lrpman

The shudder is at 1,500 to 1,750 rpm in top gear (4 speed auto) under load.
Smooths out at 2,000 rpm and up.
Will be putting in a new
Dist. Cap
Rotor
Acell Race coil
Wires
Plugs are new already.
This will take the ignition out of the equation.
Compression test WET was less than 3% variations among all 8 cylinders.
Will get the parts in on Monday and see what I have.

Imo000

I've tried an Acell coil on my '83, 10years ago and the car ran like crap, switched it back to stock and it ran fine.

If the shuddering is sharp, it's an ignition problem, if it's soft, it's a fuel system issue.

lrpman

Sharp you mean TEETH RATTLE, that is it up to 1750 then if you jump on the throttle it will happen.
Bosch Coil would be an option to check but I had (3) Chrome Accel on the shelf so we will try it.
Maybe and MSDS is it's future.
It did cough up a fur ball when it was first wound up. Black smoke out the pipe.
We set-up the suspension using the digital equipment for the modified. WORKED GREAT.

borland

Adding new parts now only makes the equation more complex. You might create more problems than you started with. First troubleshoot the problem, make the repair. Otherwise, you could be wasting money that could be well spent elsewhere.

You can test ignition wires and coil with a ohm meter. After you have it running properly, then consider replacing regular maintenance parts like rotor, plugs, wires.

Imo000

That's exactly what I had too. I worked perfectly on a car before and after the 928. Go figure....and it wasn't an instalation problem either. I think not that long ago someone else had the sme problem with an aftermarket coil.

MSDS?????? You mean an MSD? If it is, do a search as many had problems witting an MSD igniton box to the 928. They kept frying them. When working properly, the factory igniton system is very good and if you are near stock, you won't notice any difference by "upgrading" it. Don't start throwing parts at it, it will get expensive really quick.

Landseer

George, did you guys polish the injector harness-related ground wires? They are the brown ring-terminals that attach on the passenger side of the engine, kind of underneath the cam cover on that side. A bit hard to reach.

On the platinum 32 valve car, the analogous ground terminal was damaged (its located in a slightly different place on the engine than yours is). I blamed the shudder on 10 different things that I fixed, including replacing the B2 transmission piston with the upgraded kit, but ultimately, it was this ground that failed. This caused the ignition control devices, O2 sensor and fuel management to behave erratically, especially under load. Take the time to clean yours if you haven't already.

Here's the picture of the badly corroded ring terminal, and the wires ready to go back into a new terminal. You can see why I had problems under load:

User 4221

Not to be a doomsayer, but don't rule out the torque tube.

I had a chassis-rattling shutter in my '81 and it turned out to be the TT. Much worse under load and would not present itself in neutral.

It will be hard to diagnose in an AT car since you cannot completely isolate the TT. Another issue may be converter bearings as it is gear and speed specific

lrpman

That is an eyeopener since I did not know about it. as far as parts and money into the car 90% of them I have already on the shelf since I kep them for quick, "THROW MONEY AT IT" for the race car. Be surprised how many coils we ATE on that thing fro DUMB reason. Like bad grounds, rub wires to short to the frame and others.
Well I am going to the shop Sunday to play with the 928 since the modified went so well on Thursday when we worked on it. Just need to double check the "U JOINTS" so I do not get a Visitor Up in the seat with me.
Be GLAD the 928 drive is not like this.
You are looking DOWN from the drivers seat. On your right is the throttle
Trans is a 3 speed between your legs. The BENT rod is 2nd and 3rd, the other is !=1st and rev. Steering wheel is off. The little round bar on the left is the brake pedal. And YES that is the floor you see. (Belly pan is off the car)
Now if you get could get this kind of HP to weight in a 928, 650 hp at 1650#
Well will inform as to how it goes. Thansks for all the information ALWAYS glad to get it.

lrpman

Isolated to the ignition system. Checked all the connection and that terminal all was well.
So now it is on to the ignition system.
I will move the ignition system from the parts car including the brain and see where we are at. IF it is 100% the same then I need to look inside the engine.
WE ruled out the drive train on a test drive since it was not there when we put the car in "N" and let it roll.
My bet is the dist cap with carbon tracks you can not see OR a wire firing against ground in the engine bay. Will have to run the car at night and look for that.

WallyP

Possibilities include:
- Bad connection at the center of a coil - often the left side.
- Crack or carbon track on the top of the coil.
- Crack or carbon track on a distributor cap or rotor. A tiny (really tiny!) puncture in the rotor can cause this.
- Puncture in the insulation of any secondary wire (the fat ones).
- Faulty coil (rare, but can happen).
- Faulty spark plug - crack in the insulator, collapsed resistor, contaminated insulator.
- Faulty plug connector.

There are other possibilities, but these will get you started.

lrpman

Was the COIL. It had been soaked in power steering fluid and was just bad. The new Acell coil worked perfect. Went ahead and changed the dist. cap and rotor since I had new ones.
Car will wind up now where before it was not smooth over 3,500.
Thanks for all the help.
Now on to the electrical fun since the PO just cut and taped some wires.
Will use AutuCadd and redraw the wiring to as built in the car so I can get things like
Mirrors
Blower
and such working.Will do the Wally deal on the fuses and other electrical connection first.

Landseer

The Wally deal is always the right way to go.

Imo000

Excellent, no try putting the coil from the parts car to see it's better than the Acell.

WallyP

Porsche 928 Factory Workshop Manual, Volume I-A, Page 28-44:

"Never replace specified ignition coil with a different ignition coil."

lrpman

Is that just a suggestion or in STONE? Car does run very well.