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Car Forum / Mercedes-Benz Cars / January 20091994 MB 300 idle surges when cold
It will start fine, but with no accelerator input the engine will run up to over 1000, you will hear a solenoid or switch kick in, the rpm will cut, sometimes stalling the engine, sometimes not, then the rpm will run up again and this will go on and on only while cold. Once running a few minutes, everthing is fine. Any ideas? TIA > It will start fine, but with no accelerator input the engine will run up to > over 1000, you will hear a solenoid or switch kick in, the rpm will cut, > sometimes stalling the engine, sometimes not, then the rpm will run up again > and this will go on and on only while cold. Once running a few minutes, > everthing is fine. Any ideas? TIA Could be a cold start solenoid malfunction (clicking noise) or a vacuum tube leak? Not sure about your model/yr MB... > It will start fine, but with no accelerator input the engine will run up to > over 1000, you will hear a solenoid or switch kick in, the rpm will cut, > sometimes stalling the engine, sometimes not, then the rpm will run up again > and this will go on and on only while cold. Once running a few minutes, > everthing is fine. Any ideas? TIA A take it is the 104 engine. There is really not anything switching on/off, except the air pump swiching off after a minute or so depending on the temperature. Until the oxygen sensor warms up, the engine will run in open loop mode (without feedback from oxygen sensor). I open loop the mixture is controlled by inputs from throttle position sensor and temperature sensors. And in idle the throttle is controlled by ECU. So something is disturbing the control. It could be the throtte position sensor (potentiometer), the temperature sensors or a vacuum leak (intake manifold and vacuum lines). Since it works in closed loop, I would suspect a small vacuum leak (the ECU is still able to correct the mixture according to the oxygen sensor feedback). > > It will start fine, but with no accelerator input the engine will run up to > > over 1000, you will hear a solenoid or switch kick in, the rpm will cut, [quoted text clipped - 19 lines] > (the ECU is still able to correct the mixture according to the oxygen > sensor feedback). Might try pinching one of the vacuum line tubes or pulling off line ends while the car is surging and you might find that one has no or little effect which might point to the culprit. Sometimes the vacuum tube ends get stretched and leak when cold where they connect to engine. You can snip off stretched tube ends and reconnect. Keep us posted if this worked.... Clean the idle control valve... if this doesn't work, the it is the idle cotrol computer. |
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