Yes, I'm drowning in sorrow for my ailing 87 Dak... and because I have to pass a smog test before the end of the month.
Where to begin?? Well, it started with stalling at stops, first when stopping going downhill, then stopping level, then slowing down with foot off the accelerator. Adjusting the idle speed higher would cause the engine sometimes to surge at idle and other times to idle at idle.
I then rebuilt the carb. This is when I discovered the feedback solenoid. I got up to speed on the system through cruising around the net and learned some diagnostic techniques, including measuring the duty cycle of the solenoid with a high impedance meter, which I went out and bought. I also have the Haynes manual -skimpy- and bought a subscription to Alldata's online manual -less skimpy.
So here is what I have discovered. The solenoid works okay. It responds to 12 volts as it should.
When reading the duty cycle from a cold start, the computer begins controlling the solenoid immediately and tends to close off gas flow through it as the engine warms up. The duty cycle percentage starts below 10 and gradually increases to over 90. In between, the duty cycle fluctuates around 50% for a short time.
This means that the solenoid jet starts mostly open and ends up mostly closed. With foot off the accelerator, the scale goes to under 3, meaning the solenoid jet is fully opened. This on a zero to 100 scale. When in warmup mode, I have no drivability problems
At some point after about 10 minutes, the duty cycle goes to 20% and sticks there, plus or minus
1/2 of one percent and this is when the stalling at stops occurs.One would think that the computer
- is trying to compensate for a stuck lean condition
- has fallen back on a limpalong mode
- is broken
When this occurs, disconnecting the feedback solenoid cures the drivability problem completely. No stalling, no surging and more power than the damn thing has ever had. I could live with that except for two reasons: smog test and compulsion to fix things.
Along about this time I sprung for a new O2 sensor, seeing as I could not recall ever having changed it and I am the original owner. No change in symptoms. I hooked up the meter to the O2 sensor to see what it was reporting to the computer in both "solenoid on" and "solenoid off" mode.
The readings were very similar, if not the same: very high voltage output, about .95 volts, with the throttle open and very low, almost none with the throttle closed. Of course, these readings are after warmup. From a cold start until the duty cycle of the solenoid sticks at 20%, the voltage does fluctuate in a midrange as if the system was attempting to adjust the mix. But this only lasts until warmup.
The air pump and switch valve appear to be working correctly. I clamped off air to the upstream and nothing changed so I unclamped it. The engine temp sensor works okay.
Okay, I will end here. If you have any insight, I sure could use some.
Jack