A wideband might be in my future...seems like I have some decisions to make.
- Hot start issues are very common on 924s. Porsche struggled with it all throughout production. At first there was the "WOT crank" instructions in the owners manual. In 1979 only they added a solenoid valve in the intake. Starting in 1980 through the end of production, they had a valve in the control pressure line that dropped control pressure very low on warm-ish restarts. None of these things totally fix the issue, and any leakdown in the system makes it far worse. These cars have a fuel accumulator that maintains system pressure, knowing that there will be some leakage that a simple check valve like a modern system can't really handle. Mine is not working. I did check the injectors and they don't leak, so I probably have a leak in the WUR or in the regulator valve in the metering head. Both have a return to the tank. The issue is compounded by the fuel pump only running when ignition signal is seen. So there's no pre-crank priming pulse. Right now, I believe my most grassroots option is to add an intermittent switch that will allow me to trigger the fuel pump prior to starting so that I can purge any vapor locks without cranking a dozen times. Cold starts, on the other hand, are great.
- One of the local auto parts stores had injector orings. I replaced them all. Massive improvement, it idles like it is supposed to now. Cheap and easy.
- I figured out why it intermittenly runs like crap. Unfortunately, I don't have a fix for it. These cars, starting in 1980, have a lambda system that uses o2 sensor input to vary the duty cycle of a frequency valve, which modulates signal pressure from the WUR. It constantly operates, whenever the fuel pump relay is latched, and runs at a 60hz frequency. It's fairly loud, to the point which many CIS people seem to refer to it as "the chatter valve". Mine only chatters sometimes, and when it does, the car runs great. When it doesn't, it feels like it makes about 50 horsepower. I tried to clean the valve, but it seems like it's just weak or something. Cleaning didn't really seem to make a difference. The forums are littered with people with this problem, but none seem to have a fix. The advice they get from people who seem to be the resident experts goes completely against what I've read in the manual I have as well as what I have measured on my own car. The resulting poor information leads people in circles, and eventually the threads die.
Of course, the frequency valve is NLA. There are a few out there NOS in the ~$700 range, which is not something I am going to jump at unless there is really no other option.
My hope is that I can delete the frequency valve and basically make this thing think it is a 1979 model without the frequency control. But that will require more research, and possibly a different WUR.
When the valve is working right, the car runs pretty well. It's not fast, but it goes 75 down the highway just fine at ~3100 rpm just loafing along. I've just touched 5k rpm a few times, still waiting on that water pump so I can do the timing belt. It should be here either tomorrow or Monday. When the valve isn't working right, it is nearly undriveable.