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I have purchased a Mazda 323 which has approximately 90k km on the clock. While driving in the rain some water will come in above the brake pedal, approximately 5 or 6 drops during an average local journey (~10 km). A mechanic has said that the firewall has deteriorated and will cost a lot of money (~AU$800) to weld up.

This seems unlikely to me, however I am open to correction. Does a car’s firewall deteriorate enough over time to allow water to leak in?

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Firewall in a car .. Is it a different name for windscreen frame? –  Krom Stern Feb 22 '13 at 5:15
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No. I believe it is the divider between where your knees are located (when sitting in the car) and where the engine is located. –  Nippysaurus Feb 22 '13 at 5:49
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Suggestion: a picture of the area in question would help us make an assessment. I'd recommend a shot from the engine's side and one from the pedal's side. –  Bob Cross Feb 22 '13 at 13:11

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up vote 4 down vote accepted

The firewall is typically made up of metal with a layer of fire retardant material. There are holes through the firewall for necessary pipes, wires etc from the engine bay to the passenger compartment.

With old cars (much older than a Mazda 323) I have seen heavily rusted firewalls, but not seen anything like that recently. It certainly shouldn't be a periodic repair, in fact as @BobCross commented - first check that all of the grommets that seal the holes that go through the firewall are in place and in good condition, as it could easily be one of these that is leaking.

But if a weld between the firewall and the frame has gone it is a very good idea to get it fixed, however it is not the easiest thing to get at, so it probably would require lifting out the engine, so it can be time consuming = expensive.

My advice would be to shop around - get a couple of opinions from garages.

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Might want to point out that one of the pass-throughs might be loose: could be water sneaking past a grommet. –  Bob Cross Feb 22 '13 at 13:12

My first concern would be getting the firewall inspected by a bodyshop. If it is that rusted the vehicle may not be safe to drive. In the event of a serious accident the firewall deflects the engine under the passenger compartment. In my experience most leaks are due to either a leaking windshield or as @Rory Alsop has stated a bad grommet where a cable or linkage passes thru the firewall. You can try to isolate the source by spraying the glass area with a hose while someone looks under the dash. The mechanic may have been refering to the channel that the glass sits on. If this is the case a repair may not be that extensive.

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