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Thank you for your post 97. Please bear with me here.........
The 2.3 ltr 111 engine has a 'a/c style clutch', Detroit Blower type compressor that boosts intake pressure as well as acting as an AIR pump. It only comes on a 2500 rpm if driving gently and is activated by the ECU. At idle and warm up only when ECT is under 60C......or so says the MB site.
At warm idle, it is activated every second or so for a brief moment and cycles like this all the while engine idles.
I checked swing of O2 by running engine at 2500 rpm steady until I got a normal cross count of 2 a second from 110mV to 800mV. Then going to WOT very briefly and letting off the throttle. MB says then the O2 should drop very quickly to under 100mV within 20 seconds. This test was passed okay.
I cannot find a vac' schematic for the 3-way change over valve on the front of the valve cover and can only assume it is connected correctly to the three vac' hoses.
Hoses on the 2 pin electric change-over relay are... from blue one-way check valve carrying intake vac' signal to the 3 o'clock position nipple. Boost hose goes to the 12 o'clock nipple and hose to the AIR flow check valve ( Japan made...DENSO (??) ) comes of of the 9 o'clock nipple. This check valve is open (flowing air ) when vac' is applied to it.
I have activated the relay by letting power go to nearest pin to valve cover (Brown/Red.) and grounding the (GREY.) pin. Applying vac' to the 12 o'clock nipple allows flow but vac' to the 3 o'clock nipple allows no flow activated or not. So in any case...the is no vac' signal getting to the check valve to allow AIR flow.
Attaching a pressure/Vacuum gauge with a tee tap at the 3 o'clock nipple shows only positive boost pressure when engine revved but never vac' suction at any time, cold through warm, at any throttle position or idle.
If the hoses are on correctly, this seems to point to a ECU problem or a bad change over valve relay. There is 12 volts to the relay KOER and KOEO.
History is: Mercedes installed an exhaust gas back flow check valve and hard line to block 2 years ago for same code. It is still working and not blocked or corroded.
This code came back 2 weeks ago.
The Grey wire to the relay lights a test lamp too, but not as bright as the Brown/Red pin when connector is correctly connected ....which is consistent with a component Ground....but I cannot confirm wiring anywhere.
Relay 'clicks' if grounding it, when 12vdc applied to other pin.
regards
dk
PS.....I noticed #3 cylinder is always 3% to 5% and ranging, unlike other three cylinders on engine smoothness check. You can feel the slight miss at idle but no P0300 codes. O2 does not seem to react to miss.(?)
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