Power Loss 1980 911 SC

Help! I have a 1980 SC showing 104,000 miles and I have had an intermitent power loss problem since I have owned it (2 years).

The following has been replaced: fuel filter (twice), plugs (twice), plug wires, muffler, cat bypass installed, new injectors, etc.

I have run 44K and other high-quality fuel system cleaners and sometimes I think it kicks it into "fast mode". That does not work lately though. When the car is in "fast mode" it runs awesome and very strong, but the idle revs up and down at idle. When the car is in "slow mode" the idle is perfectly steady. When I start the car I can tell how it will drive based on the idle.

The car also misses at the mid-rev range. I don't know if it is connnected with the power loss or not. The timing is good. Adjusting the mixture has not helped in the past either. Temperature seems to have no effect on the condition either (25 or 100 degrees does not matter).

Any suggestions? I have taken excellent care of this car since I owned it but I do not know much about the history of it. Thanks!

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Donald Weintraub
1980 Porsche Red 911SC Coupe

i think you have a vacume leak somewhere that affects the idle.
I have had the same problem until my mechanic found the vacume leak and fixed it. My car would behave exactly as you described until this problem was fixed.
I have 82SC

Here's a trick to check for vacuum leak in the fuel system. When it's idling, remove the oil filler cap. That should affect idle as it is part of the closed system. If it does not change the idle characteristics, it indicates a vacuum problem somewhere in the maze of hoses, venting, and controls.
As your problem is intermittent, it may be something else, possibly the CIS.
Good luck all I can add is start with the basics and work from there.

Originally posted by m325ix:
i think you have a vacume leak somewhere that affects the idle.
I have had the same problem until my mechanic found the vacume leak and fixed it. My car would behave exactly as you described until this problem was fixed.
I have 82SC

Thanks! I will look into this and post my results.


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Donald Weintraub
1980 Porsche Red 911SC Coupe

Originally posted by FRED/LI:
Here's a trick to check for vacuum leak in the fuel system. When it's idling, remove the oil filler cap. That should affect idle as it is part of the closed system. If it does not change the idle characteristics, it indicates a vacuum problem somewhere in the maze of hoses, venting, and controls.
As your problem is intermittent, it may be something else, possibly the CIS.
Good luck all I can add is start with the basics and work from there.

Thanks for the information. When I remove the oil cap the idle characteristics do change. This leads me to believe that it is the CIS like you suggest. However, m325ix says he had the same condition I have and it was a vacuam leak. I will keep you posted. Thanks again!



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Donald Weintraub
1980 Porsche Red 911SC Coupe

I had a similar deal on my 81. The vacuum line the the de-accel valve was disconnected along with the line to the cruise control motor. Once I replaced the vacuum line, she ran like a top with a perfect idle.

I have an 82 and I have been trying to figure out why one vacuum line from my cruise control servo goes through the firewall and just ends, connected to nothing. The cruise has never worked for me, but I have some other things to check and also a hunting idle now. How is your cruise motor hooked up?

Olivier
82 SC

First, I believe that SCs' factory spark plug wires are starting to wear out. And unfortunately the stainless steel makes too good a ground, so you're less likely to hear the snap or see the arc. This is not the problem you are wondering about but thought I would throw that in.

I too am having some subtle surging, and unless it's my imagination I can also tell at startup whether my car will drive in 'fast mode, or 'slow mode.' It does not idle hunt, but it does have rhythmic thump when in 'slow mode.' I am suspicious of my distributor bushings and timing advance mechanisms.

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'83 SC

Originally posted by ohecht:
I have an 82 and I have been trying to figure out why one vacuum line from my cruise control servo goes through the firewall and just ends, connected to nothing. The cruise has never worked for me, but I have some other things to check and also a hunting idle now. How is your cruise motor hooked up?

Olivier
82 SC

I'm wondering if I have the same type of problem. When I start the car cold everything works great, but once the car is warm I start to get idling problems. Also the car tries to hesitate under load (pushing 5k in 2-3 gear) the car tries to die in idle. Yet if I take off the oil cap I see no change. Could anyone help me find which vacum lines to check? I do not know how many there are and were to look.

thanks
Scott
scott@wirestone.com
81 SC Targa

I plan on bringing my car to a Porshce shop soon. Does anyone have any suggestions on what I should ask about before I pick a shop? Is there any type of equipment that would help diagnose the problem I am having? I am thinking that if I bring it to a shop that is better equipped than it might save me on labor costs during the diagnosis process. Any suggestions are appreciated.

Also, has anyone had any experience with Mayos Porsche shop in Euless (or Bedford I am not sure), Texas?

Oh you are absolutely correct. I have enjoyed some very good deals on professional repairs by low-overhead shops, but generally speaking, the more you TRY to spend, the less you spend. That is, a fancy shop with all the equipment, and properly trained personnel, will be done with your car in half the time, and you won't need to make return trips to complain. There is no "bargain" in professional mechanic services. There is only the difference between those who know what they are doing, and those who do not. The folks who do not, are far more expensive.

You might print this thread and show it to them. It's only fair that they understand how many folks are watching, and that the results of their efforts will be evaluated here.

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'83 SC

you might want to hang a new relay under passenger seat to power the lambda brain. if lamda brain is not working, car runs poorly. dome light fuse powers relay. do your dome lights work?

Our 911 had a sticking plate in the throttle body, which also caused the idle to surge up and down about 50 to 75 rpm. Not too much of a problem except on cruise control, then the idle surge caused a speed change, too. After our shop lubed the throttle body plate (no, I didn't actually see them do thism, but they have always delivered good results) we seem to be better. Has anyone else had this problem? I also checked taking the oil cap off and the engine definitely reacted, so I think the vacuum system is good.

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Hal & Linda
'83 911SC