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Old 05-31-2004, 11:52 AM
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Join Date: May 2004
Posts: 15
72 SL engine surges when its cold

It stops after about 5-10 min.
Any help would be greatly appreciated.

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Old 05-31-2004, 12:39 PM
Join Date: Mar 2000
Location: Gainesville FL
Posts: 6,844
Is it a 3.5 or 4.5 motor? The problem is especially bad on 3.5 motors.

The problem I refer to is the unintended decel cut-off at idle. The system is designed to cut fuel when coasting. Coasting is defined in the software to be any time the throttle switch is closed (throttle closed) and the engine speed is above some figure, probably between 1500-1800rpms. It turns back on at about 1200rpms and will continue this cycle untill the engine slows down as it comes off the auxiliary air valve.

Since the car isn't supposed to do this it is always some form of high idle that causes the problem. Bad timing control is a real problem with 3.5s. Aux airvalves also can cause the problem. Improper setting on idle speed also can do it.

One way to verify the problem is as I state is to remove the airfilter and disconnect the idle switch. With the idle switch disconnected it WILL NOT do what I am saying because the computer isn't seeing closed throttle.

The reason 3.5s were more sensitive is because of their idle speed sensitivity to timing and probably a lower threshold in the software.
Steve Brotherton
Continental Imports
Gainesville FL
Bosch Master, ASE Master, L1
33 years MB technician
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Old 05-31-2004, 01:02 PM
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Join Date: May 2004
Posts: 15

I am pretty sure its a 4.5 engine.

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