What causes Duty Cycle to decrease from fluctuating 50% at idle to fluctuating 27% at 2500 rpm?

I have a California 1990 300e with a 103 engine. The car has 121k miles, has got new cap, rotor, plugs, wires, OVP, injectors, fuel pump, accumulator, sensors on the head and filters.

The engine starts, idles and runs well. My concern is low gas mileage -17.5 MPG mixed city and highway driving.

Duty Cycle Measurement @ Operating Temperature:
Key on: 50%
Idling: inititally fixed 50% for @30 seconds then fluctuating +/- 3%
2500 RPM fluctuating 27%

I had a problem where my car would fluctuate around 50% at idle but upon any application of throttle, the value would slowly drop to 0%. I thought that my car would we puffing out white smoke when it reached this value as I believe this means the mixture was getting way lean... anyways, a good used replacement EHA solved the problem.

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2006 BMW M5 "Heidi" @ 109,000 miles
2005 MBZ C55 AMG "Lorelai" @ 165,000 miles
1991 MBZ 300E "Benzachino II" @ 165,000 miles
1990 MBZ 500SL "Shoshanna" @ 118,000 miles

(On the hunt for a good used M103 engine as of 6/10/23, PM me if you have one to sell!)

I hate to see you guys buy $200 parts that might only need adjusting.

Would it help you guys if I bought another wideband O2 sensor and then loaned it out for $20 until it was paid for? Then it could move around for the cost of shipping only.

With a wideband O2 sensor you can see so much more and tune the EHA and CO/Idle.

EDIT: Ohm the EHA. If you get around 20 ohms it should be working electrically and just needs a mechanical adjustment.

Thank You .... I have 19 Ohms across the terminals. What direction is the adjustment screw turned and how much? My problem is low ratio at RPM.

It's not that easy I'm afraid. CCW on the EHA screw will lean you out but then you have to go CW on the idle/CO screw. You really need to see your A/F ratio to understand how the computer is working, or not.

Once in the range the computer will keep you near 14.7 (in closed loop)until you continue your adjustments out of the range it can handle. That range is about 1/2 a turn on the EHA and a bit more on the idle/CO...again, in my single case.

You can try 1/4 turn CCW on the EHA screw and 1/4 to 1/2 CW on the idle/CO screw and see what happens or if you want to contact me about using the wideband give me a call through the website...appliedracingtechnology dot com

Thanks - Im not ignoring the broadband O2 sensor idea, just a bit over my level of expertice.

So trying to size up what I hafve so far in a procedure:
1. Turn EHA screw a 1/4 turn CCW and reinstall.
2. Start and wait until the fixed 50% becomes fluctuating signal and adjust back to 50% should be CW.
3. Check pressures and see if difference between lower chamber and upper chamber is .4-.5 bar.
4. ?

Check?

Thanks

You might as well check your fuel pressures before you do anything. I saw almost no change in upper and lower pressures with a full turn on the EHA.

If you only turn the EHA and then wait for the motor to warm up before adjusting the idle/CO screw you may end up with an unstable idle, but give it a go.

If you can do all of that, you can work with the wideband O2 sensor. Attach the two leads to the battery's terminals, feed the wire through the passenger compartment (in passenger front window, out driver's side rear window) and slide the sensor into the tailpipe. The display will end up in the passenger compartment. Adjust the EHA and Idle/CO and watch the display as you take the motor through the lower RPM range.

Time to tinker with the car is slim this time of year. Between the parties, shopping and honey doo that just won't end, I'm going to have to wait until Thursday to meaure pressure etc.

What do you actually see with the Broadband O2 sensor?

XX.XX from 9.00 to 16.00

9.00 rich
16.00 lean

If you see either constantly you are out of the range of the device, not good. Within the range the number just jumps around. Once I got mine in the right range, I saw it jump around +- 0.1 or so. In my case that was 14.5x to 14.6x at idle.