OK I might have got you on the subject line.

My 300D has been running too cold. I know the engine was too cold because I verified the temp gauge with a digital laser thermometer. Gauge is reading correctly based on the temp of the head of the sending unit via the laser. Temp gauge was reading about 70 degrees.

NOTE: I believe and have read on these forums that our 20yo gauges quite often don't read accurately. My previous 300D didn't. Just because your gauge reads at the correct temp doesn't mean its accurate.

I pulled the old 80 degree Behr thermostat out of the car. I replaced it with a new 80 degree Wahler that I managed to find (at an Autozone) AFTER Autozone stopped carrying them.

Tested the Wahler in the kitchen prior to installation.

The car now warms up faster and runs at the proper temperature.
Temp gauge is now reading at 85 degrees. (80 degrees at the thermostat and 85 degrees at the sending unit) My mileage is up about 8%.

With diesel at $2.25 per gallon here in SO CA, that 8% payback will take 5-1/2 tank fulls to pay for the $16 thermostat.

For those complaining about poor mileage, check your temp gauge and determine if your thermostat is too cold. The engine runs more effeciently when properly warmed up.

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84 300DT Puke Yellow. Totalled after 438,000
84 300DT Orient Red. 169,000 (actual mileage may vary)
2002 Explorer EB (wife's)

Eric,
Your diagnosis inspired me to do this on SWMBO's ride, the 300D, which runs like a top but never seems to be able to get any better than about 22mpg in town and 24 on the highway.

I had a spare thermostat in stock and did the swap yesterday... the old thermo would run exactly 80C on the gauge. The new thermo runs the gauge slightly below 100C.

I'll run it this way for a few tanks and report on the results.

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Never a dull moment at Berry Hill Farm.

Did you test it in the kitchen before installation? I think a standard meat thermometer is more accurate than our 20yo combined sender/gauge systems.

I heat the water slowly and watch the thermometer and opening sequence of the thermostat. I also watch the cool down sequence and see if the thermostat is fully closing.

Remember the thermostat should open at 80 degrees but by the time the coolant continues through the head and reaches the sending unit the coolant temp rises. Generally 5 to 10 degrees.

I don't trust the sender/gauge system unless its been verified by a digital laser thermometer.

__________________
84 300DT Puke Yellow. Totalled after 438,000
84 300DT Orient Red. 169,000 (actual mileage may vary)
2002 Explorer EB (wife's)

Yes, I tested with a meat thermo in a saucepan of heated water.

This particular replacement thermo first starts to open at about 180F (82C) and is fully opened by 190F (88C).

This weekend, running the car at both highway and in-town speeds with the a/c on and an ambient temp of approx 75F, indicates a steady 100C (the mark between the 80 - 120 digits on the gauge) It never fluctuates more than a 'needle width' above or below that indication.

Prior to this, the car ALWAYS ran at 80C w/exception of idling with the a/c on in +100F temps where the gauge would slowly creep up to the 100 mark.

Last fall, the cooling system was flushed 3X w/MBZ citric acid flush and MBZ coolant was added with tap water to achieve freeze protection down to -20F. All new hoses too.

__________________
Never a dull moment at Berry Hill Farm.

Sounds like your sender/gauge system is reading too hot. Since you tested the thermostat, I'd be inclined to trust the thermostat and thermometer and not the sending/gauge system. Good luck and let us know if you mileage gets better.

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84 300DT Puke Yellow. Totalled after 438,000
84 300DT Orient Red. 169,000 (actual mileage may vary)
2002 Explorer EB (wife's)

This morning, while waiting for the coffee to brew, I decided to test Lilly's old thermostat and see what temperature it took to open.

The doggone thing was fully open by 160F....

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Never a dull moment at Berry Hill Farm.