What's under your distributor cap?

[mjk1040]

So last fall the neighbors 7.4L Merc is giving him some hard starting issues. So for shit's and giggles I pull his distributor cap and low and behold the rotor end and everyone of the distributor firing pins is corroded with that green build up scale. So we put a new cap and rotor in and change out all the spark plugs. Starting issue much better. So later this fall I'm changing my oil and filters and for shit's and giggles I decided to pull my distributor cap. Low and behold if my distributor cap and rotor looked as bad as the neighbors along with an accumulated amount of rust dust. So this spring I got new plugs, caps and rotors and went to work on my ignition system. The plugs appeared to all be burning perfect with a medium brown color on every spark plug I examined. Getting the rotors off the shafts was a chore, they had rusted on pretty good. I guess I hadn't noticed a starting issue in my case, though it did seem to take a little longer to fire up each time we went out. So I guess the marrow of the story is, take a look before its a real issue. Since I'm the 4th owner and found no service record of plugs, caps or rotors, for all I know they could have been originals! Question, the caps are I believe the HEI style and they had no gasket on them and the new ones I got had no gasket, but the neighbors cap did have and came with a gasket but yet had a moisture vent in the cap. Should these have a gasket or should we try to seal them with silicone or not? <Crusaders>

[Midnightsun]

What distributor cap?

[km1125]

Mine don't have gaskets.

If you see ANY signs of actual rust in there, then DEFINITELY make sure your mechanical advance is working, if the distributor is equipped with it. I have an older classic car and pulled the cap once and I was SHOCKED to see that the advance was rusted up and one of the springs broke. That car RARELY sees any rain and NEVER sees winter. Took several hours to clean the distributor up and get it running correct again. A periodic spray with some lube would have avoided that if I had any inkling that there was an issue going on!

[mjk1040]

Yea Midnightsun, If only I had diesels, life would be real good. km1125; electronic distributors, no mechanical advance to my knowledge. Under the rotor appears to be a plastic or fiber gear, I believe the ECM does the advancing.

[bud37]

Mike, look in the parts blowup for your engines ignition parts and see if the original specs a gasket.....if so get one, the height is established properly with the gasket in place.......rotor to cap clearance.....good practice at the least.

People will say you don't need it but if it was there you do.

[SplashyLady]

MIke - are you sure you got a marine distributor cap? The automotive one is slightly different and is not spark-proof like the marine one. You don't say what kind of engine you have - but if it has centrifugal advance - check those springs and weights! Replacement springs are cheap but you have to get the right one. You can get a kit with an assortment of springs if you want to try fine tuning a bit, or just replace with the original. Mercruiser with the Thunderbolt ignition system does not use springs, as the advance is electronically set by the controller/amplifier.

Here's an example of a spring set: https://www.summitracing.com/parts/mrg- ... refilter=1

[mjk1040]

Crusader 7.4L, checked parts diagram and see no gasket! Also saw no mechanical advance, just electronic module!

[bud37]

Well there you go , if the OEM specs no gasket then you are good to go using their cap....I would'nt worry further.....

My merc uses a gasket.

[mjk1040]

Yes the Merc's do use a gasket, as I notice on my neighbors 7.4L, but the Merc cap also has a vent plumbed into it also if I recall correctly. Go figure!