G Man

My car has been running a bit rough lately. I thought that perhaps I had a plug that was bad, so I put in a new set of plugs today. Finished that up and the car is still running rough. I did a quick check of the injectors using the end of a long screwdriver to my ear like a stethascope with the tip on the injector. All injectors are cycleing so I figure they are ok. I figured I would take a look inside of the distributor to see if anything looks damaged in there. Found a nice sized crack in the cap. I have no idea how this could have happened. I have only had the cap off once since I have had the car when I was installing a new "green" wire, and the car ran fine after that project. Do the caps just crack on their own over time? Got a new cap and she runs nice and smooth once again. The old cap had a rubber cover. The new cap does not. Does this cover come off or is it a part of the cap. I tried to remove it but was unable to.

EC928NC

Just looks like a fatigue crack to me as it started by the holddown clip. From the corrosion on the contacts (which was probably contributing to your rough running condition), it looks like it was time to replace it anyway. No harm no foul.... 10 extra horse power from a new cap right? lol

IcemanG17

I also think the corrosion looks really bad on that cap.....it was time to replace it!!!

dr bob

'Tightened unevenly' is my initial amateur guess-timation of cause. Fit the cap up snug before you put any tension on the screws, and this problem won't reappear.

mark kibort

I dont think the screws can be tighten on the early caps. they have a spring loaded keeper, like a race body panel fastener. i had a couple of those. felt great to touch them when the car was running, if you liked shock treatment.
rotor could have hit the contacts. looks like a contact area by that one electrode.

my S4 cap cracked once, but i also hit a wall at 80mph, so it had an excuse!

G Man

Yeah, it was probably time to replace anyway by the looks of the contacts. Trying to get everything in order prior to putting it up for sale in a week or so.

dr bob

Keerect! I wasn't paying enough attention.

The arcing erosion on the visible cap electrodes show that there were even gaps between the cap and rotor for quite a running while. The impact point that Mark sees may have been a one-shot deal where the cap wasn't seated correctly on install or re-install at some point, but it moved to correct position after the impact.

Mark-- if you were getting buzzed through the hold-downs, obviously they weren't latched down to the metal distributor housing. Or maybe the housing was 'floating' electrically off the grounded block. Unlikely that the housing was not grounded through the drive end and the clamp/adjustment bolt, but stranger things have happened.

karl ruiter

My 944S2 cracks caps. At some point in the car's life the timing belt must have came off and wadded up inside the part of the belt cover that holds the cap in place. So the cover is not quite the correct shape, and when you tighten the caps, snap. I pulled the cover and cleaned it up a bit, but it is a stopgap fix at best.