Can engine overheat without overheating coolant?

[Omahaug]

Apologies if the title sounds stupid.

Basically I'm wondering if it's at all possible for an engine to overheat without the coolant overheating. I think most engines measure temperature through coolant (an ECT sensor).

Is it possible for there to be a localized hotspot somewhere (let's say an air bubble) that could overheat the metal and warp it without the coolant being heated?

Or is it a fact that any excess heat localized in a single spot will spread out through the rest of the block ("soak" the heat), before it can do any damage? Therefore the coolant will always heat up to its limit before the block reaches its limit and warps?

I always wondered this, when burping a coolant system some people let the engine idle while some people recommend you rev the engine to 3000rpm or so and let it warm up faster. If the system has air bubbles by the head after a coolant change, can it heat up and distort the head or block if you rev instead of idle it?

 

[Tman220]

There are many engines out there susceptible to air bubbles forming in the coolant system if not properly bled. I cant speak on the specific vehicles, I do have some experience with one in particular that is susceptible.

Chevy's GenII small blocks the L99 (4.3 liter), LT1 and LT4, produced from 1992-1997 (perhaps longer in marine applications, they made it into a few mastercraft ski boats I know)

Anyway, these small blocks unlike previous, and present chevy V8's are reverse flow cooled, meaning the coolant flows through the heads first and then down through the block. As you might imagine this creates a difficult situation for purging air from the system. In stock configurations, air is purged in two ways. 1) two air bleeds, one on the heater lines and one on the thermostat housing. 2) There were formed steel lines running from the back of the cylinder heads, joining them together, then a line ran to the radiator neck area.

Anyway, sometimes people would not bleed the system properly, or the steam lines would clog leading to steam and air bubbles forming in the heads. This usually resulted in blown head gaskets on this engine.

localized hot spots are always a concern in engine design and cooling system design.

So yes I would say that it is possible to have a localized overheat condition without a coolant overheat in certain scenarios.

 

[Nick1994]

Maybe if you don't put coolant in, then the nonexistent coolant won't overheat while the engine is...

 

[edyvw]

Yes!
Especially at high altitude where air is thin and engine cannot be cooled off as fast as on lower altitude.
I think Redline and Motul are even making some products that are addressing that issue you are mentioning.

 

[69GTX]

The issue of bubbles forming along the coolant channels can be aggravated by chemical deposits for one, causing less heat to be transferred to the coolant and resulting in higher engine metal wall temps. Continue the process and eventually you can reach the point of failure in some components. As long as the bubbles are carried away quickly the metal is safe. If a blanketing of steam bubbles consistently covers a particular area, metal failure from overheating could result. Engine designers try to design these things out. Other things that could contribute to the rising metal temps could be degraded or out of spec oil, operating the engine beyond design limits, or coolant that isn't quite up to snuff in removing heat as designed (ie too high a glycol vs. water concentration, sludge or deposits in your coolant, etc.).

I wouldn't think you'd run into this issue at 3,000 rpm during engine warmup. It takes a while at idle to bring an engine to steady state operating temperatures....could take 20-30 minutes or longer. Now an endurance run of 24 hrs straight at 4000-6000 rpm at 110 deg ambient temp could get you there.

 

[Ohle_Manezzini]

Or you could have a pocket of air around the sensor, giving lower readings.

 

[Jetronic]

pockets of air aroud the sensor give higher readings. The sensor is heated by the head material, and cooled by the coolant.

 

[andyd]

Originally Posted By: Jetronic
pockets of air aroud the sensor give higher readings. The sensor is heated by the head material, and cooled by the coolant.
air doesn't conduct heat as well as water Temp sensors are insulated. Only the tip in the water passage conducts

 

[eljefino]

Originally Posted By: Ohle_Manezzini
Or you could have a pocket of air around the sensor, giving lower readings.


Yup this. And they often put the sensor behind the thermostat, so the thermostat has to open to burp the air bubble. And the 'stat needs hot coolant, not hot air, to (best) activate its pellet.

 

[andyd]

Over heats can be created by extremes in fuel mixture,timing, low oil.all kindsa things. Temp gauges will read lower in a boiled dry head AMHIK

 

[Carbuff]

I always thought that the practice of higher revs after a coolant change was to purge the air pockets due to the higher flow rate. Not only to warm it up faster...

And regards reverse-flow cooling systems: On my Hondas, I have often needed to turn off the engine after running it for 5 minutes or so, to allow the thermostat to heat-soak and open up. Otherwise, you can see really high temps on the OBDII readout 'cause it is reading 'air'.

 

[edwardh1]

many oem thermostats have a small hole in the plate to allow air to pass, many aftermarket thermostats donts

 

[ecotourist]

I'm told that some BMW engines have had a catastophic coolant leak and have overheated without the temperature gauge rising - the temperature gauge doesn't see hot coolant because the coolant is gone. Never seen it myself.

I don't see why this problem would be limited to BMWs either. If you had the lower radiator hose fitting break off the radiator, you'd lose the coolant pretty fast.

Just one of those rare things you hope doesn't happen to you.

 

[DoubleWasp]

I've witnessed plenty of airlocked engines get solar hot and not reflect it in the temp gauge. Definitely possible.

 

[Omahaug]

Originally Posted By: eljefino
Originally Posted By: Ohle_Manezzini
Or you could have a pocket of air around the sensor, giving lower readings.


Yup this. And they often put the sensor behind the thermostat, so the thermostat has to open to burp the air bubble. And the 'stat needs hot coolant, not hot air, to (best) activate its pellet.

But behind the thermostat is coolant right? The air bubble would be in front of the thermostat in the lower radiator hose? So wouldn't the sensor be "accurate" since it's immersed in coolant?

So I wonder then if it's the best practice to just not rev at all? The problem is I waited almost thirty minutes and the fans wouldn't run so I don't believe the engine got up to temp. But if I rev the engine I can get the fans to run and the engine to hit operating temperature fairly quickly. I'm torn on what to do, wait an hour or just rev it?

I also don't understand how air bubbles in the head even make their way out. Is the pump strong enough to move air bubbles and not just circulate the coolant under the air bubbles? How do engines "self-bleed"? My engine for example has no bleeder bolt on the engine. Some people don't even bleed, they just squeeze the hoses and then drive it. If there were air bubbles in there wouldn't that cause a problem?

I'm beginning to wonder whether I've been doing it wrong the entire time. I always thought you were supposed to rev it to get it up to temp as fast as possible. I never thought it was to chop up air bubbles or move them or something.

 

[SHOZ]

I've seen the coolant so low it doesn't register as hot as it really is. It was when there was a sudden loss of coolant, yet the gauge read near normal. But factory gauges are not too reliable.

If the system is free of all leaks the air will self purge over several cycles. But all it takes is a minor leak to stop that from working.

 

[69GTX]

The coolant temp sensor on one of my cars is located in the upper 1/3 of the radiator. It would take a pretty large bubble to occupy that entire upper third, or approx 1 to 1-1/2 quarts of fluid.