Surging '92 500SL 119.972
 

Just after the engine comes out of Cold Start (around 7 minutes of driving) and if it's below 10 degC my '92 500SL rhythmically surges when stopped at a light, or at a slow crawl.

In drive and stationary, the tachometer bounces from 450 to 550 RPM or so, every second or two. No effect is present above 600 RPM. A very slight increase in accelerator pedal cures the surging immediately - but the revs end up about 550. In reverse the surging is much more pronounced.

I've read many threads here and most point to air leaks or the idle stabilisation valve but this
thread in particular caught my eye.

sublettd replaced the heater matrix - very strange! This is interesting because I have recently noticed gurgling noises from behind my instrument panel just after switch on, and there's a TSB about this. But it just mentions replacing the expansion tank.

Can surging and the cooling system be related in any way?

I think I will check for leaks first - I've read several posts about leaky intake manifolds and one about a leaky throttle actuator (but that's expensive). And I have the odd ASR light condition which also points to the throttle actuator.

Can anyone point me in the right direction regarding a specific sequence of checks and the best order? I have access to Diag equipment and a garage.


Thanks
Lea

Two causes of surging that I experienced in my other cars are exhaust leak, and bad O2 sensors. There are more than 2 thou I believe.

I thought the LH system had an Idle Speed Stabiliser - strange I can't seem to locate mine. Perhaps the latter LH v5s manage the Idle speed by advancing the timing? If I haven't got one I can't clean it - guess that eliminates this device from the equation??

;)

I'd suggest the following:

1. a vacum leak, anywhere, including at the intake manifold gasket.

2. mass air meter

3. throttle actuator