Blogs Car Info Our Show Deals Mechanics Files Vehicle Donation

Driving Downhill Causes Engine Cool Down

I have a 2002 Toyota Corolla 1.8L engine, bought used 6 months ago. I have an aprox. 6.5 mile drive to town. The first 3.5 mile portion of the drive is flat or slight uphill terrain. The car is generally fully warmed up at this point except in colder weather. The last portion of the drive is downhill, initially a slight grade to fairly steep grade. As soon as the downgrade starts, the car begins to cool down. The colder the outside temp and the steeper the grade, the faster the cool down and the colder the engine gets.

I took the car back to the dealer. Initially they said it was low on coolant. It was topped off, and I still got the cooling phenomena. Next was thermostat replacement. No change. This has been going on for 6 months. In really cold weather, by the time I get to town, after 6 plus miles of driving, the car is almost completely cooled back down, complete w/ meager heat and fast idle. No check engine light or anything, and after driving on the flats the car warms back up. I don’t see much of this phenomena when the temp outside is above 70. Other than that, the car runs well, gets great milage (41 MPG Highway). It does have a couple other oddities: occasional jump in idle speed at stops, weird engine speed increase/slight racing when I upshift (5 speed manual). Whisky Tango Foxtrot?

The thermostat is what should control the temperature of the coolant in the engine block. If it was replaced, maybe the replacement is bad as well. Or, the heater in this car is super efficient, and causing the cooling effect by pulling a lot of heat out of the engine. But, I’d check the thermostat again. Was the other thermostat replaced with a flush-n-fill of the coolant? If this wasn’t done yet, your overdue. I’d suggest replacing the radiator cap as well.

A lot of modern cars completely shut off the fuel injectors when the car is coasting and the engine doing nothing but braking the car. The engine will cool off while this is happening.

All things are possible, but: the car initially warms up normally, so I think the thermostat is fine. Heater - I don’t even crank it up high, I don’t think it’s super efficient, and regardless I would think the thermostat would still regulate the temp. All work was done at a Toyota dealership, I bought the car used from them and they really went above and beyond a couple other items, I tend to trust them and the workmanship. They did this as warranty work. I appreciate the input, but there is something really unusual about this. System, both radiator and overflow topped off. It’s only downhill. I have 2 different routes to town, differing degrees of slope. Steeper Slope = quicker cool down. And during winter, cools to point of near complete cold start. Weird. Believe radiator was flushed and filled.

The dealership said that the car should not be cooling down. I doubt my car completely shuts off it’s fuel injectors, and I haven’t kept up on things as I should have since high school auto mech, but my gut feeling is this is not the cause. And if this was what was going on, the car would still cool down when it’s 60-70 degrees outside, as the normal operating temp of the coolant is 212 Farenheit. Although I suppose friction of the pistons alone causes much heat. It’s as though the thermostat is wide open, but only when going down hill. On the flats and going up hill, all behaves normally.

Years ago, after climbing a high peak and the engine temperature near the danger mark, when we hit the downhill descent, we would pull out the hand choke. The raw gasoline hitting the piston tops would serve as a refrigerant and as it evaporated, and bring down the engine temperature. The vehicle, was, of course in gear and not coasting. We don’t have hand chokes or carburetors today, but the only thing I can think is that too much fuel is entering the cylinders and not burning off, thus duplicating what we did in supplying an over-rich mixture by pulling out the choke.

When it’s 60 or 70 outside, you probably aren’t running the heater. The heater is another radiator for your engine that does not have a thermostat.

Deceleration fuel cut off AKA DFCO is a common feature in fuel injected cars. When I’m coasting towards a red light in my Toyota Yaris, I can feel the DFCO switch to normal idle at about 17 mph in fifth gear. It’s like the engine braking suddenly decreases at that point. I believe that the idle air bypass also closes during DFCO to keep the engine from pumping a whole lot of unburned air through the catylitic converter so it will stay hot.

Very unlikely I suspect. This is a modern vehicle w/ everything controlled electronically. No check engine light or anything like that.

Regardless of any of this, I have never seen a vehicle go from normal operating temp. (gauge half way between C and H) to almost bottoming out at C just from going down a hill. And neither has the dealer. After 3 visits, I gave up. Most of the time during the week I live in a flat area, and now it’s warmer, so I never see the problem. It is a very pronounced, unusual phenomena, not explained in my opinion, by fuel cut out or heater use. It’s as though the thermostat is not even installed, but only when pointed downhill.

You seem to shoot down anyone who tries to help you. I guess I’ll try anyway.

You were low on coolant and the mechanic had to discover it for you? Where did the coolant go? If it is leaking, did you fix the leak? What is the level of the coolant now?

Regarding BustedKnuckles’ suggestions, I think you should reconsider his ideas. Modern thermostats fail in the open position. I also suggest replacing the radiator cap. Like the thermostat, It doesn’t cost much to rule it out.

