W124 engine cuts out on overun


Jul 31, 2006
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Hi hope someone can help

My J plate W124 300TE4Matic auto has started to cut out when on the overun approaching roundabouts & right hand turns, i.e. with foot off the throttle and coasting up to roundabouts it dies, normally whilst crossing the give way lines! loss of servo brakes and power steering coasting to a stop and starting immediately when put into park. 140K miles Lubed and serviced every 6K
any ideas?
Thanks in advance

Alex Crow

Senior Member
Jul 9, 2009
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Super Suffolk
Your Mercedes
W169, W124, w202, W203, KTM 250 EXC, VW T25 camper and a Polo in a pear tree
lots of possibilities here i am afraid. how does it idle generally? good when shifting into gear and turning steering full lock?

here is a list in no particular order.

purge valve sticking - you can disconnect to test it.
throttle valve idle contact - pin out at three pin plug on manifold.
throttle plate potentiometer on the flap housing - harder to test, pin out at ecu & check voltage.
idle control valve - can get carboned up and sticky.
overrun microswitch - grey one on throttle linkage.

fuel supply problems like relay, pump, filter etc.
fuel metering head problem - very hard to test for.
electrohydraulic actuator.
lambda sensor - yours should have one on a j-plate.
ht problems.
heavy engine breathing.
bad carma.

i would concentrate on the first five though.

i did have one many years ago that would occasionally stall on the slowdown after first setting off. on restarting it would often be fine, but if not it would always stall on the first deceleration period. it turned out to be a dodgy inhibitor switch, the ecu thought it was in neutral and that as it was doing, say 2,000 rpm, on a closed throttle it ought to close the idle control valve a little. as vehicle speed dropped it just could not pick up the valve opening quick enough to stop it stalling. when shifted to p or n to restart and then back to d to drive it was 9/10 times ok again - all depended on whether or not it thought that it was in neutral still after selecting drive. took a hell of a time to work that one out and i have never seen it since.

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