Exploding Distributor Cap

[jrcotner]

The truck was running well after more tweaking on the on the motor, though a little hard starting. After a stop in town I tried to start it and nothing. Did this for about 10 seconds and let the starter cool. After the second try I heard a bang under the hood, and thought it was a carb backfire. Another try on the starter and still nothing. So I removed the access cover to the engine and saw the distributor cap hanging by the plug wires. I removed the hood and found that the distributor cap had broken off at the hold downs, which were still holding fast to largish pieces of plastic. I replaced the damaged cap with my spare and drove home without incident. It was still a little hard starting but ran fine. The destroyed cap was one month old.

Any idea on what caused this to happen? I've seen distributor caps crack, but never had one completely self destruct. I don't want to install a new cap without trying to address the root cause first. I've got Pertronix ignition, NGK BPR6ES plugs (one month old), and timing set at 8 degrees BTDC.

[Jimm391730]

This is common on vacuum advance distributors when the vacuum diaphram gets a leak; sometimes fuel/air mix migrates up the hose and into the dizzy, and when you go to start it the arc at the rotor flashes it and it will blow off the cap (especially those with the spring clip hold downs as was common on US iron from the 60's-80's).

But the Pinz does not have a vacuum advance, but it DOES have a small vacuum line that is intended to slowly purge air and moisture from the cap through the little bead restrictor. Is the restrictor bead in the line? You could remove the vacuum hose (and plug it) to prevent any fumes from getting into the dizzy for a test.

[pcolette]

I'm assuming (maybe I shouldn't) that you have the civilian ignition kit installed as well as the Pertronix. Depending on who's kit you have the adapter plate for the dist. cap was made in a couple of different thicknesses. One uses the rotor as is but the other adapter requires that the rotor shaft be shortened by about 1/8". If not shortened it sits against the Pertronix inductor magnet and places excessive pressure against the inside of the cap and can cause the cap to break at the ears, as yours did.

Here's a service bulletin on Scott's site: http://www.expedition-imports.com/categ ... tallation/

Note #19.

Hope this helps.

[David Dunn]

Check to be sure the base of the rotor was trim to fit over the Pertronix...
Otherwise the rotor sits too high and may have come in contact with the cap.
Look for wear marks on the Pertronix module and inside the cap and on the rotor.

[jrcotner]

The Petronix was installed by Scott before i got the truck. The rotor was trimmed and replaced when I replaced the distributor cap. However, when I did so I noticed the vacuum line was cracked and I replaced it. I didn't know about the vacuum restricter, but I probably have the old line in the trash can and can fish it out. Since the line isn't for vacuum advance, is there any problem with plugging the vacuum ports at each end and ignoring the vacuum line altogether?

[totaljoint]

[quote] is there any problem with plugging the vacuum ports at each end and ignoring the vacuum line altogether?[/

I had lots of problems with condensation under mine. Lots of removing seats and dog house to get the dizzy wiped out. Then again might be a northwest thing. I think that's functioning vacuum line is there for a reason. Won't have to worry about it for long though with the Goatwerks EFI conversion!

[Jimm391730]

Won't have to worry about it for long though with the Goatwerks EFI conversion!

Oh, yes! Distributorless is the way to go! I don't expect to worry about water/condensation ever again! And it starts well at ANY temperature, from single digits to triple digits so far!

[jrcotner]

The old vacuum line went missing, so I checked on the SAV site and they have a vent kit but not the little valve for the vent line. any idea if anyone sells just the little valve?

[undysworld]

I'd see if you can't find a used one from somebody tearing apart a wrecked truck.