Not meaning to shoot anyone down, I appreciate the suggestions. I’ve been pondering the cap suggestion. Re: low level of coolant - they stated it was about a cup low. I did not think that would be a significant amount low to cause such a pronounced effect. The car had about 75,000 miles at that point, don’t really know it’s history but it appeared clean and well maintained. One cup low over time doesn’t seem significant. They “discovered” it because I mentioned the cool down problem, they surmised that low coolant must be the cause. The coolant has maintained a steady level since, I’ve been checking. When that didn’t fix the problem, they said it was the thermostat. They replaced, refilled, and I still have the problem.
I am aware in my driving experience that vehicles temps will fluctuate w/ load, uphill, downhill, outside temp, so I do understand where some of these suggestions come from. But the degree of cool down is stunning, abnormal.
And I do think BustedKnuckles suggestion is sound, as are others. That said, while I know that it’s possible the new thermostat was bad from day one, I find it unlikely, if for no other reason the car warms up quickly on flat terrain and stays steady (except for downhill). I’ll probably replace the cap, but I don’t really suspect this is the problem either, because I’d think I’d have boil over issues. But it’s worth a shot.
And I really do appreciate the suggestions. My responses weren’t meant to shoot anyone down, I was hoping it would inspire additional suggestions.

I suspect the thermostat too. When going down a long down slope the motor is essentially not firing, and not generating any heat. Even if the thermostat closes, if there is no heat being generated then the motor will cool off as the heater core (small radiator) is taking away more heat than the motor is generating.

To test, next time on a cold day coast down the hill with the motor running either in nuetral or with the clutch in. An idling motor has to generate some heat and you should not experience the cooling off of the motor.

If it is not the thermostat sticking open then you just have a very efficent cooling system in you car. This maybe a characteristic of this car that you just need to accept and live with it.

Modern engine blocks are designed to hold very little coolant, gone are the days when a car’s cooling system held neary four gallons of coolant. The 1.2 liter four cylinder engine in my motorcycle holds less than a gallon of coolant in the entire system.

The reason they hold so little coolant is so they will warm up fast so that emission standards are more easily met. An engine that warms up fast also cools down fast, even with only the heater cooling it.

The original poster does seem to have an unusually long downhill in his commute. A two+ mile section where the engine is coasting in DFCO mode will certainly make the temperature drop even if nothing is wrong with the car.

There just may be nothing wrong with your car but you might try a new thermostat just to be sure.

If the heater is set to “defrost” then the AC compressor is running cooling the air, which the heater has to rewarm and that turns on the electric fan to cool the radiator and evaporator. This maybe part of the problem with cool air in the cabin coming from the heater.

I find it strange that anyone thought that low coolant might lead to excessive cooling. That seems counter-intuitive to me.

Has anyone explored that your temperature gauge or its sensor might be faulty? I would want to confirm the engine is really cooling down and that you are not getting a false reading.

I just thought of another idea. Perhaps your radiator fan isn’t cycling off. If it is running constantly instead of cycling on and off, it might cause this symptom.

When the temp is below normal stop and check the top radiator hose. It should be cold. Otherwise the thermostat is not fully closing or it’s missing a gasket to keep coolant from seeping around it.

Otherwise, I can imagine a lot of frigid air blowing around a small, aluminum engine that’s un-fueled and cooling it off noticeably in a few minutes.

My 2006 Matrix uses the same 1.8L engine, and if it’s below freezing in stop and go traffic turning the heat on medium or high fan will make the temp gauge drop a little.

Small, efficient engines can indeed be cooled down simply by operating the heater. The heater is removing BTU’s from the coolant faster than the fuel-starved engine is replacing them…

Original poster here. I do appreciate all replies, BTW. To respond to the last few comments: I asked the dealer if this was a characteristic of the car or if it was unusual. They seemed to think it was pretty unusual, the amount of temp. drop. They could be full of it, but, I asked. The fan is not running constantly, I have checked. The temp of the engine is really dropping, I’ve checked the hose when this occurs and it’s cold/cool. The engine goes through the entire warm up routine at the flats, i.e., takes the heater a while to get fully hot, idle RPMs increased. I don’t the use defrost setting, specifically to avoid using the AC when I don’t feel it’s necessary, I use the mix setting. And the hill I go down starts out gently, it’s only the last mile that’s relatively steep, but the cool down starts immediately, on a section where I’m not just engine breaking. I’ve just never heard of this or seen this before, I was hoping someone else had and either had an “a hah!” or that it’s just the nature of the beast. Or that there was some mystery by-pass I’d never heard of. Not shooting down anyones ideas here, but I think I’ve checked what I thought would be the likely causes. Guess I’ll try another thermostat, flush/fill, throw in a cap, and probably end up with the same phenomena.

I was pondering the same thing after reviewing some comments here. May have to wait until winter for the real test. Won’t be long in Central NY.