What is proper coolant temp?

[Quattro Pete]

The instrument cluster allows me to monitor the exact coolant temperature. When driving around town, it oscillates betw. 90-96 deg. C. But when I drive fast on the hwy, it drops down to 80C. Is this considered "normal"?

The actual coolant temp gauge doesn't budge, but that's because it is programmed to stay in the same spot within a certain range of temps (75C-113C, from what I've read).

The reason I ask is that recently I noticed poorer hwy MPG (about 27 vs 30 previously). In addition, excessive moisture was detected in my recent UOA sample. I thought that maybe my thermostat was stuck open, not allowing the engine to operate at an optimum temp. I replaced the thermostat, but it made no difference. So, what else would you recommend I check?

One other possibility is the CCV system, mainly the oil separator valve. But from what I've read, if it was clogged, the engine would exhibit rough idle. Mine doesn't.

FYI, this is the 3-liter in-line six (M54) engine with 42k miles.

Thanks.

 

[Merkava_4]

Quote:

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But when I drive fast on the hwy, it drops down to 80C. Is this considered "normal"?

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That would be normal for my car; especially this time of year.

 

[JHZR2]

80C is a common thermostat temperature for some engines. On my MB diesel, it is equipped with an 80C thermostat, but on my BMW it is equipped with one that is set to 88C. GIven enough mass flow and delta T across your radiator, the steady-state water temperature should approach your thermostat temperature, so if you have an 80C stat, you might be doing what you should.

That said, there is a LOT of variability in the opening temperatures of thermostats, including OEM ones. Did you replace it recently? Has it ever been replaced? You really need to know the OE spec for it.

Your excessive moisture could be from short trips or lack of highway mileage. not necessarily because of a steady state 80C water temperature.

Good luckj,

JMH

 

[Quattro Pete]

Yes, as I mentioned. I had the thermostat replaced, but the temps I observe are still the same.

As for moisture, I have a 30 minute commute to work and being in a warm climate, this should be sufficient to burn off water vapor, I think. Also, just before taking the sample, I took it out on the hwy for close to 100 miles, so again, that should have been enough time to burn off moisture. Alas, this is Florida - ambient humidity is really high. I don't know if that has anything to do with it.

 

[eljefino]

Is the coolant temp sensor near the thermostat or on the other side of the motor?

Reason I ask is b/c on my saturn, they're opposite. I theorize with more heat being put into the coolant on the highway, the thermostat opens further, but the greater flow means the CTS sees cooler temps.... but that only affects that half of the engine.

I bet your oil is getting plenty hot.

One of those infared laser thermometers would tell you a lot.

 

[Quattro Pete]

I think the sensor is on the same side as the thermostat. Assuming the oil is getting hot as it should, what else can be responsible for my excess water vapor accumulation?

 

[mechtech2]

The important question is if the coolant was at the same temperatures when the mileage was higher.
If it was similar, then your gas mileage problems are from something else.

 

[Sienna Driver]

It's normal for coolant temp to drop at highway speed. From what I read, oil temp behaves in an opposite fashion.

 

[va3ux]

Mine does exactly the same thing. Different numbers probably, but the same behavior. My lowest steady coolant temperature occurs a highway speeds.