Heres my problem after driving for a short while and slowing down for a red light, or stop sign the car will stumble hard and almost stall out, although it never has. Does not do it every time but at times will do it twice in a row. I have noticed that my air pump will engage after I hear the switch over valve click and then the hard stumble and a good puff of black exhaust. I find it strange the air injection comes on while the car is fully up to temp. Is it possible that a faulty coolant temp sender is sending a bad command to the air injection and the O2 sensor reads a lean condition then commands the injection to over richen causing the stumble? What do you think? Thanks.

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1989 560sec with a few mods.

First thing, try removing the air injection relay & see if the issue goes away.

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MERCEDES Benz Master Guild Technician (6 TIMES)
ASE Master Technician
Mercedes Benz Star Technician (2 times)
44 years foreign automotive repair
27 Years M.B. Shop foreman (dealer)
MB technical information Specialist (15 years)
190E 2.3 16V ITS SCCA race car (sold)
1986 190E 2.3 16V 2.5 (sold)
Retired Moderator

Idle air control valve. Just replaced mine on my 300e after pulling it and testing and seeing that it was malfunctioning. 15 minutes to get it out and a 9 volt battery to test it.
300e is just like this one for the 560
http://cgi.ebay.com/ebaymotors/MERCEDES-420SEL-560SEC-IDLE-AIR-CONTROL-VALVE-86-91-OEM-/180627942762

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'88 300ce black/gray
'88 300e 3.0 5 Speed! gray/gray

Thanks for the help guys, I went ahead an pulled relay #k17 and the air injection stopped cycling but the stumble persisted. The IAC is pretty new less that 5k miles new so I'm not ready to say thats it yet. What I did do was disconnect the connector for the switch over valve and it has not happen yet. I'm going to drive the car tonight for a while and I'll see if it happens again. Is it possible for a bad switch over valve to cause these problems?

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1989 560sec with a few mods.

Well it still happened with the switch over valve dissconnected. I ohm tested the coolant temp sensor and it was a bit out of spec with the engine cold, but in the range when hot. I'm leaning towards O2 sensor, but I think I'll unplug my IAC motor force a high idle of course but see if it happens again. If it does then I'll know the IAC has no part of it.

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1989 560sec with a few mods.

Idle air circuit will NOT cause that type of problem on a M117 (5.6) engine.

Idle circuit has it's own controller & only adjusts air quantity into the engine.

Fuel mixture has both electronic & mechanical controls.

__________________
MERCEDES Benz Master Guild Technician (6 TIMES)
ASE Master Technician
Mercedes Benz Star Technician (2 times)
44 years foreign automotive repair
27 Years M.B. Shop foreman (dealer)
MB technical information Specialist (15 years)
190E 2.3 16V ITS SCCA race car (sold)
1986 190E 2.3 16V 2.5 (sold)
Retired Moderator