Idle Oscillating when warm and lights on 83SC
 

My 83SC has a very pronounced oscillation during idle. It seems to only happen once the car is warmed up a bit and the lights are on. Other than this, the car is running very well.

Fuel consumption seems fairly normal, the lights dont flicker. Car was tuned up approx 2500 miles ago, with new cap and rotor, plugs. Fuel injector seals replaced, O2 sensor new. Permatune box is new, as is coil.

It seems that when the lights are on, there must be a voltage drop on one of the CIS components which operates when the car is warm running.

Where should I start? I've had to set the idle to approx 1600 RPM otherwise the oscillation will stall the car. When the problem started, it started all of a sudden. It didnt slowly get worse that I can recall.

Lights on means more load on the alternator which could lower the engine speed a little. The CIS compensates and speeds up then oscillates. Therefore it may not be a voltage drop problem as such. Maybe try a slightly higher idle speed to start with. A digital volt meter will give you accurate info on the voltage situation.

I have already increased the idle speed to 1600 rpm to keep it from stalling completely at a stop sign or red light. Idle speed is definitely not the issue, I have tried numerous positions.

Sorry, I missed your 1600rpm idle speed info, thats way high. I think an alternator starts putting out current at around 1000/1200rpm so I would think any voltage drop from having the lights on would be compensated for well before 1600rpm. I am no CIS expert but there may be a clue that its only an issue when the car is warmed up (leaned out from any cold start circuit) and the lights are on. If its not an issue when the car is cold and the lights are on then it sounds like its only getting too lean at normal operating temp causing the oscillating/ stalling. I can see why you suspect a voltage issue, can you put a DVM across the 12V to see whats happening?

Have you checked the fuel mixture,fuel pressue and air leaks?

dumb qustion here, but have you had the alt checked?

have you had the battery checked? how old is it?

check batt voltage with lights on, then lights off, then lower the idle back down and check the voltage again.

Is this a ROW car without the frequency valve? If it has a frequency valve and the voltage goes low you can have issues with the lambda system dropping the FQ valve out.

Its a US car, no air pump. I will try all the suggestion so far, beginning with a battery and alternator check. Then will go with a fuel mixture and fuel pressure check.

I appreciate all the input so far, and will update as I rule things out. Thx!

Battery, though old is still holding a full charge. Alternator output just over 14v and smooth, consistent.

After thouroughly reading the Bentley manual, I went through all the electrical connections in the engine compartment. I jiggled the connection to the 15 Degree temp switch, and it solved the problem. It seems the connection - though it looked good - wasnt making enough of a connection possibly due to corrosion. With the headlights on, the slight extra load was enough to make the problem more evident.

To prove 100%, I disconnected the switch entirely, and it proved to exhibit the same oscillation.

Wow, what a small simple thing that drove me crazy for months. Hopefully this can help someone else in the future.

Thanks for all the suggestions.

RPM changes when warm are not caused by vacuum leaks. "surging" like the PO had is a too rich condition, vac leaks are a lean condition, if lean enough, usually goes along with backfiring or popping, thus the need for popoff valves.
a vacuum leak will cause the RPM's to drop, and stay there. thats when most people idle it back up and keep driving.

15 degree temp switch? are you talking about the switch on the side of the throttle body?
that is a throttle position switch and that would do it. the ECU would go open loop making the mixture go rich, thus making it surge.
glad you have it fixed.

Definitely not a vacuum leak. It was the 15 degree switch on the right side of the engine, on the Cam Chain cover.

Driven the car a few times now, and everything working and running normal as it should. It was definitely an intermittend connection on that switch. It must have been barely making any contact due to corrosion build up or something.