Coolant Temperature - random spike

This site may earn a commission from merchant affiliate
links, including eBay, Amazon, Skimlinks, and others.

Joined
Jun 25, 2003
Threads
701
Messages
4,176
Location
Rixeyville, VA
Searched but couldn’t find a similar story.

Driving home from work yesterday, I had been driving for ~40 minutes with no issues (mostly highway).

At one point, I come to a stop light and am still for 1-2 minutes. A/C is blowing cold all this time. I pull out onto a local highway and with 1/2 to 3/4 throttle I row through the gears (4500 rpm max) to get up to 65 mph.
I immediately notice my ac is blowing hot. Temp gauge is pegged in the red.

As quickly as I put my eyes on the gauge, I notice that within 5 seconds it falls from its pegged position to halfway; AC comes back.

The remainder of the trip I try and replicate the situation but cannot. I stop at a parking lot and let it set for several minutes with AC blowing to see if temps will rise; nothing.

How quickly the temp gauge fell, makes me wonder if it’s just a faulty coolant temp gauge and there is really not underlying issue.

Outside temps were about 95*. Truck has 210k miles, small lift and 33’s.

No visible radiator leaking of course. Today I will make sure coolant is topped off and add scan gauge tool so I can see more accurate temps if it repeats.
 
I’m not in a place to check the wiring diagrams, but an important detail would be whether our engines have separate temp senders for the ECU and water temp gauge like Toyota’s used to be set up. If so, it is very unlikely this was a gauge fluke as the ecu shouldn’t have shut off the AC. However if there is no separate sender for the gauge and it is just relayed info from the ECU, a sender/wiring issue is possible.
 
I’ll recheck belt condition but it’s about 3 years and 80k miles old. Replaced with OEM back then.
 
What I suspected to be the original belt on my LC failed at 99k. I plan on replacing for the second time at 150/160k, so to me 80k on a belt is a good bit. From a purely mechanical perspective, your scenario sounds like you just put the system through a test with the 0-65 and a/c on, nothing it shouldnt be able to handle but possibly is showing you there's a weak link when you ask for a little more than usual. Belt, tensioner and thermostat are cheap enough to eliminate quickly at least. Hopefully its just a belt slipping/stretching from what you asked of it. If its a electrical/computer issue I have nothing to suggest. Curious to see what it is. Good luck
 
At 210 if you do a belt I’d also do a fan bearing bracket. At 150 mine was just starting to make noise.
I think if it were contributing to your problem you’d know, as it would be making noise or something.. but with your mileage you should be thinking about it.
 
Ran some errands and same thing repeated. Got home and sitting in park, I could rev the engine and watch the needle creep upwards.

Got a scanner on it and with the temp needle at about 3/4, I was 231*. Still odd how quickly it came back down though, and totally stabilized.

Will likely follow suggestion for belt, T-state and any other easy items.
 
Coolant in the reservoir with hose hooked up properly?
Level at the top of the radiator when cap removed?

Possibly an air bubble forming then moving around?

Either way this is not a common issue. Please keep the board informed with what you find.
 
Reservoir was empty this AM so I filled it up before I left. When I got home and popped the hood, it hadn’t taken any of the fluid. Hose hooked up properly but will check again.

Once again cools will check radiator cap.

Will keep thread updated since the random spiking and then stabilizing is the funky part about it all.
 
Here’s some extra data. Just went out to truck. It’s been off for 15-20 but still hot.

Start it up and let it idle with AC running. After 5-6 minutes it’s fine and holding at 203*. I get in, rev the motor and hold the rpm’s at almost 3k with AC going. Temps totally stable. As soon as I let off the gas, temps steadily rise to 248* in at most, 30 seconds. Put my foot back on the gas and hold 1500rpms and temps plummet to 200* in less the 10 seconds.

Not sure if that would/could indicate anything else like fan clutch, water pump (replaced at 60k), etc.
 
