Hi Robert,

Have been away for a few day........ here's the latest:

Thought I was on to something with my switchover/purge valve theory….. Just before Easter I swapped the switchover valve for another one (both had the same part number: 001 540 86 97) as the original appeared to be cracked on top – initially this seemed to make a difference. Went away for 4 days over Easter leaving the Merc behind, returning last Tuesday evening full of enthusiasm, convinced I was finally making progress…… Then:

Car started perfectly Wednesday morning after its holiday and drove for 5 minutes to a shop. Parked it for 5 minutes. On returning and restarting, idle was really rough, revs in ‘P’ fluctuated between 600 and 800, soon as I put it into ‘D’ revs fell to zero and it stalled. Restarted it twice, rev’d it to 1500 to smooth out the idle before putting it in ‘D’, but again both times once in gear the revs dropped to a point where it stalled. From then on, car would crank but refuse to fire….. Waited the customary 20 mins and ‘bingo’ - successfully started it, and while at low revs it was certainly not well, if I put my foot down and got the revs over 1500 it ran perfectly. Drove straight home (5 mins), parked up leaving engine running, idle was still rough with a ‘hiccup’ every 10 secs or so, opened bonnet, removed air filter housing and started to investigate….

1) Could activate the switchover and purge valves as expected (pulled the throttle cable so revs up around 800 and decal switch disengaged) – could hear/feel them pulsating
2) Didn’t seem to be any correlation between activating the purge valve and the idle hiccups – I think thus ruining my thoughts of this being the root cause of my problems
3) Disconnected the ICV wires – idle revs went from 600 to 1000 (I think as expected)
4) Pulled a few vacuum lines, blocked them off (as proposed by “trants4md”), but nothing seem to make a difference

After about 10 mins the idle ‘hiccups’ started to become less frequent until finally the engine idled perfectly smooth. Put air filter back on and took it for a spin - all was fine. Parked it with engine off for 10 mins, restarted and took it for another spin – again perfect. Also since then no problems what-so-ever!

As I mentioned before this stalling/20 min wait is the exception for me with the norm being my hesitant/hiccup’y idle. For me it could be that leaving the car for 2-3 days gives a rough idle while 4+ days leads to stalling.

Car’s going to be left for 5 days again soon so shall see if the same behaviour results. If so at least I’ll know (hopefully!) that the way to avoid the dreaded stall is simply to leave it idling for 20 mins before taking it out for the first time. If that works though it’s still baffling as to what is actually degrading over time (‘n’ days in-activity) to cause the problems to occur, but then can cure itself over time (20+ mins idling).


Robert,

One thing I did read about recently is a ‘Safety Switch’ feature on the fuel pump relay. What I found says: “There are two safety switches, one in the accelerator pedal position sensor and one in the actuator. These two switches, in combination with a switching transistor using circuit 15 to control power to the fuel pumps via the fuel pump relay or Engine System Control Unit (MAS). The purpose of these switches is to shut off power to the fuel pumps (thus stopping the engine) if the throttle setting is higher than idle while the accelerator setting is idle. Thus, if a malfunction occurs where the driver has his foot off the accelerator pedal (thus requesting the engine to idle) but the actuator output shaft is not at idle, the engine shuts off.”

Could it be that our fuel pumps are being shut down deliberately by the FP relay because some kind of malfunction has been correctly/incorrectly detected? Note, I’m presuming that our cars have this feature and that once ‘tripped’, if left for some time (20mins?) everything get reset….. Just a thought.

Let me know of any progress.

Cheers,
Neil