'86 300E rich mixture - Duty cycle help

[bigschot]

My '86 300E had been idling reasonably until I installed a new cold start valve in attemp to cure a hard starting problem. Prior to this, the mixture had been tinkered with and probably wasn't right.

Foolishly, after installing the new CS valve, I started tweeking the mixture without going through the formal diagnostic procedure, making the idle problem worse. Now it's running worse than ever and the mixture's obvious rich (more so than before).

I've read all the posts about using a Sears DMM duty cycle feature to set the mixture, but haven't been making much headway. Here's what I've got so far.

Key on, ignition off, O2 sensor connected: 97.61% duty cycle
Key on, ignition off, 02 sensor disconnected: 97.61% duty cycle

Voltage readings using pins 2 & 3 were around 6.85. Turning the mixture screw 1/4 turn clockwise made no difference in the voltage.

With the car warmed up, the O2 sensor voltage was all over the place.

What am I doing wrong?

[sixto]

Here are a couple of good articles on the matter:

http://www.peachparts.com/Wikka/EngineControls

http://www.landiss.com/mixture.htm

Apologies if you're familiar with them.

Sixto
95 S420
91 300SE
87 300SDL
83 300SD

[psfred]

First, make sure you have the red lead in #3 on the diagnostic connector and are set to read % duty cycle, not frequency -- the frequency stays the same whatever the duty cycle is.

Next, you should get a fixed 50% duty cycle on startup with cold engine. This will change to some other number when the O2 sensor warms up in a couple minutes. Should have a fixed duty cycle with key on, engine off (O2 sensor doesn't affect this reading) of 60%, I think. May be different.

Duty cycle of 98% indicates seriously lean, I think.

Once you get a reading that "floats", adjust the mixture slowly to get close to 50%. You will press down some on the linkage when you engage the spring to turn the screw, so it will always go rich when you adjust, then return to the new setting after 15 sec or so. Don't get in a hurry, and don't turn the screw very much at a time. Wait after each adjustement until the reading changes and settles down, or you will do what I did and end up way rich or way lean when it does.

On the cheapo meter I bought, grounding the black lead causes the meter to read either 98.xx% duty cycle or 0, you must leave it free.

Peter

Should have a fixed duty cycle with key on, engine off (O2 sensor doesn't affect this reading) of 60%, I think.

Key on engine off is 70% for Federal cars and 85% for Calif cars.

Cheers.

[bigschot]

psfred, you were right, I had my DMM set to Hz, not %. Now I'm getting a consistent 30.6% at key on, ignition off.

Gary

[Duke2.6]

Reverse the leads and you will get 70 percent.

Duke

[bigschot]

When I got 30%, that was with the leads connected as per the DMM owner's manual. Why would the leads need to be reversed?

As you suggested, I reversed the leads and got 69.3%.


Am I OK to proceed now?

Gary

You're good to go!

[stevebfl]

Its a matter of convention, similar to stating the cup is 70 percent full or 30 percent empty. Depends on your point of view.

[bigschot]

Steve, thanks for the explanation.

Started the car this morning and got a duty cycle hovering around 48.3% as it warmed up. Idle was fairly rough and emissions looks smokey (white smoke).

When it hit ~60C, I revved it a bit to get the temp up to the typical operating temp of 80C. Duty cycle went to around 92.3% and stayed there for a few minutes until I finally shut it off.

I backed off the mixture a 1/16 of a turn in an attempt to get to 45% duty cycle. The car wouldn't turn over (too lean?), so I turned the mixture back to its original position and then it started right up. Duty cycle was still hovering at around 92.3%.

What am I doing wrong?

Thanks,

Gary

92.3% would indicate that your car is already running very lean.

Of course a slight turn anti-clockwise and she wouldn't start.

Try going the other way.

From my own experience and Steve's words of wisdom when I had problems with my car, the car should run well between a wide range of the mixture adjustment.

At this point in time, seems to me that you are at the very end of the lean side of the mixture adjustment, which is why the car will not start, and if it does, will probably barely hold idle.

Let us know how it goes.

Cheers.

[stevebfl]

Sounds like good evaluation and advise to me.

You're on the right track and almost there, Gary!

Keep going!

Don't worry if you get it wrong along the way, Steve will beat us in the head till we understand what is going on!

[psfred]

Turn the screw the other way in small increments until you get very close to 50% and it's set correctly.

It will go rich every time you adjust it since you have to press down slightly to turn the screw -- this presses down on the flow sensor and adds more fuel. It will return to "normal" running state in a few seconds after you lift the allen wrench.

Check the wires for the O2 sensor heater, too, it should heat up faster than the 60C switch, I think.

Peter

[bigschot]

Picking up where I left off a few days ago, I let the car get up to temp before making any mixture adjustments. Duty cycle read ~92.3%.

Turned car off, turned mixture 1/16 of a turn back, restarted car (it was a little harder to start), let idle stabilize for a minute or so. Duty cycle again settled at ~92.3%. Another 1/8 turn back made it even harder to start, and again the DC settled at ~92.3.%. As a number of you suggested, it seemed to be very lean.

So I turned 1/2 a turn clockwise and DC went to ~30%. I backed it off a little bit (1/16 of a turn) and got around 36% DC, with a lot more DC fluctuation than earlier. Idle seemed a little better and starting was easier. A couple more tweeks and I got it to around 45%. Idle seems pretty smooth and it starts easily.

Well, I think the problem has been solved (fingers crossed). I'll know for sure tonight when I take it out for a 100 mile trip.

Thanks again for everyone's help - and especially your patience and encouragement. I feel a little like a total novice.

Gary