My mechanic welded in some new converters and when he looked up the emissions he noticed that the car was running rich.
After checking almost everything that could be related to it, we are a little bit confused. It runs fine, no stumbling or so but when warmed up the car runs sometimes in open loop and then in closed, back and forth...
no check lights.
Car is internally stock, just headers, new converters, catback, adj. fuel pressure regulator

What are we overlooking

please

What thermostat? Does everything else seem to work in the car, gauges, etc? In closed loop, the AFPR won't have any effect on the final mixture since it will be using the O2 sensor to set it. In open loop it certainly will though and would result in a rich condition if the pressure is higher than normal. I don't know for sure, but I would expect the ECM to throw a code if it couldn't maintain closed loop operation for some reason. Unless it is caused by something intermittent that would normally cause the ECM to go to open loop such as WOT indicated or something like that.

Thermostat is 195 or 180, not sure about that.

temp gauge is reading a bit low, but not sure if it is accurate

could it be that if the thermostat is stuck open that the engine doesnt heat up enough so its always on the border between "warm" and "cold"

headers => O2 not getting warm enough, read O2 voltages and check if they linger around center and then give it some throttle see if they migrate away. Wrapping the headers or a heated O2 will fix this

I really don't know what the minimum temp for closed loop is, I've read 106F and also 160F. I really don't know enough about the ECM software to be able to help in this area. Hopefully someone that really knows the way the code works will be able to help. Sorry.

This.

FSM says 155* which is why a 160 in cool weather is dicey.

With the engine at operating temperature, my scan tool will toggle between open/closed loop at idle. It's normal behavior.

I'm using an OTC 4000e scan tool.

Actually what he found was that the settings for the throttleposition was not accurate witch affected idle control too. Now the emissions are within tolerance but not where they should be. Next step is to drive it more to burn all the crap in the engine... If that doesnt help: new plugs