Mixture problems.

I have a customers car(Mercedes W114,M110-986) which is giving me a strange problem. i use a laser type exhaust analyser to check the C/O readings and idle it is .075% but at 3000 RPM it is rich running at 4% . it appears to be reveresed from what I would expect. has anyone else experienced a fault like this? All sensors etc are working fine and engine has had a recent top end overhaul.

Hi Unknown from Down Under,

it would be nice if you could sgn with your first name so that we can personally address you.

Now regarding CO: In idle-run it is set by both MAP sensor and idle-run rheostat in overlay to each other. Rheostat has 22 clicks. Where does it stand now? Or in other words: Which idle-run CO values can you achieve by turning rheostat? As soon as idle-switch is open, ECU will only listen to MAP sensor and ignore rheostat. Also; How is HC when CO is at 4%?

Secondly type 3 MAP sensors have a manufacturing problem that turns out to be more and more problematic the older they get. Sometimes I have not even been able to recognize any linear curve any more. Type 2 aneroid cells are from a different manufacturer and do not have that strong effect. So it could well be that your customers MAP sensor is such a case that has run out of limits. But before you test another 0 280 100 111 MAP sensor, you should verify that you have proper components installed.

Regarding car models: Please be aware that they shall reflect the models you OWN not the ones you are interested in.