Car went crazy after being serviced HELP!!! - PeachParts Mercedes-Benz Forum

mike690003 · #1 08-15-2003, 09:58 PM

Hello Board,

Today I took my car to a well known and very reputable shop to get my air/fuel mixture adjusted. It was previously tampered with and caused my car to run very rich. My car was put on all the specific computers and it showed my car was indeed running a little rich and had a slight rough idle. The mechanic adjusted it to spec and then my car idled smoothly and accelerated great. After reaching close to home(drove about 15 minutes) the car began to run weird. It hesitated really really badly and stalled a few times. I was almost rear ended TWICE because of this. It was as if the car began to run very lean. Well I called the mechanic and took it back to him. He then richened the mixture and now the car pours black smoke out the exhaust when I accelerate (like before).

Why did the car react negatively when the mixture was adjusted to spec???

I have replaced:
Cap and rotor
Plugs and wires
Air filter
Cleaned idle control valve
All vacuum lines
Fuel filter
Injector seals

What could it be??

ECU??
O2 sensor??
Fuel injectors??

HELP!!!

#2 08-15-2003, 10:16 PM

i seem to remember that you recently replaced all the vacuum hoses on your car? might want to recheck all those connections.

if you didnt get your idle above 500 rpm as you reported earlier, you still have some other prob in the idle circuit.

mike690003 · #3 08-15-2003, 10:19 PM

I re-checked all vacuum lines.
Should I re-clean the Idle control valve??

pesuazo · #4 08-15-2003, 10:25 PM

Try the O2 sensor

mike690003 · #5 08-15-2003, 10:38 PM

Are the O2 sensors that important in the 300E??

I thought it only was needed for the first 5 minutes when the car is started??

1991300SEL · #6 08-16-2003, 12:43 AM

Seems to me the guy who "turned the screw" screwed up.

Take it back and talk to the shops owner. If they really are reputable, they'll fix it.

#7 08-16-2003, 04:31 AM

Quote:

I thought it only was needed for the first 5 minutes when the car is started??

The O2 sensor warms up after about 5 minutes and starts sending voltage signal to the ECU thereafter for as long as the car is running.

It senses the oxygen level in the exhaust and signals the ECU to lean/richen the mixture when necessary.

If you use a voltmeter to check pin 3 of your X11 connector, you will see that on startup, the duty cycle is 50% which indicates that the O2 sensor is not working (coz it hasn't warmed up).

Once the sensor has warmed up, you should see a fluctuating duty cycle on pin 3 which can be anywhere between 0 to 100% depending on your current mixture.

Fluctuation at less then 50% (say between 35% to 45%) is rich and fluctuation at more than 50% is lean.

My 2 cents.

Cheers.

psfred · #8 08-16-2003, 04:36 PM

Make sure the system is in closed loop mode (see Steve Brotherton's DIY article on electronic engine controls) when setting the mixture. If the heater on the O2 sensor is bad, it will be in open loop mode when you set it, but will go into closed loop when the exhaust heats up while driving, and I think this is what happened to you -- it was set way too lean in closed loop.

A defective o2 sensor or a bad connection can do this too.

The O2 sensor is used all the time to control the mixture except when it is too cold on initial start up -- that is, it is only NOT used for that first couple minutes until the heater gets it hot enough.

Peter