Cold Start Valve & Electrical Explanation

[fireant911]

As of September 22, 2003 I became a proud owner of a 1981 Porsche 911 SC (factory Sahara Beige). I have two questions that I hope the experts here can help me with an answer.

My car averages 17.90 MPG (smooth highway driving) and I was under the impression that beginning in 1980 that these cars were rated at 29 MPG for the highway. I am using Chevron Premium fuel (91 octane). I have just replaced the Oxygen Sensor with no improvement. My first question is: what is the likelihood of the cold start valve leaking during normal operation / has anyone else found their valve to be leaking? I certainly do not wish to remove the valve unless there is a strong possibility that this could be at fault. The car starts well both warm and cold.

Second question concerns the Bentley 911 SC Service Manual: when looking at the wiring diagram, the letters "NCA" is listed connecting to the Oxygen Sensor. These same letters are used in many different areas of the electrical diagrams and I can find no reference to this in the legend. What does "NCA" represent?

[vash]

i hate acronyms so i cant help you there.

but my book list three possible causes of bad gas mileage:
leaking injectors, leaking cold start valve, and improperly adjusted air fuel mixture.

so line up the easiest first and work down the list. but i get crappy gas mileage, because i drive like a fool.

[vash]

oh yea, i think you are supposed to post some pictures. hurry before the picture police show up.

[kstylianos]

Check your plugs. If the cold start valve is leaking, you most probably are running rich and your plugs will show this (dark and soory). Remember, a rich mixture will is not only caused by a leaking cold start valve, idle CO% and incorrect system and control CIS pressures will also cause a rich mix.

Sorry, don't know what NCA is either.

[Mike Kast]

I think you can check the cold start valve for leakage the same way you test the injectors for the correct spray pattern. But I also read there are some worse symptoms caused by a leaky cold start valve than bad mileage. I'm not quite sure if I have experience with that or not though because checking mine for leaks is still on the to-do list. If i get motivated I'll look at the CIS trouble shoot book in a little while and post back. or hopefully someone with practical experience will post and teach us both something