Join the Car Talk Community!

Broken Distributor Cap

The Puzzler


RAY: Here's the answer: This wouldn't have happened to older cars or even a newer car with a distributor cap, because they are missing a piece that older cars have. That is a vacuum advance. The vacuum advance is simply a vacuum operated device which contained a diaphragm and a rod, and when the engine vacuum was high it would pull the plate and advance the timing.

TOM: An elegant little device I might say.

RAY: It's rather crude. It's all done electronically now.

TOM: Well, compared to electronics. But it was elegant in that you could see something happen.

RAY: Well, it's analogous to beating on drums, but crude compared to a telephone call.

TOM: Or email.

RAY: And the vacuum advance diaphragm had a little hole in it. So that it really didn't work, but it also didn't affect the performance of the car all that much.

TOM: Don't forget where it's getting the vacuum from -- the manifold.

RAY: And when the thing was shut off...

TOM: What's in the manifold?

RAY: A mixture of gasoline and air would waft into the vacuum advance, penetrate the diaphragm, and get on the other side of the diaphragm, where it would be in the area of the cap and the rotor. Where there are sparks of high voltage! And when that mixture of gas and air - and you turn the key, that first spark exploded the cap.

TOM: Badabing, badabang!

RAY: So he needs another 800 bucks for the vacuum advance.

[ Car Talk Puzzler ]

Support for Car Talk is provided by:

Donate Your Car,
Support Your NPR Station

...and get a tax break!

Get Started

Find a Mechanic


Go



Submit


Rocket Fuel