Radiator Coolant Temp In vs Coolant Temp Out

[4Gears4Tires]

So before this 8 day trip (which I will eventually write up a trip report for, hah) I wanted to add the coolant temp out gauge. I had already added the coupler and sender a long time ago, so all I needed to do was run some wires and mount the gauge. Of course I was a little pressed on time so you will notice the lovely rear locker knob doing double duty as a gauge hanger. Works perfect and I am sure I will mount it more permanently very soon. Very sure.

I was already fairly confident in the cooling system. I had struggled with little air leaks, displaced coolant, overflowing expansion tank, and such constantly over the last 2 years. And, I think a lot of it was actually overfilling the system. I would get the system 100% filled and then turned the van on, hold 2000rpm and add more coolant. What happens when the system is hot? Coolant expands! And then my expansion tank is super filled. Thinking something is wrong I would bleed the coolant system and repeat the whole process! Fun stuff. But going into this trip the coolant level in the expansion tank hadn't moved in a month or two so I figured I had bled this system 100% correctly this time.

So I added the gauge the night before, checked to see if it moved with power (it did) and then went to bed. The next day, as soon as we started out I honestly thought something was not hooked up correctly because the outlet gauge never really moved from just over 100 degrees F. It stayed at 100-105 F. Usually if a temp gauge does this it's not grounded properly. Coolant temp in has stayed at a very steady 209-210F for 6 months and the engine hasn't died so I figured that was where it should be.

Traffic then got bad on 495 crossing the Potomac River into VA and guess what finally started moving? The outlet gauge! Temps crept up to ~175F and stayed around there as I slowly progressed through stop and go 0-20mph traffic. At this point coolant out also started to raise from 209-210F up. Once we got above a steady 40mph temps dropped back to 100-105F in and 210F out.

This might sound dumb, but I have always added temp gauges to the thermostat housing and I really just assumed coolant temps were one temperature. Initially I thought the van was overheating at 210F radiator in and then I realized that no, of course not. The engine makes the coolant hot, the radiator cools it down, and then cooler coolant goes back. Duh. However, I really didn't expect the coolant temp going back to the engine to be this cold. I assume the thermostat must be working constantly to regulate the temp. It doesn't just hit 194F and opens, it has to be constantly adjusting the flow of coolant.

Around +180F out it seems the radiator fan kicks in at low. Hot crowded parking lots seemed to be the place where this would happen, especially immediately after shutting the van off. The loss of coolant flow immediately heat soaks the radiator and the fans then would kick in for 20-30 seconds.

Below 40mph outlet temps will creep up. If I was in easy city/suburbia driving temps would stay around 125F out for example. Obviously it's July, but temps during driving ranged from high 60s to low 90s and this seemed fairly consistent. Above 40mph (and maybe outside death valley) the air flow kept the outlet temps at 100-105F, again, it didn't seem to make a difference whether it was 68 or 92F.

Long climbs really benefited from lower temps though. The combination of slower speed and high load really saw temps climb. I never saw temps past 175F in +80F weather, but I also pulled over at nice vistas whenever I temps start to get around there. I just wasn't willing to find out if it would stay at 175F or if the fans would kick in while driving. The Smoky Mountains has lots of really long climbs and plenty of pull outs. Can't forget to enjoy the scenery anyway! With temps in the low 70s, the van seems like it can climb forever with radiator out temps never exceeding 150F. I can imagine that if outside temps were 95F and over and the grades were steep, the coolant temps really need to be watched.

I had made a mental note of whether the engine cools down/coolant temps stop rising in a lower gear at partial throttle vs a higher gear in full throttle but I seem to have forgotten the results. Apologies. All in all, if there's any takeaways from all this it's that over 40mph you should not be overheating unless something is wrong.
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'87 Syncro
Ferric Oxyhydroxide Superleggera Edition

[space]

Dumb question alert: where are you getting your "in" reading from?
I have one gauge and holds pretty steady at 190-200
T
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82 westy
w:
2.5 JDM Subie
Rear discs

[4Gears4Tires]

Both gauges are plumbed into the rubber lines right before the radiator with aluminum couplers made for T-ing in a temp sender. https://www.amazon.com/Aluminum-Temperature-Radiat...&psc=1

If you are seeing 190F I am assuming your gauge is in the thermostat housing.

It would be interesting to see if someone else plumbs in 2 gauges and documents their temps.
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'87 Syncro
Ferric Oxyhydroxide Superleggera Edition

[hardway]

When I am checking cooling systems I use my Raytek infrared thermometer. I can produce consistent readings with it. The color of the surface being measured might influence the reading a little. The thermal conductivity of the material being measured should be taken into account. After all rubber is a reasonable insulator.

Once you start paying close attention, the temperature differentials can be pretty amazing. The 1.9 wasserboxers can have very large differentials across the engine itself. The 2.1 is obviously much improved.

Another thing that you see is that the thermostat really needs hot water to open it. As the complete system reaches heat soak the coolant leaving the engine is actually cooler.

A friend of mine is an engineer. He has a Subie 2.2 with a kennedy install kit in his Westy. He has temperature sensors everywhere. He switches between them so that he can read each of them at a time at the readout.

One of my busses now is fitted with an AWP with Stephan's kit. I leave the Scan Gauge On the coolant and air intake temperatures. Reading the OBD II data output from the engine is way more precise than analog gauges. The coolant temperature at the output flange goes down below the thermostat rating after full heat soak. Of course it goes up when you load the engine. Watching the analog gauge and the Scan Gauge simultaneously is quite enlightening.

I do not feel the need for measuring the return coolant temperature personally. But all of newer BMWs have a sensor in the return line. They call it an auxilliary fan switch. But I bet it has something to do with the MAP thermostat control logic as well.