reason.

Same exact Temps as mine.

This makes a huge difference and make sure to "burp" it on an incline.

I have this same problem, very very similar in temps. I tested my coolant for exhaust gasses and found that I am getting some combustion byproducts in my coolant, causing the temp to do weird things and the reservoir to boil, but never truly overheat.
I tested using block test fluid as well as a neighboring shops combustion leak tester. I believe I have a breach in my head gaskets causing a mix of the two. I hope this isn't the case with you, but it is a possibility.

Have the same symptoms just as described with my 96, but only when I'm near and in Colorado. Doesn't happen here in MO. Have not changed the thermostat as the timing belt is due soon and am planning to do it then. Did have an oil analysis done, no coolant in oil. Can't see any oil on rad cap or in coolant. Have not done a pressure test yet. No noticeable loss of coolant or coolant leaks. Newish radiator and rad cap and have cleaned the condensor/radiator fins. Manual trans so no tranny cooler. Have a coolant temp gauge is how I know the temps.

Interested in what this turns out to be, obviously. Since I can't get it to recreate here I'll have to live vicariously. This is me sub'ing.

my temps hit 96C to 98C Almost boiling while reading scan gauge on a hot 95 degree day. Seems @mtbtim made a good point about the jiggle valve. I wondering if that is the issue.

after doing some reading here, I thought these temps were okay for our 4runner. As long as your radiator cap is holding pressure it shouldn't overheat exactly. or are at these temps we are playing with fire?

I am wondering if anyone has change the position of the jiggle valve and noticed the ~10 degree drop. I heard this a few times, just not see the source.

Lots of posts here on the jiggle valve flip to 6 o'clock position & results. I personally have seen the 10* temp reading drop but it still defies conventional logic.

Sent from my SM-N900W8 using Tapatalk

What people are missing is his temps are not that high. 230 and above is when damage can happen. I would hit 230 in my hopped up turbos and was never boiling. He is getting excessive pressure. Usually stuck thermostat, bad cap. Clogged radiator or trans cooler. Or the worst failing head gasket.