Duty Cycle Madness

[LloydMd]

Hello,

I have a 1990 300se/ M103 engine. I bought the car with a blown head gasket which I repaired. For the last year I have been doing routine maintenance as is the standard for these cars. Thus far I have put about 8,000 miles on the car and the odometer reads 189,000. I need expert help.

Either as a preventative measure or due to failure, I have replaced the following with Bosch/first quality parts:

Plugs, wires, and distributor cap and rotor
Injectors, holders, o-rings and seals
Fuel pump, fuel filter, and deteriorated rubber gas lines
EHA
OVP
All deteriorated vacuum hoses and fittings

Currently, the engine measures 16 inches of steady vacuum and the gauge behaves normally when revving the engine. With my old EHA the duty cycle was around 65%. When I put the new one on I adjusted the duty cycle to about 60% by adjusting the mixture. Through all this, the car seemed to run fine. I periodically check the duty cycle and for the last month or so, the duty cycle reads 98% at operating temperature. When the engine is cold enough, it will start at 50% as it should, then the loop opens and it will show a normal duty cycle, then as the car warms further it will quickly run up to 70, 80, 98% and stay. It does all this with the usual decrease in idle as the engine warms. In all other aspects the engine runs normally. From the dash tachometer the car idles at about 750 in park and 600 in gear. The idle is no rougher than what I have been told is normal for these cars.

Now, I am aware of the relationship between the fuel distributor pressures and the EHA and the throttle mixture. I do not have the equipment to measure the upper and lower chamber fuel pressures and I am reluctant to mess with the EHA since so far I have only touched the mixture and it was OK at the time, and the car still seems to run fine. So, what could be the reason for the high duty cycle readings and why does the engine seem unaffected? Any ideas will be greatly appreciated.

Thanks,

Lloyd

[Frank Reiner]

LloydMd:

Just as a refresher first, the duty cycle (aka on-off ratio) of the engine control unit (ECU) is a means of checking the response of the mixture feedback loop to fuel/air ratios as sensed by the oxygen (O2) sensor.

When the O2 sensor is cold immediately after engine start, the system is not receiving valid information from the O2 sensor, and it remains in the open loop mode. In this mode a fixed cycle/on-off ratio of approximately 60% will be observed.

When the engine and O2 sensor have warmed sufficiently (2 min. or less), a valid output from the O2 sensor will cause the system to switch to closed loop mode. When the loop is closed the ECU operates with feedback information and continuously varies the fuel/air ratio to bracket the stochiometric ratio (Lambda = 1).

When the system is operating in closed loop the duty cycle/on-off ratio will be observed to swing between 45% and 55%. If the ratio is not cycling there is a fault in the system. The very high ratio of 98% corresponds to either a failed O2 sensor, or a wiring fault that is interrupting the O2 signal.

[LloydMd]

Hello Frank,

Thank you for your input. I did install a new Bosch plug and play O2 sensor about a month ago, so I hope it is not bad. Since it is new, I did not consider this as a likely problem. I will check the voltage and trace the wiring to make sure that the circuit is good, and then reassess the situation.

Lloyd

[S-Class Guru]

I agree with Frank; O2 sensor would be the first place to look. Easiest test might be to actually monitor the sensor wires under the passenger floorboard with a good voltmeter. Sensor should generate approx. .2 to .8V, varying when warm. A DC of 98% corresponds to very lean, which would correspond to an O2 sensor voltage very low, say 0.2V. I would think that a mixture reading this far off might set the check engine light - apparantly a bad O2 failure is the only thing that does set it on this model. It might also show up as a fault code. Do you have access to a code reader? Really easy to make one.

An actual DC reading of 100% indicates a faulty O2 sensor, or no current to the EHA. Might try unplugging the EHA an see what happens.

Now, just to verify; many of us who have used the Sears-type duty-cycle voltmeter on pins 2 & 3 of the round connector, have wired it backwards: the red wire actually goes on the GND pin #2 to obtain the correct correlation of the DC (less than 50% for rich, higher for leaner). Of course either way, it should read around 50% when warmed and in closed loop.

DG

[LloydMd]

Hello,

Thank you for your post. This weekend I will start fresh and systematically check everything again. I have a code reader and last night I made a breakout plug to check the voltage on the EHA. I will check the O2 sensor as well. Thanks for the reminder about the voltmeter. I believe I had it the right way but I have been known to get it reversed…

Thanks,

Lloyd