I had to replace the bypass hose on my 300TE and decided since the job was so involved to replace the original water pump, tension pully, thermostat and hoses. Also replaced that little shock absorber thing as well.
Now the engine is running hotter and sometimes the aux. fans don't come on even with A/C on and very warm engine.
Everything was operating good prior to this work. Oh, sometimes the aux. fans will turn on if I turn the engine off and on. Seems to be a loose connection, but I don't know where to look.
Any ideas?
I'm still pissed I had to do so much just to replace a bypass hose.

Was the water pump OEM quality? Some after-market pumps have less than efficient impeller blades and do not flow water as well as OEM pumps. How about the tension on the serpentine belt - snug? And the t-stat - when you reinstalled/replaced it did you place the air bleed valve at the highest position in the housing where it sits?

Sounds like you stillhave an air pocket in the cooling system. Let it run a cool down while adding coolant when you have it running. You may need to bleed the air out of the system. My 103 motor has a air bleed on top of the thermostat housing.

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1990 190E 2.6
1996 Grand Voyager 3.3
1985 Mustang GT 5.0 5 SPD
1982 Suzuki GS 750T

Used MB pumb from dealer and belt is tight. I'm going to check the thermostat. I still lean towards something electronic. I drove around all day yesterday and the Aux. fans didn't come on even though engine was hot. It's kinda hit and miss with the fans.

Sure sounds like an air pocket to me. That will cause both the hot running and cause the fans to cycle strangely. Run it, let it cool, add more AF, run it, etc. Takes a bit to get the air out if there is no bleed valve.

Brian

If air-in-the-system is the problem, this post may help.

Cooling system "burping"

On mine, with no bleed valve, the only way to get the air out is to fill the reservoir to the top and let it cycle through hot and cold coolant a couple of times. I agree with the above posts that you should start with the assumption you have trapped air.

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Steve
'93 400E

Thanks for the lead and thanks, Donnie, for the great tip. I just did a quick search on the web and found the AirLock for ~$84. It's at this link:

http://www.thetoolwarehouse.net/shop/UV-550000.html

Richard Easley
Waco, Texas

Now I know what to ask for for Christmas.

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Chuck Taylor
Falls Church VA
'66 200, '66 230SL, '96 SL500. Sold: '81 380SL, '86 300E, '72 250C, '95 C220, 3 '84 280SL's '90 420SEL, '72 280SE, '73 280C, '78 280SE, '70 280SL, '77 450SL, '85 380SL, '87 560SL, '85 380SL, '72 350SL, '96 S500 Coupe

Hi 2LeftGoofyFeet,
The aux fan is working in two steps. first between 80-96 C (slow speed), and above 96C (high speed).
And If you want to check the aux fan system, just look for the aux fan thermostat, it is located aprox. half way on the top left side of the engine besides the valve cover. It is the bolt with two contacts on it. Remove the socket (socket only, not the thermostat) from the thermostat at engine high temp., and the aux fan should run at high speed. It means the aux fan is okay and thermostat is not working (remove the thermostat at cold temp engine only).
Or you might check the ceramic resistor behind the left headlamp. It is for the slow speed aux fan. Using a short piece of cable try to jumper the two terminals accross the ceramic. If the fan run at slow speed, the ceramic resistor is broken (usually coroded). This happened to my 300E'88. Hopefully you can find the trouble, be carefull and have fun.