Hot Running Problem Solution

By Ed Milich
April 2001

I have a submission for the K-Bike tech articles. It’s on a K100 hot running problem.

This problem has happened to myself and a few other K100 owners that I’ve talked to. We never found the exact source of it, but we gained some insight, and effectively cured it.

The problem can be described as a fuel problem during hot conditions. For example I was riding through the desert @ 85MPH in 115 degree heat. Bike loses power for a few seconds. I struggle to keep it running, bike gains power again, and the problem repeats until the bike finally won’t run.

After sitting on the side of the road for 5 minutes, the bike will start up again. This happened 2 or 3 other times to me. Each time, it was relatively hot outside (upwards of 90 degrees F).

The bike in question was an ’85 K100 with 50k miles on it. At the time, the bike was in a good state of tune, with a relatively new fuel filter. Upon examination, the gas tank was hot to the touch.

First thoughts: Gas tank vacuum problem? No- I opened the gas cap when the problem first occurred, but this didn’t solve anything. Gasoline is boiling somewhere in the fuel injection system? Weak fuel pressure? What we narrowed it down to is some kind of problem caused by gasoline that is too hot. Possibly gas boiling somewhere in the FI system. A quick internet search determined that gas boils at a range of temperatures (from 130 degrees F and up), due to differing formulations.

BMW added insulation to the underside of their tanks early in the K-bike series. Obviously, when the fan blows hot air from the radiator, some of the heat is transferred to the underside of the tank.

Add to this that the black tank cover that I was using at the time was absorbing a lot of heat from the midday sun. It’s easily possible that the gas temperature in the tank could have reached upwards of 130 degrees F.

All of the people who had this problem were using a large tank bag and/or tank cover. In any case, I took my black tank cover off and the problem hasn’t occurred again.

If the problem persisted, I was going to begin a thorough diagnosis by running a fuel pressure gage from the fuel rail, in order to check the fuel pump operation. Also, if your bike is not already equipped, some insulation under the tank may help protect it from heat transfer by the fan’s hot air flow.

I hope this helps someone. The problem can be very frustrating.