Plenum drain cleaning
From Knowledgebase
The Problem
A problem in the early 90's 100 (C4 platform) is clogging of the drain holes inside the air plenum in the engine compartment just in front of the windshield. Leaves, pine needles, and other debris can collect in the plenum, and work their way into the plenum drains, plugging the drains. When that happens, water will find it's way into the interorior of the car, dripping down somewhere behind the dashboard. A major problem is water dripping down the right side, following the wires down to the ECU (Engine Control Unit). The water can corrode the connections or get inside the ECU and short it out. When that happens, you will be left stranded.
The Fix
The fix is easy (provided no damage has been done to the ECU. If so, see ECU wiring). All you need to do is clean out the air plenum, and make sure water flows freely through the drains.
- Open the hood (bonnet to the Brits).
- Remove the air plenum cover. The air plenum cover is held in by clips. Working from one side to the other, lift the front edge of the cover straight up, freeing it from its clips. Once the cover is freed from the clips, pull it forward to free it from its channel at the rear of the plenum.
- Clean the air plenum of debris. With the cover off, you can see the air plenum, and the debris that has collected in it. You can vacuum out the air plenum, blow it out with compressed air, or use your hand, although there isn't much room for a hand to reach in and under the HVAC blower and other bits.
- Clear the drains. There are two drains; make sure they are free of debris.
- One is an oval opening on the left (when seated in the driver's seat) by the fuse box, on the vertical panel near the fender.
- The other is circular and rubber covered, located on the right side on the bottom of the plenum in front of the blower motor.
- Test them. Pour some water into the plenum. The water should flow freely out of the drains. The fuse-box side drain exits under the fender. The blower motor side drain exits into the engine bay. I used a stream of water from a garden hose. It started to fill the plenum until I got the big drain (in front of the blower) open; then the water rushed out into the engine compartment.
Summary
That's all there is to it. You may want to make this procedure a standard part of your spring cleaning. It will help prevent problems with the ECU.
-- Main.KentMcLean - 28 Jun 2004
