by James Sly
photos by PETER WU

       We receive a number of tech letters from VW owners experiencing poor idling engines.

      The VW guru we most often refer our questions to is Ron Wood at VW Specialties in Huntington Beach, California. Ron tells us that he has at least two of his VW customers a week coming in with similar problems. We suspect that most VW shops have such expenences. Quite frankly, the 1.8-liter Scirocco 16V we drive has the same problem. It was time to look into this!

      The CIS-E Bosch fuel injection system, such as the one in our 1.8 16V, is extremely similar to the CIS-E systems on 1985 through 1987 and a few '88 GTIs, GTs and GLIs, and on all 1.8-liter 16V engines. Later GTs and GTIs had Digifant injection, while later 2.0 liter 16Vs have their engine management handled by a Motronic injection system. Some steps may apply, but the procedure is for CIS-E cars only.

      Required tools: Spray carburetor cleaner, duty cycle or dwell meter, volt-ohm meter, test leads, normal hand tools, and patience to carefully diagnose. Also, a Bentley Manual for your car.

      CIS-E stands for Constant Injection System, Electronically controlled. It's a terrific injection system, simple, and easy to understand. It doesnt require too much in the way of sophisticated tools to diagnose most problems.

      On to the idle problem, then. This article deals specifically with the idle problems on cars with this fuel injection system. Many symptoms and cures will be similar for other systems like Digifant or Motronic, so it wouldn't be out of place to read on, no matter what type of injection you have.

      As we mentioned, our patient for this diagnosis was a 1.8 16V in an '87 Scirocco. The symptoms? The idle would drop below 1OOO rpm, come back up and drop back down again before settling to idle. On occasion, the engine would not even bother coming back up to idle, but would simply stall. Not an uncommon situation for these eight-year-old cars, but it definitely wasn't something that we wanted to live with! The problem occurred with a warm engine, by the way.

      The CIS-E engine control unit ("brain") or ECU, adjusts the idle with a device called an idle stabilizer valve according to engine temperature, speed and mixture. The oxygen sensor will always cause a CIS car so equipped to vary slightly in idle rpm as it strives to keep mixture under control. Don't expect an absolutely rock steady idle like the later cars. As the oxygen sensor gets older, it will waver more, too. We were talking about a much more serious problem on our 16V than a little waver.

      According to Ron, the most common causes of a poor idle are vacuum leaks, a defective idle stabilizer (either dirty or not getting enough voltage), the basic mixture adjustment or duty cycle out of adjustment, or idle switches and switch wiring needing adjustment or replacing. If all those are found to be functioning correctly, then the problem is either from the ECU-and Ron has never seen an ECU incapable of handling the idle function or a bad fuel injection harness, and is beyond the scope of this article.

      The first step is to make sure that ignition timing is correct to spec. The 8-valves seem to benefit from a little more advance, to as much as 12" BTDC, but 16Vs perform best at the factory setting of 6" BTDC. Have the timing correctly set, and the engine warm. We would also recommend using a contact cleaner like Caig DeOxit, available at many good computeT stores, to spray each connector from the wiring harness to the sensors. This assures a good electrical connection. Simply spraying the connectors can solve many problems. If things are adjusted right and the connections are good, but the idle problem still exists, it's time for the diagnosis to begin.

Diagnostic Flowchart