one more time, I need a clue
 

I have done a bad job explaining my problem. If you don't mind, I would like to try again.

78SC with 66K miles
new plugs, new wires, new distributor
new fuel filter, new fuel connector (check valve didn't occur until 79)

Car starts fine and runs fine at first.

Once warm, constant speed triggers engine shudder.

Once engine begins shudder, it doesn't stop until it idles or is off for about 10 minutes.

I now drive long distances (~10 miles) by accelerating and coasting, accelerating and coasting... (I'm learning how far 50 mph gets me to my house - Denis Jenkinson would be proud).

Here is my list of culprets in order of best guess:

1. Fuel pump going bad
2. Aux. Air Regulator stuck open
3. Clutch going bad, maybe it's not the engine.

I'm trying to avoid changing every part on the car in search of an answer. I'm thinking about mounting a temperory fuel pressure gauge in my car.

Does anyone have a better idea of what could be causing this problem. I am so looking forward to a long, sweet drive.

If this is one of those competitions where the winner gets a free beer; my entry is the problem is a blocked fuel filter/fuel supply.

If it's a fuel problem (and I'm not saying that it's not) why wouldn't the problem happen straight off? Why do I have to drive constant speed before the problem starts?

Kinda' sounds like that old timey problem of vacuum building up in the gas tank because the tank venting was clogged. Offhand, how DOES a Porsche tank get vented in this day of Smog Controls? I don't imagine it occurs at the gas cap anymore --but how exactly?

SmileWavy

I ran without the gas cap and the problem still occured.

Clogged or Bad fuel injector(s)???? Or a bad CIS thingy - the fuel distributor, maybe....

I have used two bottles of fuel injector cleaner with no signs of improvement.

Can these injectors be pulled and run into a jar??? It seems that a bad injector could be most obvious when trying to maintain a steady speed. But I know NOTHING about CIS, really.

Razor--can you provide add'l description of the shudder? Does it sound like the engine is missing on a cyl or two? Your mention of it possibly being the clutch makes it sound totally mechanical. True? Would you characterize it as sounding like it's running "lean" or "rich"?

SmileWavy

This shudder is felt more than heard. I actually drove 150 miles to Austin, Texas and back with the problem. If I kept the speed constant, it was like dull hesitations.

I also have to engage the clutch slightly to accelerate after the problem begins.

Have you tried pulling all 6 plugs right after a "shudder run" to see if there is a significant delta in plug condition? You are running pretty new plugs, right?

What is delta?

Does the shaft on the distributor feel like it has lots of slop ???

It's a new distributor and the maintenance records indicate the PO was searching for this problem for several years.

Please keep your ideas coming. I might be buying several beers. It's just the delivery costs are going to kill me. But with a the pent up desire for a long drive, I might just make it.

Razor,
I'm sorry. By delta I meant a significant difference between 1 or more of the plugs and the others. That could indicate lean or rich fuel condition in those cylinders.
Delta in this sense means "change".

Tell us more abut pushing in on the clutch to enable acceleration, please

The hesitation / lack of power increases upon acceleration. In order to get going faster, I have to push in the clutch, thus reducing the load on the engine. If I don't push in the clutch, if feels like the engine might die.

Razor--Is "Cruise Ctrl" involved? (not sure whether your vintage had it) Also, Delta refers to "change" or "difference".

SmileWavy

No cruise control.

MY vote is plugged fuel line. From your description, it sounds like the symptoms are similar to when a vehicle starts to run out of gas. Like the engine quits running for a half-second at a time. It's like someone (someone very strong) is tugging the vehicle briefly in the backward direction. If this is your experience, and if there is no backfiring, then your engine is losing its fuel source briefly.

I have at times been a kind of "Dr. Frankenstein" of vehicles. I re-animate them. When resurrecting an otherwise dead vehicle, one of the 'must do' steps is to disconnect each and every fuel line at both ends, and blow compressed air through them. A six pack of Heineken says you will see rust flying out of those lines.

Several times I have done this, thereby resolving the problem, after the PO had replaced all the fuel parts ($).

Your on superman. What parts will I have to remove from the car to get to all of the fuel lines? I have removed the pan that covers the fuel pump. Will I have to do major parts removal to get to the fuel lines?