I was thinking faulty thermostat not opening properly but water pump not circulating flow properly fits the symptoms also now. When was the last time the coolant system was re-done? Could be a combo

I missed you said 60k. Is that 60K or 150k ago?

A failing fan clutch can affect A/C operation so sh**, any of the main components of the cooling system may be going. If that jives with your timeline then you know what to do.
 
Last edited:
Water pump shows to be replaced at 60k by dealership. I bought it at 120k miles and knock on wood, have never had to touch the cooling system, heck anything on the truck for that matter (except normal maintenance).

Maybe it’s time to bite the bullet and do them all.
 
If your fan clutch is slow to respond for some reason it may account for this.

Either way with the temps you are seeing a major cooling system rehab is in order. I'd plan for some down time if you don't want to pull parts and inspect before replacing, or just have fan clutch, water pump, fan bearing bracket, thermostat, idler, tensioner, belt, and fresh coolant ordered and ready to go in. Possibly even radiator too.

One thing to consider is whether damage to the engine has already occurred. Maybe look into testing coolant for exhaust gasses. If you are looking at a head gasket or something that may change the equation. I'm not as familiar with high temps on toyota engines but this is all aluminum, and that helps a bit due to head/block expansion rates being similar. I know LS3 guys aren't too worried about 230F on track. But almost 250 is a different story.
 
The speed of the spikes (seconds) seems to indicate a coolant circulation issue. It‘s not good the reservoir was empty; that would indicate something serious. I’m still mulling over these unusual symptoms, but I should mention that when the head gasket blew in my 80, the symptoms had some similarity
 
More data.
Let engine cool. Removed radiator cap (good suction and lots of bubbling in reservoir as I removed). Took almost one half gallon of fluid. Restarted and let idle with AC running full blast form 5 minutes. Temps now steady at 185 f.

Tried to replicate earlier scenarios with high revving and did not go over 195f. Put in 1st gear, 4 lo and drove around in a field with ac blasting. Again, did not go above 195f.

Obviously nothing definitive here but shame on me for not checking coolant level since it did take so much. Could be sign of other issue but wanted to provide update.

Oil levels are good and inside of cap is normal; nothing milky if that’s what you mean.
 
Last edited:
The speed of the spikes (seconds) seems to indicate a coolant circulation issue.
I didn’t say that explicitly but that was my gut as well. A thermostat just doesn’t go from closed to wide open that fast.

OP look into the coolant valley leak that happens on some of our trucks. If it’s not leaking out the water pump and not the obvious radiator crack that would be my next step in diagnosis. Hopefully it hasn’t been burning it.
 
I didn’t say that explicitly but that was my gut as well. A thermostat just doesn’t go from closed to wide open that fast.

OP look into the coolant valley leak that happens on some of our trucks. If it’s not leaking out the water pump and not the obvious radiator crack that would be my next step in diagnosis. Hopefully it hasn’t been burning it.
This^^^^^. It would explain everything. Loss of coolant/air in system/not easily visible/fast temp spikes/known (but rare) issue.
 
About 30k mike ago, I noticed a few traces of pink fluid “staining” on the transmission, while I was in the process doing a drain and fill. Should have done something with it but since everything else seemed fine and there was no dripping, I just decided to keep an eye on it.

Trying to get any other visible proof that it is the valley leak seems next to impossible though. Even with a flash light and climbing all over the engine bay I can’t spot a thing. Seems like a likely culprit though.
 
About 30k mike ago, I noticed a few traces of pink fluid “staining” on the transmission, while I was in the process doing a drain and fill. Should have done something with it but since everything else seemed fine and there was no dripping, I just decided to keep an eye on it.

Trying to get any other visible proof that it is the valley leak seems next to impossible though. Even with a flash light and climbing all over the engine bay I can’t spot a thing. Seems like a likely culprit though.
You might need to dig out some of the foam block under the front of the intake manifold around the PCV hose and valve. It’s not expensive to replace but does require pulling the manifold. It’ll give you a good look at the valley with a small flashlight though.
 

Users who are viewing this thread

Back
Top Bottom