87 300D running hot - Mercedes-Benz Forum
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#1 (permalink) Old 04-16-2013, 03:54 PM
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87 300D running hot

I have a 1987 300D with 223,000 miles on it. It is running hot when I drive the vehicle over 55mph. it will sit at idle and not run hot. I have replaced radiator, water pump and thermostat. Waiting on new fan clutch, but I don't think its bad. Does anyone have any ideas about what it can be? There is no water and oil mixed and we did a CO2 test from radiator cap and it passed. Is it possible the head is cracked and runs hot after pressure builds?
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#2 (permalink) Old 04-16-2013, 04:06 PM
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Since you have a '87 diesel, do you have the dreaded head#14 or some later revision???

Usually running hot at speed indicates a bad WP, but in your case it sounds like either clogged coolant passages, or lack of proper pressure. The fan clutch does very little when the vehicle is at speed.

IF you do replace the fan clutch, ONLY use a genuine MB fan clutch. All the others are junk, including the BEHR fan clutches purchased from places other than MB. BTDT

Also, does your radiator cap have the proper rating??? I believe for your year it should be 1.4 bar. If you use a 1.2 bar it will prematurely overheat. Only use genuine MB radiator caps. Water pumps should only be Graf, Laso or rebuilt by MB. The water pump impellers have a tendency to spin loose (on old or substandard pumps) also causing hot running conditions.

Can we assume you are running genuine MB coolant (a.k.a. Zerex G5) mixed 50/50 with distilled water and nothing else???
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#3 (permalink) Old 04-16-2013, 04:49 PM
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I do have the #14 head. I am not running MB coolant. Will that make it run hot? It has the original coolant cap.
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#4 (permalink) Old 04-16-2013, 05:03 PM
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On MBs, not using the proper coolant is asking for big trouble.

Flush and replace coolant with proper spec mixed 50/50 with distilled water and get a brand new cap from the dealer.

Then report back.

Is your thermostat a brand new MB/BEHR t-stat??? Don't use Stant or some other low quality t-stat.

Remember, local brick & mortar autoparts stores carry ZERO stuff for MBs, except windshield washer fluid.

See link

http://www.2phast.com/500e/techdocs/...ze_article.pdf
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#5 (permalink) Old 04-16-2013, 06:33 PM
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The type of coolant you run is irrelevant. I run 2/3 dystilled water and 1/3 cheap green coolant and an 80c thermostat, NO clutch fan and a 14 head. It never over heats.

Replace your water pump. The story on the 14 head is over blown. I saw a 300SDL with a 14 head at the JY with 320k miles. It had been rear ended.


87 300D turbo + 5spd
93 190e 2.3 + Megasquirt 1
92 190e 2.6 5spd + Megasquirt 2
97 E300 DIESEL
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#6 (permalink) Old 04-16-2013, 06:35 PM
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Originally Posted by tjts1 View Post
The type of coolant you run is irrelevant. I run 2/3 dystilled water and 1/3 cheap green coolant and an 80c thermostat, NO clutch fan and a 14 head. It never over heats.

Replace your water pump. The story on the 14 head is over blown. I saw a 300SDL with a 14 head at the JY with 320k miles. It had been rear ended.
The OP claims he already changed the water pump, but we don't know what kind he used.

The coolant type IS very important. Try running GM Dex-Cool in your car or any other car (even a GM car) and watch what happens.

Only MB coolant (a.k.a. Zerex G5) has a ph level of 7 which is completely neutral. Anything else is either too acidic or too alkaline.
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#7 (permalink) Old 04-16-2013, 06:49 PM
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I've been watching what happens with the cheapest green coolant I can find over the last 300,000 miles on 3 different Benzes so far. The part people seem to forget is changing it out every 2 days.


87 300D turbo + 5spd
93 190e 2.3 + Megasquirt 1
92 190e 2.6 5spd + Megasquirt 2
97 E300 DIESEL
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#8 (permalink) Old 04-16-2013, 07:03 PM
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Originally Posted by tjts1 View Post
I've been watching what happens with the cheapest green coolant I can find over the last 300,000 miles on 3 different Benzes so far. The part people seem to forget is changing it out every 2 days.
Huh??? Change coolant every 2 days??? Surely you must mean every 2 years. And don't call me Shirley
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#9 (permalink) Old 04-22-2013, 10:40 AM
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Originally Posted by sbaert View Post
The OP claims he already changed the water pump, but we don't know what kind he used.

The coolant type IS very important. Try running GM Dex-Cool in your car or any other car (even a GM car) and watch what happens.

Only MB coolant (a.k.a. Zerex G5) has a ph level of 7 which is completely neutral. Anything else is either too acidic or too alkaline.
Replaced cap and coolant. Still runs hot above 55 mph. Any ideas?
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#10 (permalink) Old 04-22-2013, 11:08 AM
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What brand water pump & radiator are you running and how old is it???
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