Distributor Cap corrodes yearly

[toddh]

Why does my cap need replaced each year? It's stored indoors and I always take the plug out each weekend. But every spring it starts missing and this is the culprit.

 

[hughespat57]

Are you using a quality marine cap and rotor?
The distributor should have a small vent somewhere with a metal screen, this is a little flame arrestor just like the engine intake. A marine distibutor cap must be gasketed. So very little air (and moisture) should be inside the distributor, and every time you run it up to temperature it should dry it out.

 

[markrsimon]

This fall before putting it in storage take a look at the cap to gauge its then-current condition. I would guess that the winter temp changes which can lead to condensation are your problem. You could try wrapping the dist this fall in plastic to help limit air exchange. Or you could do what I did (which goes against what everyone said) and seal up the cap - preventing anything from getting in there. I used a bit of black RTV. My issue was water hitting the top of the block, evaporating up into the cap and condensing - causing it to miss while underway. I finally fixed the water issue. Anyway - I dont recall the exact reasons they say to not seal it up - but thinking it had to deal with heat (spark gap between rotor-cap), air expansion, pressure increase.

 

[toddh]

Mercruiser distributor cap and rotor. I don't see a gasket. At least none have ever come with the cap.

 

[hughespat57]

This fall before putting it in storage take a look at the cap to gauge its then-current condition. I would guess that the winter temp changes which can lead to condensation are your problem. You could try wrapping the dist this fall in plastic to help limit air exchange. Or you could do what I did (which goes against what everyone said) and seal up the cap - preventing anything from getting in there. I used a bit of black RTV. My issue was water hitting the top of the block, evaporating up into the cap and condensing - causing it to miss while underway. I finally fixed the water issue. Anyway - I dont recall the exact reasons they say to not seal it up - but thinking it had to deal with heat (spark gap between rotor-cap), air expansion, pressure increase.

Sealing the bottom of the cap would be fine, but better was fixing the water problem.
You should not really seal over the vent I mentioned because as the air inside expands with temperature it will find a way out. The reverse will happen when it cools, vacuum. Either way can cause the cap to crack and fail, sometime quite spectacularly.

 

[Nater Potater]

Look for something like this brass screen in the middle (or bottom) of the distributor body:

This is for a Ford, but they're all similar. While running, the action of the spark between the rotor tip and cap contacts creates ionized air and ozone inside the cap. The ionized air can lead to cross-firing, and the ozone is quite corrosive. That, and any fumes that find their way from the crankcase up through the shaft will tend to condense inside the cap. They all gotta go away somehow.

 

[toddh]

OK thanks I will look into it.

 

[Wanderer 290]

Get Napa cap and rotor. Comes with a gasket and vent elbow. They also have brass contacts.
Used them for 20 years and no problems.