Rough or high idle after disconnecting the battery. The BOSCH Motronic ECU over time adapts to a specific engine resulting in a consistent and relatively smooth 800 rpm idle +/- 40 rpm. When the car's battery is detached this adapted information is lost and over a short subsequent running period, the information is recreated by the ECU. I have been told, but have not verified that if the car exhibits a high idle RPM, say 1200 RPM, and suspect sources of this condition such as the idle valve and the Mass Air Flow Sensor have been addressed the car can be left at idle for ten minutes and the ECU will adapt and the idle will return to normal.
Replace ignition wires – Ignition wires have proven to be long-wearing, some have needed to replace them in as little as 60K miles others much longer. In my case, I replaced them at 80K miles even though they were not exhibiting any issues or misfire codes as it was an economical add-on item to other work at the time.
If your car is not throwing misfire codes, you are not experiencing ignition problems in wet weather and the wire ends look good you can probably hold off on replacing them as a preventative maintenance item. However, if your car has over 60K miles on it and you will have the engine out or the valve covers off for other reasons the incremental labor to replace them should be low and so replacing them may be considered.
Replace plugs - Plugs are generally extremely long service life items on a 993. I would not replace them unless your car has miles in excess of 80K and you are having the engine out or its ancillaries removed for other reasons as replacing the plugs alone is labor-intensive.
If you do not have a diagnostic misfire code attributable to worn plugs avoid the temptation to mess with them. If you do have such issues check the cap & rotor first then the ignition wire involved as these are more common reasons for such codes. A non-cylinder specific or multiple concurrent cylinder specific misfire codes are usually not caused by a wire, plug or injector and are more likely related to systemic issues like a vacuum leak, a dirty idle valve or an oxygen sensor out of specification. By the way, it is OK to replace just one ignition wire or plug as needed.
Replace rotor & cap – IF there is a concern about rough idling. 993 does not exhibit a perfectly smooth idle so what is normal is learned with experience.
My experience is that if one starts from cold a 993 without giving the engine a little throttle blip once started it may settle down in a very slow uneven idle. In this situation, it may even illuminate the check engine light and throw a random misfire code. The solution to this is to clear the code and move on.
I found that caps and rotors seem to be a two to three-year repeat item on my car. If I leave them longer at some point I get a check engine light and a reoccurring random misfire diagnostic code that keeps coming back when cleared. So about every two years, I have been replacing the caps and rotors on my engine.
Having lived in Northern Ohio (wet) and now Virginia (humid) it seems the small amount of motor oil applied to the pad in the center of the rotor shaft at rotor replacement and oil changes may have contributed to the total lack of corrosion on the internal parts of the distributor assembly on my car. This, in turn, may be why I have never experienced the common corrosion-related distributor bearing failure leading to a broken internal distributor belt.
Recently I tried 1500 grit sandpaper to polish the carbon tracks off the rotor and I hosed out the inside of the distributor caps with spray electronic cleaner. This seems to have done the trick extending the life of the parts somewhat. I for some reason kept some of the old caps and rotors, and now I have a lifetime supply of two-year-old rotors and caps that can be put back into service if needed. On one occasion I got a check engine light and upon inspection found one of the distributor caps was a little loose. I tightened it up and the light went out shortly thereafter. Even though the caps are plastic and I do not want to distort or crack a cap I will need to make sure the distributor cap screws are a bit tighter in the future, a bit beyond lightly hand snug.
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