Hello group,

The '86 300E is having rought idle . I checked the Duty Cycle on diagnostic port while the Engine was running (operating temperature) , it is stuck at a very low number (7 percent). It used be hovering around 50 percent few months ago. I am not sure what would cause the wrong reading now.

I did the following tests:

- Disconnecting the O2 sensor corrected the duty cycle (showed 50 percent) .
- Disconnecting the temp sensor plug going to ECU made no difference in reading.
- Disconnecting the temp sensor plug going to ignition control unit increased the duty cycle.
- Checked the voltage at the output of O2 sensor , it is about 480 mV which I think is the correct one.

Does this mean I have a bad O2 sensor , a bad temp sensor plug or something else?

Any suggestion is appreciated.

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Zorin

'88 Benz 420 SEL
'79 Porsche 911SC ('87 Carrera Engine)
'00 BMW 540 Sports Package
'99 Land Rover Discovery

86 300E - Manual (sold)
88 260E (sold)
84 944 (sold)

Take a look at Steve Brotherton's engine controls DIY article, it explains what's going on pretty good.

Disconnecting the O2 sensor and getting 50% duty cycle indicates you are in closed loop mode, but way off on mixture. Either something is wrong with the EHA or you have another failure (low fuel pressure, etc) or a bad fuel distributor.

You need to check the EHA current to verify the ECU is working correctly, then go from there. Chances are it isn't just a misadjusted mixture if everything was working properly recently. More likely a bad O2 sensor, but it can also be a failed pressure regulator or fuel distributor, I don't know what happens when they go.

However, rough idle and way off on mixture can be a vac leak -- check all the rubber hoses (the idle valve hoses are fairly large) and the line to the booster for leaks. Also, if the engine gets rougher with the brakes on, you can have a leaking brake booster. Eliminate any vac leaks before getting excited -- vac leaks will make it run lean, so the ECU is vainly attempting to richen the mixture up.

Peter

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1972 220D ?? miles
1988 300E 200,012
1987 300D Turbo killed 9/25/07, 275,000 miles
1985 Volvo 740 GLE Turobodiesel 218,000
1972 280 SE 4.5 165, 000 - It runs!

Thanks Peter, I will try the EHA current.

As for the vacuum leak I will check that as well, but one thing I noticed is that there are two auxiliuary air valve (if I am calling the correctly) on the intake manifold close to the EHA, one of them is connected to the booster but the other one which is closer to the engine has no hose on it, so it is not connected anywhere. Is it supposed to ?

Thanks again.

__________________
Zorin

'88 Benz 420 SEL
'79 Porsche 911SC ('87 Carrera Engine)
'00 BMW 540 Sports Package
'99 Land Rover Discovery

86 300E - Manual (sold)
88 260E (sold)
84 944 (sold)

I suspect that is the pressure regulator for the fuel distributor.

Peter

__________________
1972 220D ?? miles
1988 300E 200,012
1987 300D Turbo killed 9/25/07, 275,000 miles
1985 Volvo 740 GLE Turobodiesel 218,000
1972 280 SE 4.5 165, 000 - It runs!