Why no humm???
 

I recently had my CIS engine rebuilt. During that process the other major work done was a heater back date.

One of the first things I noticed was there was no longer the high pitch humm when you turn the key on.

What is/was that noise and why is it no longer there?

Any ignition changes? Factory Capacitive Discharge units should emit a sound I'd describe more as a whine when the key is on.

yeah, the CD box emits a high pitch "whine," while the fuel pump "humm" should only be heard until proper fuel pressure is reached in the system.

No changes past what is mentioned. The CD unit has not been changed and the car has not had that whine since up and running again 3000 miles ago. I had wondered.

Funny enough the car has failed to start 3 times now. Twice on me and once at the shop where they seemed to think it was a wiring problem that they "solved".

1st time was on the road (you could tell something wasn't right when the engine turned over and didn't fire) and a few minutes later I hit the switch and it fired right up. Drove 500 miles home with no further problems.

Dropped the car off for a change to Mobile one and they couldn't get the car started. "Wiring" was said to be at fault. Drove it home and parked it after 200 miiles that weekend.

Two weeks later it fires right up and I make a run to the post office. Won't start on my return to the car. Same sound when you crank it over. Flat bed 30 miles back to wrench's shop. Lower it off the flat bed and it fires right up...twice. Pretty tired of iti all by now. Go have lunch and return thinking it will fire up again no problem. Of course.....zip....nothing, de nada.

A 200 mile tow would have been a lot worse than a 30 mile tow.

So anyone have a decent CD box, cheap, for a 78/83?

Upgrade to an MSD, you won't regret it.
Sam

Now, just what would be the point of <b><i> wasting time and money downgrading to an MSD</b></i> ... when there is obviously intermittent power getting to the CDI unit???

How about just spending a bit of elbow grease to clean the 25-year old connectors with Q-tips, 9% vinegar, distilled water & 91% isopropyl alcohol , then crimp the sides of the female Fastons, and when the intermittent problem is solved ... just put a bit of Dow Corning 4, 111, or 112 silicone grease on the connections to keep out moisture and oxidation???

Warren, you should definitely write a book about inexpensive Porsche care. You could save us thousands. :D

Ok, I just dropped by the car. Lifted the lid and took a look at the CD and wiring. Everything looked in order. Tapped on the CD box. Of course nothing happened. Cranked the car over and it fired right up.

Let it run a bit and turned it off. Again it fired right up. Drove around a bit and parked it. Just before I left it again fired right up. Still no whine from the CD, but the thing is running. Any suggestions?

How long do you plan on keeping this car? Just a suggestion, but our host sells rebuilt CD units. First, do what Warren suggested. It costs nothing but time. Then consider buying a bad CD unit at a swap meet...send it to Pelican as a rebuildable core...get one you KNOW is good...then send you old one in...presto spare CD unit. Without that whine? Perhaps your CD has something internal causing this intermittent running? Hoping Warren or Loren will weigh in more here...

Why not the MSD? Doesn't running the MSD allow you to open up the spark plug gap a fair amount giving better combustion? That reminds me, I've been meaning to ask this:

Does a wider plug gap make detonation less likely in a high compression engine?

Is so, and if the MSD allows you to do this, it may work out well in rdane's 3.4 SC

Ive got a stock Bosch CDI in my car (spare in the trunk also), and I have no complaints. If both of my boxes died tomorrow, I'd probably go for the MSD. You can probably get a new 6AL for ~$200 from a local speed shop, how much does a rebuilt 6 pin Permatune or Bosch box cost?

Sherman,

<b><i>Do you normally, as a matter of policy ... replace both front wheels and tires, and rotors and hubs ... when a wheel bearing goes out on a 25-year old car you love, and plan to keep around for a long time???</b></i>

That is the best analogy to installing a MSD when your currently working CDI unit starts acting up due to corroded/loose/intermittent wiring connectors on an older car!

Opening up the gaps beyond about 0.040" - 0.045" can cause a whole host of problems ... burned out rev-limiter rotors, burnt cap carbon contact buttons, burned cap connector insulators, burned coil tower insulators ... etc. The wider gaps put the entire ignition distribution system under higher stress, and the weakest point fails, and then a domino-effect can take place ... well documented on the hot rod chatrooms and bulletin boards around on the 'net! There is even a replacement brass 'button' available for Chevy/GM HEI distributor caps to replace the carbon one that burns out!

MSD makes some wild claims that you can use spark plug gaps up to 0.060" ... yet there is no documented benefit to doing so ... except for burnt-up components mentioned above!!!

There have been pictures posted on this board of a burnt-up Bosch rev-limiter rotor [the EMI/RFI resistor incorprated between center contact and tip] after installation of an MSD unit!

ok, thats a good enough analogy for me :). I'm still a newbie with this stuff.

Not to start an argument here, but I've never had any of the above problems mentioned, after installing an MSD. I know there are die hard supporters for both MSD and CDI on this board. I also know that the MSD I installed in my '69T makes the car start up first time, no matter if it is hot/cold or whatever the situation. Sam

No whine.....
 

Actually, Warren makes some very good points. Doing the basic maintenance "stuff" that only costs your time is always the best place to start! No sense in buying new parts if you are simply going to plug them into bad wiring. I also agree that there is really no reason to replace a working Bosch CDI with anything else. However, IF the Bosch CDI is bad, a change over to an MSD unit is a reasonable choice/decision. I had the same "opportunity" a little while ago and after determining the fault was in the Bosch unit, I chose to install an MSD 6A, MSD coil and Magnecor plug wires. With some careful shopping, I was able to do all this for the cost of a Bosch rebuild. Also, I wired the MSD into the factory master plug that attaches to the bottom of the Bosch CDI so that a change back to a Bosch unit (if desired) would be easy. My experience with the MSD equipment has been positive. The car starts and runs well and is getting marginally better fuel mileage. I guess the bottom line is...stick with the Bosch as long as it is working correctly, but don't overlook alternatives when the time comes for a replacement.

Fred Cook
'80 911SC coupe