Coolant temp drops going downhill
01-02-2016, 12:14 AM
#41
Timber Baron
Thread Starter
We were heading to the inlaws today and that requires a large downhill coast on the way home. With this in mind I ran my scanner live just to see what it was doing. During the downhill at about 80 km/h and coasting the engine dropped to idle speed and was still using fuel.
When I descend a 7% grade for 7 miles in 2nd and 3rd gears, my engine is above 2500 RPM. The key to the computer shutting fuel off is that the engine is well above idle, as the articles I linked above state.
Agreed.
My whole point is that when I am coasting downhill at 65mph in 3rd gear with my foot off the gas, the engine is turning, but is not providing any forward locomotion since there is zero fuel being delivered to the combustion chamber. No fuel being burned + lots of wind = coolant temps drop.
... It is still running it's just using the minimal amount of fuel, so it still has a combustion cycle, just a very minimal combustion cycle so that when you return to idle your engine will still be running, just enough for the engine to get by in a high rev without stalling, that's just my guess cause the truck won't start itself
01-02-2016, 12:47 AM
#42
Originally Posted by Feathermerchant
Remember that it is difficult to turn any engine. That is due to friction. Friction causes heat. How much? Well Torque X RPM = Horsepower. With the thermostat closed, I'm guessing that the engine even without fuel creates enough heat to stay at operating temperature.
Here's my stab at it..
It's not due to friction, it's due to compression. If you pull the plugs on a motor you should be able to spin the crank with relative ease..
The way I see it.. Your just "turning" the motor.
The tires basically turn into "generators" on a downhill which significantly reduces engine load. If engine load drops low enough, and the downhill motion of the tires can keep the engine rpm's ABOVE the PCM's DESIRED Idle speed, then you get no fuel.. Put the truck in neutral while coasting down a grade, and your injectors will begin to produce an idle again without the "turning" effect of the tires. Desired engine load is almost always commanded by the gas pedal (i.e. The driver), so when the PCM sees zero load input (foot off the gas pedal), it will default to maintaining engine idle. If engine idle rpm is met as a consequence of a downhill force, then you get zero fuel from the injectors.
The braking affect is from the compression stroke of the cylinders and the gear the transmission is in. The lower the gear, the higher the rpm produced by the tires, the larger the braking affect..
I suppose the lack of an actual combustion event will drop the overall cylinder head temp. While coasting down a hill with zero combustion taking place, you would also be forcing a cooling affect via wind speed over the radiator, which would then produce the much larger dip in coolant temps.
If anything I would argue that if the coolant temps dipped low enough during a compression stroke/engine braking coast, the PCM would command a higher idle to try and warm the engine back up. This would kick the injectors back on because the PCM has no other way to raise engine coolant temps other than dumping fuel to create a combustion event.
The compression stroke (air compressor) does also create heat, even without combustion, so I would still argue that the excessive cooling is predominantly a result of Mother Nature, and gravity, but lack of combustion would sure make that cooling affect even more efficient.
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Toshbar (01-02-2016)
