Can anyone provide direction with regard to setting the air/fuel mixture screw to either A) the factory intended setting, or B) the setting appropriate for an '84 Euro 500 SEC. The screw has been tampered with and the result is has been an idle that several minutes after startup "lopes" or "hunts". The throttle plate is pulsing at this point as if it is gasping for air and is rhythmically opening and shutting. How do I restore some constancy to this plates erratic and miscreant behavior? If I can reset the mixture myself what other tools/equipment in addition to a long stemmed 3mm Allen wrench will be required? Thank you in advance for your replies. William Canon.

William,

Besides the 3mm allen key, you'll need some way of checking the idle mixture. The accepted way to do this is with an exhaust gas analyzer or sniffer. On vehicles without a feedback system such as an oxygen sensor (and maybe also for carburated engines only), there's a device made by Colortune(?) that lets you view the color of the combustion flame. It didn't seem more accurate than winging it, but it has high entertainment value

Turn the idle mixture screw a quarter turn one way, let it stabilize for a couple of minutes and check for improvement. Then turn it a quarter turn the other way and check again. It takes a while for the system to equalize at the new setting. If an adjustment either way doesn't yield an improvement, your problem is probably something other than a misadjusted idle mixture.

FWIW, my guess is that whoever tampered with the mixture screw was trying to sort out the idle fluctuations.

BTW, I think the plate is reacting to the situation, not causing it. Or maybe I misunderstood that statement.

Good luck,
Sixto
91 300SE

Actually, the idle was fine prior to the misguided attempt to lower the idle speed. The attempt was a response to a harsh engagement of the transmission from Park to Drive and more noticeably, Reverse. This was attempted with the idea that a lower idle would produce a smaller "clunk" during engagement. The replacement of the flex discs has remedied this problem and now I would like to simply return the idle to it's pre-tampered state. Thank you for your response. William

William,

The feedback system makes it difficult to do by feel. Does it cost more than you feel it's worth to have a shop set the idle mixture?

Sixto

On my Haynes manual, the task difficulty for any particular kind of service is rated on a scale of 1 to 5 (with 5 being the servicing best left to an extremely skilled technician).

Guess what, setting the fuel/air mixture shows up as a 5!

Not that the task is incredibly difficult, but that the diagnostic equpment required are probably only available to a reputable service shop, and it takes training to be able to use the equipment effectively.

This is not a trial-and-error shadetree mechanic kind of service...take it to someone who really knows what he's doing!

__________________
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This is for a 300e, it could be the same figure, get a digital volt meter, pos probe in pin 3 of the diagnostic plug and neg probe to battery negative, adjust the mixture screw to get a reading between 6.8 and 7.2 volts d.c. Remember the mixture screw has to be pushed down against a pressure spring to engage the drive, otherwise your just spinning the allen scerw to nowhere, i found it better to stop the motor do a slight adjustment then start it and take the reading, because if you push down to far on the allen key you are able to flood the motor, there`s a fine line between engaging the drive and flooding the motor. 6.8 is just lean, 7.2 is just rich. Do this properly and you`ll get a perfect idle back again.

If you have an O2 sensor you can use the voltage it generates to set your mixture by probing the main lead to the sensor with a meter

intruder

From what I've read, a properly operating O2 sensor will have fluctuating voltage reading in closed loop mode such as at idle. What is the procedure for setting idle mixture using the O2 sensor voltage?

Sixto
91 300SE

Is the O2 sensor the same as the "air flow" sensor? There are two sensors located on the passenger's side of the air flow valve on a '84 500 SEC. What are these sensors and what should they be connected to. I have both an American and grey market 500 SEC and both these sensors are connected to different components. I suspect that is a query for another post, however, I would like to perform the suggested procedures that have been kindly offered above but I must identify the O2 sensor to procede. William.

On US cars, the O2 sensor is along the exhaust pipe just before the catalytic converter. The wiring runs along the front edge of the driver's seat.

It takes some doing to check since it's usually mounted to the top of the exhaust pipe (or one of the pipes).

If the car didn't come with an O2 sensor, I doubt anyone would have gone through the trouble of retrofitting one other than just to pass a visual inspection.

Sixto
91 300SE
81 300SD

Again... If you have an O2 sensor installed and if you DISCONNECT it, it can be used to measure the fuel-to air ratio with very little oscilation. Caveat, if the F/A ratio is really out you'll get some major voltage fluctuations until you are in the 0.45-0.50 milivolt (if memory serves me) range.

The O2 sensor is simply an oxygen reactive metal that produces its own voltage which varies with the percent of oxygen in the exhaust gasses (and that's why you can use it to adjust your F/A ratio by measuring that voltage). The car must be warmed up so you aren't reading the rich warm-up period and the engine should be "revved" between adjustments. This is about as accurate as you'll getting without the right equipment. Put one end of you voltmeter into the single wire exiting the sensor (any additional wires are for the heating element and will read abou 12 volts, usually) and put the other probe to ground; when you're reading less than 1 volt you're probably on the right one.

The sensors do wear out, some sooner than others. Some just start reacting too slowly. So, start with a new sensor>

Intruder

Thanks Intruder.

I've read that a digital reading volt meter will not react quickly enough to the fluctuations so use a good analog reading volt meter.

Sixto
91 300SE
81 300SD

Actually just the opposite. the digital is your best bet. The analog needle is too slow and it's not like the number are changing so fast on a digital that you can't read them.

I used to adjust my 1991 300E like this all the time. It's easy after you break the code!!!

Intruder

I tried the procedure recommended here on my 1982 280e. I used pin no. 2 of the diagnostic connector, but found that I could not get the voltage below about 8.2 (not in the same range as for georgehifi's 300e. Has anyone tried this procedure on a 280e. BTW it has a O2 meter, but I don't know if it is working. Also, I find that after making the mixture adjustment, I had to crank the idle all the way down to the point where there was no idle adjustment left (all the way clockwise).

You need to probe the O2 sensor AT the sensor!!! In the 300E, you pull up the front passenger carpet and the wire was right there on the floor pan. Disconnect the sensor at the quick disconnect fitting, probe the sensor wire (either the only wire on older sensors or the "non-white" wire (the white wires (two of them) are the heater wires) and ground the other wire. Start the car. The sensor should always be providing a voltage! It will read rich (a lower than normal voltage I believe) until the car kicks out of "start-up" mode. That's when you start adjusting the Fuel/Air ratio. P.S. Keep count of the direction and amount of turns so you can get it back to where you started!!!!!!!!! AND Write down each adjustment.

Intruder