What does normal look like... (Jeep JL 3.6 temperatures)

[ChrisW]

As we enter the summer season, @Sam Selim and I have been watching the gauges on our Jeep JL 3.6L 4x4's - which quickly raises a question...what does normal look like?

 

Our hobby means we push cars harder and in unusual conditions, often in ways for which they were not directly designed. A quick glance will tell you if everything is OK (i.e. all the needles are below red-lines), but over time many of us develop a gut feeling for what looks normal and when things are outside of what we would expect to see.

 

I decided on today's drive with @Mark B to snatch a few data points and look at things with a bit more detail:

 

 

 

Couple of major points to relate my gut feelings to what you see:

• The oil pressure on the Pentastar engine varies with RPM, with typically around 30 PSI below circa 3000 RPM, rising to 70 PSI when revving higher - the key point is just to ensure you don't have too low oil pressure (<<30 PSI).
• Oil temperature is a leading indicator: if you push hard, it moves up quickly, and vice-versa. This is to be expected, as engine oil has the dual job of lubricating the engine as well as drawing heat.
• Coolant temperature is a lagging indicator: The coolant temperature rises when pushing the engine, but at a slower rate - it also cools down less quickly too. Also on the Jeep JL, the fan will kick into max power at 110 degC and pull the temperature down again. If you see the temperature going beyond 110 degC and can't hear the fan (or the temperature keeps rising), it's time to do something (like come to a stop!).
• Transmission Temperature: It's a slow mover with plenty of margin to the limit value. However my suspicion is that once this gets hot, it will take some time to cool down again.

 

What does normal look like on your car, and what are the big red flags for you?

[Zubail]

Let me pull data from my car scanner from last week when I was on Jebel Shams and Akhdar

[Zubail]

On my Expedition Timberline, the cooling is simpler than your JL. There is no separate air-cooled transmission cooler from the factory. The gearbox uses a heat exchanger that runs engine coolant against the transmission fluid. This means the ATF basically cannot get colder than the coolant. When the engine gets hot, the gearbox has nowhere to dump its heat. I log data with CarScanner and an OBDLink adapter: ATF temp, coolant, intake air, RPM and boost.

I drove up Jebel Akhdar to the Saiq plateau and back. The trip had three very different parts:

Going up (about 38 min, around 38°C at the bottom): For the whole climb I kept the gearbox locked in 3rd gear manually. I did not let it shift higher. This held the revs around 2500-3000 and stopped the gearbox from jumping between 3-4-5 all the time on the corners. Less shifting under load means less heat, and the car felt more stable. With this, ATF went from 92°C up to 110°C, and coolant reached 108°C. You can see the heat exchanger limit here — ATF and coolant climbed together and stayed only about 2 degrees apart. Intake air went from 39 to 61°C. No knock, everything normal.

On the plateau (about 26 min, around 2000m): This was the easy part. ATF stayed between 77 and 99°C. The air up there is cool, so intake dropped to 36-54°C. The car was relaxed.

Going down (first 8 min): ATF went back up to 106°C, even with engine braking. The interesting part: knock retard jumped to almost 7°. This was on a closed throttle while braking with the engine, going downhill. High revs, hot engine, thin air at altitude. The computer pulled timing to protect itself. I only use Super 98, so it is not the fuel. Just something I did not expect to see going downhill.

What I think is normal, and what worries me:

• ATF below 105°C is fine. 105-110 I watch it. Above 115 I would stop and let it cool.
• Coolant near 100°C in summer is normal for this car. The radiator is working hard but it is okay.
• After a hard climb, the ATF temp can keep rising for a few minutes after you stop. So I let it idle 5 minutes before turning off.

One question for you all: my "Fan Speed Desired" reading stayed at 0% the whole time, even at 110°C. I am not sure if CarScanner is reading the wrong value for the Ford fan, or if something is wrong. If anyone knows the correct fan PID for the 3.5 EcoBoost, please tell me.

The biggest help was holding a gear manually on the long climb. Much less shifting and the car feels more stable.

PS. Car was loaded (Me, wife and 2 kids, and full trunk of different staff)

Edited Saturday at 03:59 PM by Zubail