1996 F350 (460)

Does anyone know why a distributor cap would corrode quickly? The original was very corroded when I bought the truck, so I replaced it with one off Amazon (maybe poor quality part?). After about a week, I was noticing my truck was having to crank for extended periods of time before it fired up. I pulled the cap off and noticed quite a bit of corrosion already forming on the cap contact points. I sanded them all off, then reinstalled the cap which fixed the problem. However, a few days later, the longer crank times started to return due to corrosion forming on the cap contacts again. Is there an issue with the voltage going to the distributor possibly? Any suggestions would be greatly appreciated!

 

Corrosion could mean moisture, do you have a coolant leak anywhere?

 

No coolant leaks, its just excessive build up on the contacts. It sands off easily, but leaves some burnt marks. No signs of moisture anywhere.

 

​​​​​​It may just be a poor quality part prone to oxidation.

 

More than likely it's aluminum contacts & the ignition in these thing are very hot, high energy despite what's considered HEI by experts.

Getting a Motorcraft cap is no gaurantee, most are aluminum.

Get a fancy MSD cap or a Standard Motor Products with brass.

They last a bit longer.

 

I use distributor caps with BRASS contacts, exclusively. The aluminum ones corrode/'fuzz-up' in a matter of days if operated in a humid environment. Just the jumping spark in a high-energy ignition from rotor to distributor contact creates heat. A spark also crates Ozone. Ozone is O3. Normal 'stable' Oxygen is O2. Ozone will try to divest itself of the extra O molecule. Heat + extra O + moisture ( H2O - more Hydrogen and Oxygen molecules) + metal will cause Oxidation of the metal. Some metals are more prone to Oxidation than others. Oxidation of aluminum takes the form of white fluffy corrosion... Brass does not corrode as easily. Gold even less so, but who would want to pay for gold-plated contacts everywhere...

 

Similar to how hydrogen peroxide works, O is a very lonely molecule, and will readily oxidize something.

 

Yeah & the vents in the cap are damn important for the above reasons.

 

THe reason for this oxidation, corrosion, and build up is the formation of nitric acid

Nitric acid is formed by Ozone, nitrogen, and water.

Ozone is created by the spark jumping, the energy creates the O3 in the simplest way of explanation,
The nitrogen is split from the ambient air when the oxygen is taken for ozone, and the moisture is either just ambient conditions, water intrusion (say some water spray got up in there) , or otherwise.

The problem is enhanced on unventilated caps. I personally have bad experience with the red MSD caps but the Oreilles blue cap I bought had brass terminals and held up well., make sure to get a matching rotor!!!

Finally, if this issue is still persisting, then ventilating the cap with a vacuum line and a fresh air supply are solutions which could be employed relatively easily based on the location of the 460 distributor. If you 460 is MAF you need to source the air after the MAF sensor, if the truck is a MAP sensor ,I think you can use the same pull source as the PCV fresh air or just anywhere after the air filter, then use a vacuum line to the cap from your tree.
But doing so would be wise to use a restrictor, flame arrestor of some sorts , to ensure leaky injectors dont cause you to blow up your cap on startup some day. Or use an alternative place to draw the vacuum, like the vacuum tank.

Note . nitric acid is more corrosive to brass than aluminum, but aluminum is more prone to create a mineral layer (passivates) of aluminum oxide, which is not conductive, which creates resistance which is lowering your spark, and aluminum is also prone to HVE (high voltage erosion) more than brass, so the contacts get "further" apart with every spark jump. This means the more gap you get, the hotter that spark jump is, creating more ozone, creating more acid, and that acid promotes that passivation layer, and then its just a cycle downward.

The brass, while its corroded more by nitric acid, is also harder and doesnt create the same corrosion that aluminum does, and it means more stable conditions. If your cap is ventiliated right, it shouldnt even build up that much under the cap assuming your distributor shaft isnt letting crankcase vapors up into it

 

Dammit! I was an Mech Eng major waaay in my past (my college experience was almost 50 years ago) I totally forgot about the Nitrogen component - 70% of our air! At least I remembered the sparks creating Ozone, combining with moisture part, and some of my Materials Science.

I'm retired now, and my brain is mush. That's my story, and I'm sticking to it...
Why do you think I have that oh-so-fitting Avatar?

 

Thats the spirit! But At least the important part of what/why/how is something you remembered, the acid part is cool and stuff but isnt functionally important to understand for the lay person

 

My wife is constantly amazed of my memory... just don't ask me where I left my reading glasses - or car keys, LOL!

 

You guys are awesome, thanks for the wealth of knowledge, and schooling behind the corrosive environment inside a distributor cap! Looks like I have what I need to fix this problem, Thanks again!

 

I know for at least my 1991 F250 that I used to have, I would buy BlueStreak because of the brass terminals. I feel that NAPA does brass terminal parts as well for cap and rotor. I don't know why Motorcraft doesn't do brass.

 

It's made, just cost like sin & next to impossible to find.