[EMAIL PROTECTED] wrote: > My 300E/91 has a very strange problem, after a month or so coming back > from the indy: > > a) Cold or warm, the rpm rises to 1300/1400 rpm by 12 sec., and after > that the rpm returns to normal cycle (750rpm). > > b) Un-pluging a two-wire connector at the idle control valve this > symptom stops: no high rpm is observed when the motor is warm. >
Well, I don't have any experience specifically with gasoline Mercedes engines, but I do have a little experience with K-Jetronic in VWs. Are you saying the idle hunts, or revs up and down excessively? Or does it only do it once? My experience is that these kinds of problems are rarely actually the idle control circuitry. Usually they happen because the mixture is wrong for some reason. (Or the base idle is off, but I think on KE-Jetronic the base idle is non-adjustable.) When that happens the idle circuit gets into odd feedback modes where it hunts and otherwise acts up. Often the only way to fix the problem is to carefully go through the system and check or adjust everything to specs until the problem goes away. One really basic, but important, item to look for is false air. Any vacuum leak that lets air into the intake without going through the airflow meter will cause idle problems. Common culprits are the bellows at the ends of the intake pipe and the injector seals, but any of the vacuum hoses can cause problems. > c) Is the auxiliary air valve different than the idle air control valve, > or two different names for the same part number? > On K-Jetronic they're separate. I'm not sure about KE-Jetronic. On K-Jetronic the auxiliary air valve is a device with a wax pellet and a heater. It boosts the idle when the engine is cold, until it heats up enough for the wax pellet to close off the air passage. After that it doesn't do anything, and the idle air control valve manages the idle speed. > 2- The fault could be in the ECU? How can it be tested only for this > problem? > It's probably *not* the ECU. While it's possible, they don't often fail. My advice is to get a dwell meter, a DVM or volt-ohm meter, a fuel pressure gauge set, and a shop manual. Those are the only tools you need to check out the whole system, including the ECU. Because the K-Jet systems have no self-diagnostics, the only way to fix running problems is usually to systematically check each component and fix anything that isn't right.
