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How do you access the plenum chamber drain in a 2004 Mitsubishi Lancer?

Asked by Peter

How do you access the plenum chamber drain in a 2004 Mitsubishi Lancer? I suspect it is blocked.

Answered by CarsGuide

27 May 2021 David Morley

The first sign of a blocked drainage system in a car is usually wet carpets. This, however, is not necessarily the result of a blocked plenum drain, as there are other causes including a blocked air-conditioning drain, a poor door or window seal and even a hole in the firewall between the engine bay and the passenger compartment.

In the case of a simple hole in the firewall, the solution is usually a rubber grommet which will cost a few cents and will sort things. For other leaks, however, you need to take the time to learn where the drain tubes live and ensure that they’re clear and free of mud or dust that could be blocking them, causing them to overflow into the cabin.

The other possibility is that the leak into the car is being caused by a faulty heater core which is allowing the engine’s coolant to escape. That’s a bigger fix as it usually involves removing the dashboard to access the heater core which then needs to be replaced. But if you’re lucky and it’s a simple blocked drain pipe, the drain holes for both the plenum and the air-conditioning drain should be visible on the firewall, below the windscreen. Undoing them and clearing them would be the first step to curing the problem.

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