Where is the most accurate place to aim the temp gun to get a coolant reading? How many degrees do you think should be added to the gun reading to account for material being between the coolant and the laser? I know these things have been brought up in a lot of threads, but I haven't found any clear consensus on where the best place is to aim the gun or how much to add to the reading...

My temp gauge has been acting kinda wacky...just trying to find out how accurate it really is...

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1984 300D, 228k, Light Ivory, Java MB-Tex

The engine side of the radiator works well for me; it's a fairly large area so I don't have to worry about what the IR sensor is "seeing." It seems to read higher than the gauge in my cluster; clearly, one of them is inaccurate. Must do some tests with boiling water one of these days.

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"Buster" in the '95

Our all-Diesel family
1996 E300D (W210) . .338,000 miles Wife's car
2005 E320 CDI . . 113,000 miles My car
Santa Rosa population 176,762 (2022)
Total. . . . . . . . . . . . 627,762
"Oh lord won't you buy me a Mercedes Benz."
-- Janis Joplin, October 1, 1970

I always check mine out by testing the temp of the head surface near the temp sensor.

Water neck/thermostat housing. Just spray a dob of black spray paint on the surface you'll be checking... My bluepoint infrared gun picks up signal better on dark surfaces that don't reflect the rays.

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Regards,
Ian White

1995 E300 Diesel w124 OM606
2014 E550 w212 M278 biturbo

2001 BMW 740i E38 M62 (past)
1981 300SD w126 OM617 (past)

So, do all of you think the reading on the gun is close enough to the actual coolant temperature without adding any extra degrees to account for the material (hose/thermo housing/engine head) between the coolant and the temp gun laser beam?

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1984 300D, 228k, Light Ivory, Java MB-Tex

it'll be off a hair, but it's the temp of the block that's important, the coolant temp is immeterial. the temp sensor in the car is for protection of the engine.

The advice about black paint is well taken. Flat black works best, you will get a reading 10-15 degrees higher than a shiny light color. Something on a metal part like the thermostat housing will be the most accurate. Cheers Dan

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It's always something simple
91 300D 603.960 (from japan) 194K
92 Toyota Diesel Landcruiser HDJ81-t 116K
02 Golf TDI new head courtesy of PO
87 300D
97 BMW 525 TDS Wagon 5spd
bunch of Onan and other diesel generators

Okay, good points all, thanks much.

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1984 300D, 228k, Light Ivory, Java MB-Tex