I had a similar problem. The issue was the EHA valve. Check this post out and see if it helps
systematic testing of fuel components

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1990 190E 3.0L

EHA checks ca 19 ohms. And voltage on pins. Need to check the current.

Any idea how to test the OVR relay? Like the zener diode in it?
Or just call it a replace and eat the cost?

I did pull the IACV connector while the engine idled (at 500 rpms) as suggested above.
The rpms did go up. 100, or maybe 200 to 600-700. When I reconnected the IACV plug, the engine coughed once and died.

Your thoughts?

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BnObob (for the fallen flag railroad.)
Artic "Snow" White 1989 300E, 124.030
w/ M103 engine, and 106K orig miles.

Have you checked your duty cycle? Engine must be fully warmed up to do so.

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