I hope this post will interest a few owners of classics that have an ECU controlling the fuel injection. I went to start my 1985 735i automatic E23 BMW. It had been parked out in monsoon rain all day. The engine started but would not idle, and the revs were surging wildly on their own. A very odd feeling. I engaged drive and tried to move the car. It moved forward and then did a series of really violent bunny hops like the engine was seizing and then freeing off. it was very scary. I had to abandon ship and go home by train. That was another disaster as it happens. I returned to the car next day with my mechanic friend. He fitted a new Coolant Temp Sensor, and that cured the rough idle, but not the violent bunny hops and surging revs. He tried a spare Air Flow Meter but it made no difference. He then checked the FI ECU which on this model is located in the front wing cavity, behind the driver's footwell speaker grill. He instantly spotted droplets of water on the ECU. Bad sign. He removed the ECU and cracked open the metal casing. He then removed and unfolded the PCB inside. It had water on it. He cleaned and dried the PCB by placing it by the heater outlet of his car for 20 mins, with his engine running and the heater on full. When the PCB was completely dry he reassembled the ECU and fitted it to the car. She started instantly and ran perfectly, and I was able to drive home. I have now purchased a spare used ECU and fitted it to the car. My old one will be a kept as an emergency spare, in the car.
Some people wrap vulnerable ECUs in plastic bags to keep water off, but make a small hole at the lowest part of the plastic wrap to allow any moisture to escape.
My car needs some welding as we found rust in the wing cavity.
Exactly the same issue had once affected the mechanics own 320i E30 BMW.
Keep your ECUs free of moisture!
Some people wrap vulnerable ECUs in plastic bags to keep water off, but make a small hole at the lowest part of the plastic wrap to allow any moisture to escape.
My car needs some welding as we found rust in the wing cavity.
Exactly the same issue had once affected the mechanics own 320i E30 BMW.
Keep your ECUs free of moisture!
