'87 560SL - Oxygen Sensor Light On after Replacement
 

Hello,

I have replaced my Oxygen Sensor and Over Voltage Protection Relay and the light still remains "ON". The car also is difficult to start cold, especially when it is below 40 degrees and runs very lean until it warms up a bit (hesitation).

BACKGROUND:

My 1987 560SL had an issue last January.....I parked my car at work the morning it went down to 4 degrees in Atlanta. This car has been a Sunday car for most of it's life and never was exposed to cold weather (garaged it's whole life in a heated garage). The car started just fine!

The next day the Oxygen Sensor light came on! The car struggled to start when engine was cold (although it was 60 degrees in the garage). I did some research on the forum. Since the car has around 100K on it, I changed the Sensor with a new genuine MB part. NOTHING CHANGED.

I changed the OVP relay. NOTHING CHANGED.

Since that time, I haven't done anything (been a busy year!!). Now it is getting cold again and it is more difficult to start if the car sits outside for several hours. It starts right up, but stumbles and shuts off. It takes 4-5 times doing this before it will eventually idle OK. If I try to drive immediately, it has hesitation, as it is running very lean!!

So...I'm wondering if the Cold Start Valve is even getting a signal to operate? What role does the Fuel Relay have in regards to the Oxygen Sensor and/or Cold Start Valve? I'm also wondering if the ECU was affected by the extreme cold weather?

ANY SUGGESTIONS??? THANKS FOR YOUR HELP!!

I read your thread and have a thought as to a possible problem that might cause your symptoms. This idea actually applies to all makes of fuel injected cars. My sister has been having trouble with her Mercury where on cold days the car will not run properly. It will start, sometimes run fine, other times will stumble and die, and other times revs up then dies. It will not idle good till it warms up. Sometimes it will even stumble and die when stopping at a stop light. On nice warm days and in the summer it runs great.

After doing web research, I discovered this can be a common problem with fuel injected cars. What happens is air will get sucked in after the intake inlet (after the mass air flow sensors, or in this case the flap on the fuel distributor) and mess of the fuel/air mixture. This will cause the engine to run lean. The problem becomes a lot worse when the weather turns cold, and may go away when the engine warms up. Common failures are cracked intake gaskets and injector seals. In the case of the SL, it has a lot of places air could get in with all the vacuum lines and such. The SL even has air injection lines that go to the injectors that could be leaking or cracked. I know first hand that cracked and leaking vacuum lines can create a rough idle.

So maybe it is not a lack of fuel creating the hard starting, but too much air. The oxygen sensor would sense a lean mixture and turn the light on.

I'm pretty sure for the 87 the O2 light turns on after XX miles and isn't part of a diagnostic system.

The O2 sensor won't affect cold start as it needs to be hot to operate.

The cold start injector only operates while cranking below 32 ish * F ( check the specs for your car ) and for a short period of time. The thermo time switch controls the cold start inj operation by looking at temp / cranking time.

It is also my understanding that the O2 light is a one time system set for 60,000 miles on the 560SL. Removing the bulb or waiting for it to burn out is the fix.
The 560 uses a three wire electrical heated sensor.

Humm....I thought the O2 sensor was controlled by mileage on '85 and earlier? I borrowed a good ECU from my mechanic. He said this could be the problem as well. Off for 10 days, so plenty of time to play with it!!! :D:D:D

I will report back when I have solved the problem!! Thanks for the suggestions!

FIXED!!!! '87 560SL - Oxygen Sensor Light On after Replacement
 

It turned out to be the ECU!! If I had to "guess"...I'd say a solder joint must have cracked in the ECU went it was exposed to 4 degrees F. My car has always been in the sun belt, and usually only driven when it temp is above freezing. But it could just have been a coincidence.

PLEASE NOTE THAT THE O2 SENSOR LIGHT WENT OUT. 560SLs O2 sensor (or check engine light on '88 and '89 models) go off when the issue is resolved. 1985 and earlier models' O2 sensor light is illuminated when the car reaches a certain mileage (from a sensor on the speedometer).