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John Orlando
12-23-1998, 06:56 PM
After two visits to a dealer for an idle speed problem - I fixed it myself, but I'm not really sure why my fix worked.<p>Here is the background: I recently replaced the original battery on my '93 525i (68K). The mechanic who changed the battery (not a BMW shop) plugged a 12V source into the cigarette lighter while changing the battery, so that the OBC and radio would not have to be reset.<p>After the new battery was installed, I noticed that the engine would search for the right idle speed, by bouncing back and forth between 500 and 600 rpm (approx, settling at 550 rpm. At times, the engine stalled before it settled on an idle speed.<p>I then decided to take it to the dealer. They, supposedly, did their computer diagnostic checks, and found a fault code that indicated a faulty throttle position sensor. They installed a new one, but when I picked the car up, it behaved exactly the same way - severe idle hunt. I left the car with them, and did not pay for the TPS.<p>The dealer next installed an idle control valve ($320 installed). Although this made the problem better, it still did not solve it. It seemed like the new valve did a better job of responding to a disturbance, but the disturbance was still there.<p>I told the dealer I was not happy, and I planned to give the car back to them 12/29.<p>This morning, I thought that the control system for the engine may have gotten confused or a changed reference when the battery was changed. Since there was always a voltage source present, the control system probably saw three distinct voltage levels, but never saw 0 voltage.<p>I tried the old ON/OFF trick by disconnecting the battery leads and reconnecting them. Eureka! Everything works fine. The idle is now rock solid, as it was before. It also seems like it's slightly higher than it was during the problem period (650 rpm approx).<p>1. Anyone have any ideas why this fix worked? <br>2. When the control sytem powers up from 0 voltage, does it establish a new set of references? Is this a process that would be skipped by plugging a voltage source into the cigarette lighter while changing the battery?<br>3. Would a different voltage level from a new battery affect the control system?<br>4. What is the correct idle speed?<br>5. Was the idle hunt the result of trying to control to an idle speed that was too low?<p>I now have to dispute the $320 charge from the dealer, and I'd appreciate any ammunition you can send my way to help me win this argument.<p>Thanks for reading this,<p>John

TBM
12-23-1998, 07:37 PM
<br>I do know that on newer cars ('84 + )with electronic engine management systems, they tune themselves. As parts get old and wear, the system compensates for it best it can. If a drastic change is made, i.e. tuneup. It is best to diconnect the battery and let the system "forget" what it has learned about the engine over time. When the battery is reconnected, it may take a few miles for the system to re-learn the engine characteristics again and adjust for them. As this relates to your situation, when the battery was replaced and power not disconnected, the unit may have sensed the some sensors were not working and tried to adjusted for them (the TPS in this case). When you disconnected your battery, the system forgot you had a problem and re-learned the engine characteristics.. This all I can think of. Hope it helps!<p>TBM<br>85 325e<br>89 525i<br>

John Orlando
12-23-1998, 09:43 PM
<i>: <br>: I do know that on newer cars ('84 + )with electronic engine management systems, they tune themselves. As parts get old and wear, the system compensates for it best it can. If a drastic change is made, i.e. tuneup. It is best to diconnect the battery and let the system "forget" what it has learned about the engine over time. When the battery is reconnected, it may take a few miles for the system to re-learn the engine characteristics again and adjust for them. As this relates to your situation, when the battery was replaced and power not disconnected, the unit may have sensed the some sensors were not working and tried to adjusted for them (the TPS in this case). When you disconnected your battery, the system forgot you had a problem and re-learned the engine characteristics.. This all I can think of. Hope it helps!<p>: TBM<br>: 85 325e<br>: 89 525i<p></i>


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