Fuel mixture becomes richer at idle

I have a 1990 560 SEL with a mystery fuel enrichment problem. Using the fuel analyzer on the X11 connection, fuel enrichment progressively increases at idle. If I hit the throttle it goes back to normal (45%), but gradually gets richer as soon as the engine drops to idle. The car stinks, and my mileage sucks. My mechanic is a Master MBZ mechanic with over 25 years experience, and he has never seen this behavior. Does anyone have any ideas?
Thanks, -rha

[dpetryk]

I would bet money that the oxygen sensor is NFG.

[Arthur Dalton]

Check the 02 heater /circuit..
Bad heater will affect 02 at idle...

A day or two after this last post, the check engine light came on and I got a code 5. I put a volt-ohm meter in the sensor and sensor heater circuits and they checked out OK. I am still running way too rich though, <10 mpg. This mystery fuel problem has been plaguing me for 9 months now (see earlier posts). In the past year the following work has been done to this car:

1. Both fuel pumps and filters replaced and associated lines renewed.
2. New O2 sensor
3. New EHA
4. New Bosch fuel distributor
5. New valves, guides, seals, cam oilers, air induction boot and vacuum lines.
6. New timing chain, tensioner and guide rails.
7. New temp sensor, thermo. vac. valve, vacuum check valve
8. Replaced every filler cap, belt, seal, hose I could find that even looked like it might leak

The car runs OK, for a few days, then the problem starts again, usually a faint fuel smell with the windows open, then rough upshifting from 2nd, loss of power, stalling, fuel consumption zooms. My mechanic has put in dozens of hours trying to diagnose this, and he has over 25 years on these cars (former shop foreman at a major Bay Area MBZ dealer). I would appreciate any advice on how to approach this problem. Should I take the car to someone else? Should I try to sell this car to someone I don't know? So far, this car has cost me over $13,000.00 (including the price), but it hasn't been much more than a shiny boat anchor.

I am not familiar with your specific vehicle engine type, but suggest you check the device that monitors air mass (MAF) or manifold pressure (MAP). These emissions controls override the O2 sensor input.

[dpetryk]

You have a CIS type injection system in that car. The system is all mechanical except for the EHA which has some limited authority to adjust the mixture. The EHA is ultimately controlled by the ozygen sensor thru the Lambda controller module. For the most part the mixture is controlled by the position of air flow sensor plate. The EHA only makes minor adjustments to the mixture. There is no MAP sensor on this engine.

The fact that you smell gas tells me that there is some leak somewhere in the fuel delivery system. Even if the engine was running very very rich you should not smell fuel inside the car. EVER. Lots of black smoke maybe, buy you should not smell raw fuel in the car. Have you checked the fuel pressure regulator?. When the diaphram fails, they leak the fuel into a small vent tube which is connected to one of the breather tubes connected to the valve covers and into the air filter housing. The purpose of this is to vent the leaking fuel to someplace safe, not on the top of the engine manifold. The leaking fuel will be inhaled by engine. If the leakage is high enough, you could get a rich mixture that might cause the symptoms you describe.

You need to locate the source of the fumes. That will most likely point you to the culprit. Check all the fuel lines under the hood. Everything.

The photo I have attached shows the fuel pressure regulator. I am missing the vent line mentioned above. But look for it (green line) on yours. the small tube should go to the crankcase vent tube on the other side of the engine.

Thanks for your suggestions for my 1990 560 sel. I will check the vacuum line on the fuel pressure regulator, and also the mass air flow emission controls. Thanks for attaching the picture! The engine looks just like mine.

[kjsabat]

check cat, it may have restricted flow.