If your 928 is idling below 600 RPM it may be the idle control valve or one of the fuel pressure regulators. I believe the pre-'86 928's with that "octopus" looking fuel distributor has a manual idle adjustment screw too. I have an '87 928, I don't think idle is manually adjustable on mine, it will be the mass air flow sensor, ICV or FPR that would need to be looked at.
Thank you for the info! According to the gauge the idle is low, but I was honestly wondering if it was actually low or a gauge problem. RPM's seem a little low when driving too. Then again, this is our first 928 and I'm not 100% sure what right looks like. I will definitely do some research on the idle adjustment to see if I can raise it a little.
My 89 voltmeter reads low below the 12V mark. Sometimes, it doesn't work at all. I use a 12V cigarette lighter USB adapter with digital voltage display. Usually above 13V with no lights on. Supposedly, it displays the alternator voltage.
My issue was usually after the car warmed up and the idle dropped down from the cold idle. After the first time we drove the car a few miles and got it warmed up we realized that the idle was way too low (200-300 RPM). We adjusted the warm idle to 700 RPM and now it shows good charge at idle. I think the issue was the alternator wasn't being turned fast enough at that really low idle. We will see how it goes on the next few trips. I will have to look into that voltmeter gadget you mentioned.
