Hot but not overheating - PeachParts Mercedes-Benz Forum

mhin1956 · #1 06-20-2018, 07:47 AM

87 300sdl 419k. We've owned for 21 years. Temp in traffic or after a while driving climbs to over 110. Ran for years at 80. Replaced so far: radiator (custom alum two years ago) rad cap, thermostat, upper hose, idler assembly, belt, water pump, and fresh correct coolant. None seem to address temp.
Car runs great, warms up normally, then after driving with or without a/c gradually starts to climb. Not related to speed or a/c (aux fan works). It IS warm summer 95 deg but has been before. Sitting on interstate at 97 degrees yesterday saw temp climb to 110 without a/c. Worried, turned on defrost and temp dropped almost immediately by 20 degrees.
At this point thinking possibly blocked radiator (internal) or has anyone had oil thermostat cause something like this? I do not think it's head or gasket related.
Really need help on this one.

Update..all parts are MB replacements w/ exception of radiator that was working extremely well for last two years. Yes, fan clutch replaced also. No debris in radiator or blockage externally. Radiator cap is new MB and system is pressurized when hot. All replacement parts that I've put on have not affected the temperature, so must be something else. Temp is hot, as have used infrared thermo on various parts. Thanks to all but still need help, please. Any info on past sticking oil thermostat..if it does not open would this cause apparent temp rise? Happens mainly after about 20 mins of driving; various conditions, freeway/town etc. It is hot (normal for us) around 95 and mostly sunny. This heat up occurs with or without a/c on, so probably not related. Aux fan functions as designed.

Father Of Giants · #2 06-20-2018, 08:12 AM

Is your fan clutch good?

t walgamuth · #3 06-20-2018, 08:16 AM

I'd clean the rad and check the fan clutch and get back with us.

Maxbumpo · #4 06-20-2018, 08:17 AM

Confirm that your temperature gauge is accurate with a non-contact thermometer, measuring at the upper radiator coolant hose right where it exits the head.

It's entirely possible that one or more of the parts you replaced (radiator, water pump, thermostat) was a faulty part and is causing your issue.

I'd say the cheap / easy thing to try first is a new thermostat, and make sure you test the thermostat in a pot of water on the stove before installing, and test the old thermostat to compare. You need to test for both opening temperature and if the thermostat reaches full stroke at designated temperature.

Next I'd say pull the radiator and pay a competent radiator shop to test it.

Last would be the water pump and the water pump housing. Did you install an MB part from a dealership, or aftermarket? Was the impeller cast or stamped (stamped impellers have a bad reputation for not moving enough coolant). The housing is installed into the block and matches the impeller profile; it needs to fit the impeller closely so that coolant doesn't "leak" around the impeller blades and so not enough coolant is moving. Look for evidence that erosion of the housing is causing too much gap between impeller blades and housing.

tyl604 · #5 06-20-2018, 08:29 AM

I just had this exact problem on my '81 300SD. Here is what I found out thanks to the forum:

The aux fan is designed to lower heat due to the AC (at least on the SD); so with my AC belt removed, it would never come on - no matter how hot the engine gets. But the viscous fan clutch is what cools the engine from normal hot temps. When the engine gets hot, it should lock up and the fan should roar; mine did not.

Thanks to the guys here they recommended I test the fan clutch by getting the engine warmed up and watching the fan blades when the engine was switched off. Sure enough the fan turned a couple of times instead of coming to a quick stop; ie bad viscous fan clutch.

Replaced it two days ago and problem seems to be solved. I am the original owner so I know exactly where the temp gauge needle should be. Over the last month it had just climbed about to or just below the 100C mark and I knew something was not normal.

Maxbumpo · #6 06-20-2018, 08:35 AM

Bad fan clutch and debris-clogged radiator are both great suggestions.

Here is a link with the thermostat specs for test:

Temp Sensors and Thermostat Fix

Diseasel300 · #7 06-20-2018, 08:49 AM

If it's been 21 years since you've bought the car and you've never replaced the fan clutch, start there. Consider upgrading to the clutch and 11-blade fan from a 98-99 E300D, they move a significantly higher volume of air compared to the stock 9 blade metal setup.

Your test whether or not the heat issues are head related is to check the radiator hoses in the morning when the car is cold. If they're still pressurized and firm, you have a head gasket breach or a crack in the head. If they're squishy, you're good there. Obviously if your radiator cap leaks or you have coolant leaks somewhere the test is invalid.

tyl604 · #8 06-20-2018, 09:18 AM

Diseasel300 - do not understand your post. If there is a radiator cap pressure leak, wouldn't that allow the radiator hose to get squishy instead of it remaining tight and under pressure? Do not understand.

tjts1 · #9 06-20-2018, 11:25 AM

Do you have a #14 head? Is the cooling system stay pressurized overnight?

Diseasel300 · #10 06-20-2018, 01:24 PM

Quote:

Originally Posted by tyl604

Diseasel300 - do not understand your post. If there is a radiator cap pressure leak, wouldn't that allow the radiator hose to get squishy instead of it remaining tight and under pressure? Do not understand.

If there is a radiator cap pressure leak, the pressure in the cooling system will never build or will bleed off quickly when the engine is shut off. That negates any sort or "retained pressure" test. A healthy 603 cooling system should never have hard radiator hoses when the engine has cooled.

tyl604 · #11 06-20-2018, 02:20 PM

Diseasel300- sorry, bear with me. You say "check the radiator hoses in the morning when the car is cold. If they're still pressurized and firm, you have a head gasket breach or a crack in the head."

If the engine cools off overnight, why would there still be pressure on the radiator hose?

Oh - figured it out. You are saying start the car in the morning and feel the hose before the engine heats up?????

Now it makes sense.

Maxbumpo · #12 06-20-2018, 02:30 PM

Quote:

Originally Posted by tyl604

You are saying start the car in the morning and feel the hose before the engine heats up?????

No, he said check the hose BEFORE starting up the car. This is the easiest way to diagnose a cracked head on an OM60x engine.

My theory is that the very high pressure combustion gasses dissolve into the coolant while the engine is running, and then when the engine is shut down, they slowly come out. Think of the cooling system like a bottle of soda, which has carbon dioxide gas dissolved into the soda to make it fizzy.

If the radiator overflow tank cap is working, it will hold up to about 15 PSI of the dissolved combustion gasses, even after everything has cooled down, because the combustion gasses keep coming out of the coolant just like a poured glass of soda will be fizzing and popping for awhile after you pour.

tyl604 · #13 06-20-2018, 02:41 PM

Maxbumpo - I finally get it. Thanks for clearing that up. Had never heard that before.

mhin1956 · #14 06-22-2018, 09:34 AM

Still no solution. Driving in this am at 80 degrees (f), car started out on interstate at apx 85 on temp guage (c). slowly climbed at interstate speeds about 10 degrees. AC not on. Came off interstate into town and stop and go brought it up to 105. driving slowly for apx 6 blocks brought it down to 100. Have checked for air in system, none found. Temp sender to be replaced this weekend, but think car is running hot. Found thread that said maybe exhaust restriction (??)..anyone know how to test or what to check? MPG seems ok, car is driven daily.

Diseasel300 · #15 06-22-2018, 10:02 AM

Temps that high at an ambient of 80F suggest you have a cooling system problem of some sort. If you have access to an infrared thermometer check your upper and lower radiator hose when you're having an overheating event. If you have a large difference between the two you have a circulation problem, start with the thermostat.