Radiator block + EOC vs. heat soak

[some_other_dave]

I noticed something interesting yesterday in my CRX.

I had to make a ~20 minute drive during the heat of the day. I have a full block in front of my radiator (but behind the bumper and AC condensor). It helps with warm-up more than aero, I think. The coolant temps were reading a little on the high end of "normal" on my stock gauge when I got near my freeway exit. I turned off the engine and went into EOC.

When I got to the end of my exit, I bump-started the car, and noticed that the coolant temp gauge read noticeably higher. By the time the engine had run for ~30 seconds or so, the temp had come down to lower than it had been on the freeway.

My guess about what happened to cause this:
I think that the radiator continued to drop the temperature of the coolant that was in it. Meanwhile, the presumably smaller amount of coolant that was in the (warm) engine got a chance to heat-soak in there, raising its temp. When I started the engine again, and the coolant circulated, the colder and warmer coolant got mixed together, bringing the overall temp down.

I am wondering if this heat soak effect could do anything bad to the engine while it is off, or when it just starts up again? I suppose you'd really want to avoid extended EOC in super frigid temperatures if your engine bay was really very well insulated (and your radiator not so), to avoid sudden massive temperature changes when can damage metal. But I think that's a really rare circumstance anyway.

-soD

[RobertSmalls]

I've studied this phenomenon on my Scangauge. It's really not much of a suprise. My coolant temperature sensor records the temp at a single point in the water jacket in my intake manifold. That temperature is about 195F. Meanwhile, the normal operating temp of my cylinder lining is probably a few hundred degrees warmer.

When coolant is flowing, the coolant keeps things steady-state, but when you switch off the engine, the water pump stops. Heat from the engine is conducted into the coolant temp sensor, so it reads hotter. But while the engine is off, heat is lost overall (just not at the temp sensor). Once the coolant has a few seconds to equalize the temperatures again, the sensor reads lower.

To switch off the water pump is to subject the car to another cycle of thermal expansion and contraction, just like what happens when you park the car. I wouldn't worry too much about it.

[cfg83]

Hello -

I've seen the same thing. EOC also lowers my HAI temps. I wondered about engine health, but I am guessing that (maybe) it lowers the lifespan of my coolant. A coolant flush is on my todo list.

CarloSW2

[some_other_dave]

Originally Posted by RobertSmalls

When coolant is flowing, the coolant keeps things steady-state, but when you switch off the engine, the water pump stops. Heat from the engine is conducted into the coolant temp sensor, so it reads hotter.

That is the "heat-soak" that I referred to.

But it sounds like none of us think it is much of an issue. Which is definitely good...

-soD
(Gonna fill up today, looking for another good tank!!)

[PaleMelanesian]

I've seen it affect my cooling fan's operation. I EOC, and the coolant at the sensor heats up, triggering the cooling fan to come on. Then, when I bump-start again, the cooler radiator coolant comes in and the fan turns off. It's backwards.

[DonR]

Your engine should be fine. The exact same thing happens when you turn the car off.

[theunchosen]

The only difference between shutting the car down in your garage and running it for EOC purposes. . .

Is the EOC requires it to start going again and suddenly.

You can also notice this on some cars that the pumps are synced to the rpm. going over I-26 in august from Erwin to Asheville My engine gets really hott climbing because its well over a thousand feet in about a mile and it does this 2-3 times. If I push the clutch in at the top of the mountain and coast down engine temps actually increase because the pump slows down and doesn't cool the block.

The danger is thermal elastic deformation. Thermal deformation is much more powerful than strain deformation. The coolant won't likely suffer much of a problem but the engine is no longer transferring heat away. In EOC you aren't running more heat like I do in coasting(they ticket almost everyone speeding for speeding and driving in neutral), but still for the initial few seconds when the engine starts back up the temps are going to soar and your temp gauges won't notice because the super heated fluid is mixing and cooling.

In the summer you don't really need the grill block, but even if your pistons and cylinders expand 1/1000 of an inch each that cylinder needs a sleeve and the piston needs to turned(100 + 50 bucks).