Thought I would share my journey to solve a problem with a fast idle issue in hopes of helping others who might experience this.
When I rebuilt the top end of my 1977 2.0 FI engine last summer, I replaced all fuel lines. Over the last few weeks I have been replacing all other hoses in the engine compartment. I had been having an intermittent problem with high idle, and thought I might have a vacuum leak. Even with the lines replaced, the idle would be ok at times, others not.
I started working backwards on the air circuit and decided to check the Auxiliary Air Regulator. When I blocked it off, and blocked the corresponding port at the S-boot idle went from 1400 to 650 and held steady. Ah – ha, getting closer.
I removed the AAR and found it in the partially open position. Photo below. The aperture isn’t like a partial moon, it is more like a scripted capital letter “P”. Following the advice here, I put it in the oven at 200F and magically, it closed fully.
Given time to cool, it went back to the partial opening in the photo. I cleaned it with Marvel Mystery oil, sloshed it around, and then cleaned with Q-tips. Black residue came out.
More reading, and I then tested it with 12 volts on the terminals. Worked perfectly. Time to go back in the engine. Once installed. High idle continued at 1400 RPM. OK, keep working backwards.
I knew it was a working AAR, so must not be getting voltage. As readers here know, the wires to this are kind of down in there, so I fished them up and found the wires to the AAR were slightly bare on both positive and negative. Time for another test. Getting out the test light, no voltage with the engine running. That’s not good.
Working further back in the process the voltage comes from 88c on the double relay. Time to pull it out and test it. I pulled the relay from the wall and decided to take it to the bench and open it up.
The mapping of the circuit board was good, just not attached at the solder point. The outer coating on the board looks poor from heat, but the underlying mapping is good.
So, I then used some solder and some heat shrink to repair the wires at the AAR- I believe what happened is that the shorted wires caused the damage to the double relay.
Started engine and.....
Problem solved! AAR working as it should and when warm it idles back down to 950. As for why the issue of fast idle was intermittent, it seems often I was starting the bus and taking it out on the road rather quickly. The engine would warm enough that the engine heat was moving the bi-metal strip in the AAR and closing the aperture without any voltage at all. So, my adjustments on idle on a hot engine were fine. However, if I was just working in the garage and the engine not really warm, I had fast idle.