Is my Temp Gauge Accurate? Stubborn High Temps!

[JacksonKotch]

Hi All!

I have recently purchased a 1985 300TD (222K mi.) that seems to have the common semi-kinda-sorta-overheat problem. The engine temperature generally settles around 95C-100C per the dash gauge. I blasted the parts cannon since I wasn't sure of some of the cooling system history, and the gauge reading didn't budge an inch. I've reviewed a ton of threads regarding this issue, but I'm looking for a sanity check before I continue troubleshooting...

Do you folks think this temp gauge reading is accurate vs what I'm measuring with my IR thermometer? See below for photos of the measurements.

Here are the parts I've replaced thus far:

• Radiator (Nissen)
• Water Pump (Kent Special)
• Thermostat (P.O. replaced <500mi ago)
• Sender (P.O. replaced <500mi ago)
• Pressure Cap
• Coolant (Green stuff replaced with Zerex G05)

Some other details:

• The cooling system holds pressure @15psi for a few minutes, @8psi over an hour, and @5psi indefinitely. No fluid leaks as far as I can tell.
• At 95C-100C the cooling system pressure feels normal. Firm-but-squeezable top hose which returns to soft after engine cools.
• P.O. did a partial flush-out of green coolant and replaced it with G05. I then drained that concoction (it was clean and clear) and added approx. 2.5gal of G05 premix during my parts replacement and futzing.
• Filled coolant tank till radiator overflowed and top hose till overflowed w/ car front-end up on stands. Did NOT see a 'burp' while running/filling. No coolant loss observed while I've owned the car.
• 80C Thermostat was boil tested and opened fully between 80C and 90C. Opened similarly to a second thermostat I had on hand)
• The car runs great and shows no other symptoms.

Here are some temperature measurements after bringing the engine to temp with a 15 minute romp around town:
Gauge Reading:

Temp sender / block area: 92.4C

Cylinder Head Top Surface: 96.7C

Thermostat Housing: 90C

Radiator Inlet: 75.2C

Radiator Outlet: 37.0C

Oil Cooler Inlet: 75.8C

Oil Pan: 68.6C