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Friday, May 9, 2014

Heat soak. Hint: its not a hot relaxing bath...

Names have been changed (repeatedly) to protect the guilty...  
 I have to use my good friend Sam as my example for this one. Its a guy thing, he gets it. Joe called me a few hours ago to ask what happened, what went wrong up at the trailhead. Now, John drives a small sedan- not a big truck- so even the fact that he got up to the trailhead is a testament to his driving skill.  
  From town, the trailhead is about 3 miles up into the mountains and has a gain of about 2000ft in that distance, from 7500ft up to about 9500ft ASL. Its not a big deal for most vehicles since the FS has repaired the road again and Bill drove fine- obviously, he got there. He got all the way up past the leftover snow and up the last steep rocky part to the parking area. Thats where he parked and shut the engine off. 

Only ten seconds later- POP and a puddle of steaming ethylene glycol all over the ground and everything under the hood. “WHY??” he asked me and I immediately visualized what had just occurred. “You paying attention?” I asked in reply, and continued with a quick version of what follows.  You get the full length explanation...

  When the car was cruising up the hill, going nice and slow to avoid the rocks and water diverting swales, ditches and humps, the engine was spinning nice and fast, and working hard. Even when operating properly, a gasoline engine produces 4 units of waste heat for every 1 unit of forward motion producing mechanical energy. As all that heat builds up in the engine coolant, the thermostat opens wide allowing the coolant to flow out of the engine into the radiator, where the air blowing through it can carry the heat away away.  

The coolant is moved through the system by a pump that is driven directly off the engine- the faster the engine spins, the faster that pump spins, thus, the faster the coolant flow. So while the engine is running at high speed and high load, the water is hot and pumping fast through the cooling loop being heated and cooled again rapidly as it circulates. Also, the automatic transmission cools itself through a tube immersed in coolant at the bottom of the radiator, adding more heat to the system.

So the engine is running fast and hot, moving lots of waste heat out the radiator into the air as a heat-trail behind the car... Then the engine gets shut down. The coolant stops circulating, but there is still alot of hot metal transferring heat into the coolant.  This heat gets trapped inside the engine and the pressure and temperature in the cooling system climb, and climb, and climb.

 The term for this scenario is 'heat soak', and it is a normal situation. Hopefully, the relief valve that is the radiator cap will release enough liquid and steam out to the overflow tank to prevent damage from over pressure. Sometimes though, the heat and pressure build up so fast that something else must fail in order to relive the pressure. In a worst case scenario, that pressure release happens in engine gaskets creating permanent water leaks that are pricey to fix. Often though,the cheap plastic radiators which have become the norm are the weak link. 

I have seen radiator caps rip off their plastic mount on the radiator, pressure tanks bloat and explode, hoses blow out, and worst of all, engines never start again after being shut off hot. Over the years, I have fixed several vehicles and explained (after the fact) why an engine, radiator or coolant hoses would blow after a trip up a steep hill. Its not the hill, nor is it a problem with the vehicle. Its a simple lack of understanding of what causes heat soak and how easy it is to eliminate.  

How to mitigate this you ask? It takes about 3 minutes to cool down an engine to safe temperatures after a trip up a hill like that, so don't shut it down right away. Turn off the A/C and hold the engine a tiny bit off idle- about 1200rpm if you have a tachometer. Just hold it there, just a bit faster than it would run normally without touching the skinny pedal. 

 The engine is not doing any work this way and is spinning the pumps a bit faster than baseline, thus shedding waste heat from the engine and out to the air or down into the oilpan. After a few minutes of running like this, the remaining heat will have been circulated out of the metal of the engine. Now its safe to shut down with only minor spot- boiling likely inside the engine which does no harm.  

So... What is 3 minutes worth to you? If you live at the top of a hill, go visit friends or go up hiking in the woods up a steep winding road, keep Greg's minor inconvenience in mind. We'll have him back on the road with a new radiator in a couple days, but others might not get so lucky. Fred and I both hope it saves some of you out there from some degree of annoyance. Cheers and cool rollin... :-)

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