||04-13-2010 01:54 PM
Originally Posted by BILLS_XLR
Cleaned the throttle body, which is quite an adventure. Condition still exists!!
Has any one experienced a hesitiation about 1500 RPM when decellerating. My throttle holds at 1200 while decelerating, with or without taping the brakes, with or without the AC on and with leaving it in drive or shifting to neutral. As soon as the speed (speed signal) goes to 0 the idle drops to 800 or so. Something is reading the 0 speed signal and dropping the idle. I have thoroughly cleaned the throttle body (that's another story) and checked for vacumn leaks with none found. The local dealer said this is normal on supercharged cars but I don't buy it! There may be some slight compensation for the additional air volume when the throttle first closes but but I don't think it should "high idle" all the way down to 0 MPH. Its definately effecting Gas mileage, brake wear and It could be considered a safety issue as simply llifting off the throttle does not generate anywhere near the engine braking effect that you would expect.
1/ Get a comparison with another V.
2/ It is possible for a failing throttle controller unit to start to fail but not necessarily set a fault code, or it could set an obscure fault code. I diagnosed a defective throttle control unit (in a F*rd, I know :rolleyes) using a fairly basic reader. By observing the actual position I could see it was making improbable jumps (because the unit's position sensor was failing) while somebody else was driving. I forget what fault code set in the end but it was a weird one. The driver symptom was sudden severe surging forward at lowish speeds or total loss of power at higher speeds. Better still the local dealer diagnosed this as 'normal' (it was mildly chipped car) but observed that the car ran very 'rough' (incredible considering that the factory program had been reinstalled while they diagnosed this driveability problem). We sent it to another dealer at that point as it was interesting that even with a car that was close to undriveable at times a dealer was capable of returning the car 'no fault found'. (Useful learning experience here.) (I might add that the dealer which solved it spent a lot of time looking at the wrong things, some faults can be less than obvious, IIRC it took about a day of labor in the end.)
It can take considerable care to investigate all the symptoms to diagnose what is wrong in modern cars, don't ever forget that the ECM does not 'know' what is wrong, it has to guess based on a combination of out of kilter readings and then set a fault code or two based on that mis-match; the absence of a code does not automatically mean everything is good either.