I thought I had bad thermastat. I changed it now 3 times, 1 time it was tested and deemed to be defective, ordered a new 1 and same issue. Before thermastat went bad the car rarely got over 40c, now w/ the new one it runs at 85c idling. take the thermastat out and it runs at 40c. Took the car to a radiator shop and he said the radiator wasn't restricted or blocked. Is there a chance I could have gotten a 2nd bad thermastat ? I'm at a loss at this point. Any help would be appreciated. The car is an 84 300d turbo diesel.

40C is way too cold, and usually indicates a bad thermostat. 85C is perfect. If it runs somewhere between 80C and 90C you're fine. It sounds like there's nothing wrong with the new thermostat.

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1982 300D, black w/ palomino leather, 203K
2001 E320, champagne, 105K (Mom's)
1999 Dodge Dakota, squeaky & battered, 142K

mine gets to 85 and sticks there unless its 95 outside and I got the AC on stun.

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'85 300 Turbo Diesel- 308K

85C is right where it should be, does it overheat when you drive it? Also I wouldn't trust a 20 year old guage, they are sometimes close but almost never right.

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1999 SL500
1969 280SE
2023 Ram 1500
2007 Tiara 3200

Like Hattie said.

85°C. is perfectly normal. It will climb to 90°C. in the summer and, it might even approach 100°C. if the ambient is 95°F. and the a/c is on full blast.

Nothing to even think about, until it gets to 110°C. Then it's time to lift your right foot.

40C at idle = too cold
85C at idle = just fine!

Given mass-production temperature gage and sensor tolernances, I would not worry +/- 5C, so 80-90 is OK. It's more about how much it moves, or does not move with changes from idling to highway cruising. If it idles a few degrees warmer, and cools right off, you are doing fine.

Best Regards,
Jim