Engine braking in cold temps drops engine temp?

Wondering if this is normal? I have a 2020 TRD Off Road and noticed that in cold weather during engine braking 5-7%+ grades, the engine temp drops from dead center on the gauge, to just above the low acceptable mark. This even happened with a grill cover installed, cover vents closed, at 30 degrees. Is that expected with no/minimum fuel injected and the low viscosity oil? or do I potentially have a stuck open thermostat?

 

how are you getting your engine temperature readings ? aftermarket guage? also if there was no fuel the engine would turn off, and by low viscosity do you mean 0w20?

 

I’m just taking a guess here: truck runs Atkinson cycle so I think you’d have a net loss of energy from the air as it makes its way from intake to exhaust (assuming no fuel is being injected). Sounds plausible but honestly idk.

 

very plausible, there was 7% grade I has to go up and down daily for a couple of months and this never happened to me actually going up I would shift into s4 mode,im mean op said his engine temp was fluctuating, it could be that he is right , sounds like he googled it

 

There is no reason I can think of that Toyota couldn't set injector duty cycle to 0%. The engine will still run, NOX levels will be 0.

 

unless it was a hybrid

 

no but yes your 100% i just remembered lol

 

Stock engine temp gauge lower left corner on the cluster. No numbers so I described markings. Climbed over an 11,000' pass at 45mph and ~25 Fahrenheit exterior temp, descended at 40mph and noticed after a couple minutes I was getting cool air from the heater vents. Temperature from the external temp display and confirmed by wunderground.com for Monarch Pass. Engine temp returned to center of gauge while idling when I pulled over at about 8500' and 30 exterior temp. Dropped during engine braking with the grill cover on. returned to normal on gentler slopes under light engine load.

Yes, 0w20. my last vehicle was a similar displacement V8 with 5w30 and never experienced anything similar

 

hmm but its not engine temp gauge its coolant temperature gauge

 

Have you checked coolant levels? Low coolant can cause poor heating in some models of vehicles. How long do you coast down the hill? I could see some cooling happening but your heat shouldn't go cold. Maybe a Toyota thing? I don't have any mountains here to deal with to check if mine does it.

 

My 2020 sr5 v6 basically does the same. I'll stop at a traffic light with the temp gauge reading mid way...sitting idle there for a couple minutes the temp gauge will drop to low. When I get going again with more engine rpm, the needle with go back to mid way. Been that way since I got the truck new. Btw, this truck seems to take forever to get up to temp...same for producing good heat.

 

I wouldn't say this is abnormal. On my 2005, I had an ultragauge to monitor temps and when I would engine brake down a hill, the engine load would drop to ~7% (comparable to idle) but the RPM's would still be a couple thousand, meaning there's tons of air flowing over the radiator because of the fan. My engine temps would drop 10-20 degrees in a short period of time and that wasn't even during particularly cold weather.

Moving the needle that much seems interesting but I don't think it would raise any serious red flags for me.

 

It would be interesting to know what the actual temps are with an OBDII reader. Typically manufactures build a pretty good deadband into the gauge on the dash. The fact that its dropping to almost the low mark seems weird

I know on my 2nd gen the needle starts to move right around 100*F.

 

Fuel cut when you lift off the skinny pedal.

..and yes, the temp guage in the dash will read "normal" across quite a wide temp range. I don't recall the exact numbers, but I watched that PID during warmup a while back.

 

As I understand it, tacos (as well as many modern vehicles) have a deceleration fuel cut off feature that turns the gas off when decelerating or engine braking. The lack of fuel burn is going to cause the engine temperature to decrease, however engine braking should cause it to warm up a bit due to pumping losses and friction. If it didn't, the truck would not slow down. Engine braking works by converting kinetic energy into heat analogous to brakes.

What is this "grill cover" you speak of? Do you have an aftermarket thingy blocking the air entering your grill? If so, why? I haven't seen one of those since the 1980's.

 

When using engine braking down a hill my temp gauge stays at mid way. It only happens in cold weather while at a stop sign, or traffic red light when the truck is at idle speed. Within seconds underway, all is back to normal. I've had several toyota pickups in the past with various 4, 6, and 8 cylinder engines, but nothing like this 3.5 has been.

 

Only in this isolated case, would I rather be leaning left (colder) and than right (hot).

 

If your coolant temp gauge is in the middle, the thermostat is expected to be open. There's probably some hysteresis or lag with the thermostat so it can't close quickly.

Most likely it's as @EatSleepTacos described: engine rpms go up with engine braking, so the radiator fan speed goes up as well and cools the coolant down more than usual.

 

That's what I might expect, but the temp actually rises again when stopped and idling or moving and not engine braking

 

I thought the work of air compression would warm the engine as well. The grill cover is a fabric cover blocking frontal airflow to the raidiator. I noticed the truck had trouble maintaining temp driving around town when temps are -10 or below. These covers are not uncommon in the Rocky Mountain states.