Date registered: Aug 2002
Vehicle: 190E, 400E, SLK350
Location: Chesapeak Bay
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You need to measure current going through the EHA, not voltage!
The cold start valve isn't going to be an issue in terms of idling but it does have a role in terms of rough start.
If I were you, I would actually check the OVP's fuse or simply replace the OVP all together. In my book, it's a maintenance item with a life span of a few years even if the fuse looks good. If the OVP (or fuse) is fubar your car will have a hard start and will die as soon as you start it. If you manage to keep it alive it will eventually stall the car at random.
At this point don't waste your time measuring devices until you replace the OVP. You need proper voltage and in a steady delivery mode before you can even conclude that the EHA is bad or cold start valve has issues. Besides not getting proper voltage to these devices does not mean the devices are bad, it's whatever delivers power to these device that should be suspect.
It will not hurt to check your voltage regulator, heck actually replace that one since it too is a maintenance item, and so is your battery.