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Overheating Solutions

K75 Overheating

Things to check for an overheating K75

From the BMW list:

Date: Thu, 13 Apr 95 18:01:07 MST
From: Rob Lentini
Subject: BMW: Re: Art Jacobson’s Hot Ks

Art: It is unusual for K75s such as you and Katherine ride to get so hot to the touch, as you report. Things to check:

1. Verify operation of the cooling fan. Start the bike up and let it fast idle until the fan kicks in–about 10-20 minutes depending on outside temp. These fans are known to seize up and burn the commutator brush assembly. Clue? Your horn doesn’t blow because the fuse that powers it and the cooling fan is blown due to the seized fan.

2. Change the fuel filter in the tank. A plugged filter will slow down (eventually stop) fuel flow and overheat the fuel in the lines and fuel rail, giving you the “boiling” sounds you hear, and will eventually cause vapor lock.

3. Install the foil-like heat insulator blanket on the underside of the fuel tank. Common on K100s, many K75s didn’t come with it.

4. Ride faster!

Rob Lentini
’87 K75S
Tucson, AZ

From: (Joe Senner)
Subject: Re: BMW: K-Bike Overheats!
Date: Wed, 31 May 1995 10:28:57 -0500 (CDT)

]I heard a muted "pop" and noticed steam rising from the right side
]of the bike and hot coolant running down my pant leg. I pulled off
]to the side of the road and as I did so the "hot" idiot light 
]came on for the first time. There appeared to be about a quart of
]coolant in the road....
]pressure-relieved itself. Now I'm not so sure...why was the overflow 
]cap seemingly blown off, especially considering that it's got a vent hole? 
]Why no idiot light until AFTER the incident occurred? Any theories here?

you’ve probably gotten a bunch of answers to this already, I’m so far behind in my mail, oh well, I’ll pipe in anyway.

things to check:

  • 1) the fan. make sure it turns _freely_. spin it with your hand to make sure.
  • 2) the fan circuit. check the fuse. [same circuit as the horn – wd]
  • 3) the radiator cap valve and seal. pull the radiator cap and check the rubber seal on the bottom. it should fit well. make sure the screen in the bottom of the cap is clean. I wipe mine off and try to blow/suck air through it. you should have easy flow when sucking on the cap, but you shouldn’t be able to blow air through it. this lets coolant into the system easily but makes sure it’s pressurized to get out.
  • 4) pull the reservoir tank hose off at the cap and blow on it. make sure it’s clear. look for bubbles in the reservoir when you blow on it.
  • 5) brush your teeth after 3 and 4 above. yeech…
  • 6) make sure the level in the reservoir itself is _correct_. as the bike heats and cools near the extremes it’s going to be using that tank. too much to start with and you can expect some overflow.
  • 7) make SURE SURE SURE your coolant is fresh, AND mixed properly. if in doubt, change it! use the correct antifreeze! many US sold brands have additive packages designed for cast iron engine blocks. this does NOT work well with a full aluminum system! this US trend is changing, but better safe than sorry. I never let my AF go over 1 year old, and always use distilled water in the system (except when flushing).

one thing to remember about K bike idiot lights is that the sender for the light is mounted near the bottom of the cooling system as a whole. this means it’s going to get a reading from the coolest point in the system. the other thing to remember about idiot lights is that they are just that, idiot lights, and are simply a guide. knowing when your fan should be on and isn’t is a better warning system.

Joe Senner

From: “keith (k.p.) hanlan” <KEITHH@BNR.CA>

I just wanted to one more item:

  • A cracked breather hose can introduce unmetered air causing the engine to run lean. This breather hose is a short S-shaped hose running from the secondary airbox above the throttle bodies down into the crankcase.

[FWIW- I had mine replaced under warranty – wd]

Thanks for the service you are providing.
Keith Hanlan Bell-Northern Research, Ottawa, Canada 613-765-4645

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