'77 300D is getting to the red mark 250 mark on the thermometer just from idling for a while, coolant levels seems good, could it be a problem with the water pump? Any ideas to help figure out what it is? Also, unrelated, but the emergency brake light on the dashboard isn't turning off when the parking brake is off, brake fluid levels are full. I'm new at this so I might be just missing some easy thing, thanks for the help!

How long is a while? A few minutes? 15 minutes?

I think first step is always to confirm the gauge reading by reading the actual engine temp using an infrared thermometer from the thermostat housing on the side that touches the cylinder head, or the side of the cylinder head where the temp gauge sits. It's entirely possible your temp gauge on the instrument cluster is not reading correctly.

You can get an infrared thermometer at any hardware store.

If you can confirm that the gauge is correct, then there is a list of potential issues:

Thermostat is stuck closed.
Radiator is plugged up.
Water pump is dead.
Big air pocket somewhere in the engine.

All of those would cause poor coolant flow or no coolant flow which can lead to this.

I believe your car does not have a clutch driven fan, I think it's always on, so that wouldn't be an issue. If I'm mistaken and someone corrects that, then it could be that your engine fan is not turning when you're idling and the engine is hot.

Best way to test the thermostat is to remove it from the engine while it's cold (you'll lose about 1/2 gallon of coolant, but you can catch it in a clean drain pan and re-use it) and put it in a bot of water, heat it up, and see if it opens at the temperature rating engraved on the thermostat body. Usually that is 80°C.

While your engine is cold, you can open the cap on the radiator, then turn on the engine and look to see if you can see the coolant moving. If not it may be the water pump.

You can test the radiator by using the same infrared thermometer. You can aim it at the radiator to see if there are hot spots, or areas where there is a drastic temp change all of a sudden going from top to bottom, indicating a blockage internally.

P.S. - don't drive the car for any period of time while the needle is in the red, especially if you confirm that the gauge is accurate. This spells quick death for your engine.

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Does the heater work? If it does, that verifies circulation and that you at least have some coolant in the system. If it doesn't, it could be a low coolant situation and/or a bad water pump.

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'83 300D, 126K miles.

If your positive your not leaking coolant, I would check the thermostat....don't drive it until you figure it out!!

You have the evil servo unit, they are known to crack and leak coolant...

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I haven't seen any trace of coolant leaking whatsoever, but I will be on the lookout for that. I'm going to run the diagnostics uberwasser recommended tomorrow. And don't worry, it won't be driving yet, I'm still working on getting it to the point where it can, getting close though! (Family car that hasn't been driven in a while, I finally decided I wanted to fix it up). Thanks for the advice, you guys are the best.

Okay, so I replaced the thermostat and put in new coolant. I also replaced the thermometer sensor because it seemed to be reading higher than the reading I was getting with an infrared thermometer. After all of that I'm still getting a higher reading on the thermometer (About 250(F) when the infrared tells me ~175) and after a while the reading from the infrared climbs up to about 230 just from idling for a while (I turned it off before it kept going). What should my next step be?

What is the temperature at speed? Mine was above 80C and would spike when stopped. Turning on the heater full blast would get the temps back to 90+.
I changed the coolant, sensor, fan clutch and it still ran hot. I bit the bullet and changed the radiator (that looked perfect on the outside) and now it runs around 80 all the time no matter what. That old radiator looked good but was clogged inside.

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I haven't driven it yet, this is one of the last kinks I have before I'm pretty sure it'll be good to go, but it needs to be inspected and what not so I can't really drive it. And okay, that sounds like I'm gonna want to buy a new radiator

Have you pointed the thermometer to the oil cooler? and see if the oil is circulating through the cooler? There is a thermostat in the oil cooler, I have heard that some times they stick shut and cause the car to over heat.

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According to what?

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Check radiator top tank/hose and bottom hose temperatures and also run it up from cold with the cap off. The bottom should be cooler than the top, but not a whole lot. I got something like a 25 degree (F) difference at idle after a few mile drive last time I checked. If the top is blazing while the bottom is still cold this is usually a sign of a blocked radiator. However, read on.

Usually a bad water pump will leak, but in some instances the impeller fins erode to the point of the pump being ineffective without developing an external leak. You should be able to see coolant moving in the radiator after the thermostat opens. Also, it seems from Pelican that a '77 300D does have a fan clutch. Verify the fan is operating. If not this will lead to eventual overheating at idle even with everything else working right.

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83 300D Turbo with manual conversion, early W126 vented front rotors and H4 headlights 401,xxx miles
08 Suzuki GSX-R600 M4 Slip-on 26,xxx miles
88 Jaguar XJS V12 94,xxx miles. Work in progress.
99 Mazda Miata 183,xxx miles.

There is an oil thermostat in the oil filter housing. If when the engine is warmed up and the oil cooler lines are still cold, that thermostat could be the problem. Apparently the oil thermostat is not available anymore.

Regards,

W.

Hmm overheating at idle... never putting the car on the road, JUST idle... have you pulled the block drain, and checked to see if it's full of rust? where are you located? I've got a pair of good radiators... (engines, cars, whatever...) the NA 617 is a PAIN IN THE REAR to get all the air out of the head, be SURE you have properly drained the coolant from the block drain on the starter side of the block, and filled from the upper radiator hose with the car pointed nose up as high as possible... there is a vent on top of the thermostat housing usually, there has to be ZERO air at that point on this motor...

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Use distilled water. Never use hose water. Even with the 50/50 mix I sometimes add distiled water to help run a little cooler. On the old dirt track we ran 90/10 (90% distiled water)

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1986 560 SEL
1960 Ranchero
1970 Cougar XR7
2002 Crown Vic Sport
1992 F150

The red mark on the cooling temperature gauge is 120C. Where are you reading a 250 mark? Please quantify what idling for a while is. Is it 5 minutes from a cold start? 15 min? 1/2 hour? 1 hour? Is your water pump belt on and tension correct and the water pump is turing? Is coolant flowing in the overflow tank?

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