Early 86.
When starting when car is cooled down. Car is hard to start and idles at 750 to 800 rpm's. Had to give it a lot of gas. When it warms up to normal operational temperature it idles at 1050 rpm's. What could be causing this? I replaced the temperature II sensor. Thanks
The 32-valve 1985 - 1986 Porsche 928s require setting the base idle speed and air/fuel ratio (CO%) which enables this first generation digital fuel injection to properly function. I would consider this part of a tune-up to be done when spark plugs are changed.
Once the base levels are set, the idle speed control and lambda (O2 sensor - air/fuel adjustment) will function properly. Attached is the section of the factory workshop manual instructions. The proper settings are:
The most troublesome part of this "tune up" is measuring the CO%. First, as shown in the factory workshop manual, the exhaust must be sampled before the catalytic convertor. Secondly, locating a shop that has an exhaust analyzer may be difficult as no modern cars require air/fuel adjustment. A shop that has a chassis dynamometer will have this capability. If your mechanic doesn't have this equipment don't change the air/fuel ratio.
The 1987 and newer 928s used a second generation fuel and spark control which has no external adjustment for idle speed and CO%.
Thanks Rich . The idle was on cold start 1050 to 1100 and 800 when warmed up. It changed to 850 on cold start and 1050 when warmed up. What could cause this ? I changed the
temperature II sensor.
To me it sounds like your auxiliary air valve might be stuck or failing. In theory it's supposed to allow extra air during cold starts for higher idle, then close as the engine warms up.
If it's staying open when warm or closing too early when cold, you'd get exactly the symptoms you're describing.
I'll explain the cold start air/fuel enrichment strategy that may help discover what is going on.
A cold engine needs a richer air/fuel mixture (more fuel per unit air) as the fuel doesn't atomize (mix) with the air and combustion isn't as efficient.
The engine temp sensor provides a resistance inversely proportional to the coolant temperature so the LH fuel module can inject the proper amount of fuel. The fuel injection module initially increases the amount of fuel per unit air when cold and gradually decreases it as the engine warms up.
There is a second system, in this case it is the idle speed regulator (ISR) which is a small electrically power vane that opens and closes by the LH Fuel Module control signals. When the engine is cold, this vane opens wide and allow more air to enter so the richer mixture is combined with more air and there is a faster idle speed when a cold engine is started. As the engine warms up this vane decreases the air flow and the idle speed falls. This same unit moves enough to provide the proper idle RPM.
What happens if this idle speed valve doesn't move? A slow idle when cold and possible a faster idle when warm (if it is frozen in a more open state).
To properly diagnose your problem you need to be confirm that the wires to the temp II Sensor are not broken or shorted. A vacuum leak from somewhere in the intake system also cold cause this problem. The fuel pressure regulator and damper system needs to be checked, if they leak too much fuel in the system and there is a vacuum leak (a common problem on all the older 928s!!). Only once all these other possible causes are confirmed not to exist is the idle speed regulator to be suspected.
The idle speed regulator for the 1985 - 1985 32 valve 928s is no longer sold by Porsche/Bosch, it was obsoleted. There is an aftermarket valve sourced by Loewe. Some 928 owners have and success with this valve, others have not. So before replacing it the effort of cleaning up your ISR may be enough to bring it back into to service.
