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Old 06-20-2010, 03:06 AM
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Join Date: Jun 2010
Posts: 14
Steve Brotherton did this to me...

I recently read a post by Steve Brotherton of Florida who explained in a detail I have not heard before about the operation of the CIS-E Lambda control.

Essentially, in a closed loop stable idle, any Duty Cycle setting within reasonable boundaries of within 100% will have the same fuel air ratio. The EHA will obey the commands of the Lambda control and trim the fuel flow to maintain the correct setting.

This is news to me (slightly painful news).

My 1988 560SEC (US Federal 70%) has a rough idle.
I have replaced, literally, everything. At great expense.
This is my "fun" car and not a daily driver so this is a DIY adventure.

I thought by trimming the Duty Cycle towards richer I made a better idle, but the car wouldn't start well and bogged on acceleration.

Now I think I see that the hard start was due to too much fuel since the mixture screw was too far CW and the Lambda control starts in Open loop which basically follows the mixture screw alone. Hence too much fuel for start.

Bogging on acceleration happened because the system is already "maxed out" trying to lean the mixture to compensate for the incorrect screw setting. When the acceleration enrichment (AFP overswing) kicks-in the system cannot lean out enough since it is already maxed out and the mixture goes too rich for a moment.

If all this is true... I been barking up the wrong tree.

A lean mis-fire at idle then can only be one thing. Vacuum leaks.

My duty cycle swings as much as 5% to 10% either side of the average.
Is that too much?


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Old 06-21-2010, 02:44 AM
compress ignite's Avatar
Drone aspiring to Serfdom
Join Date: Feb 2004
Location: 32(degrees) North by 81(degrees) West
Posts: 5,554

Are you "OFF on The WRONG Foot" ?

(Your Mis-Conceptions are Steve's Fault ???)

[You're surely not putting your "Best Foot Forward"
(Or anywhere else other than in your own mouth)]
'84 300SD sold
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Old 06-21-2010, 09:54 AM
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Join Date: Sep 2007
Location: Fort Worth TX
Posts: 151
I've said the same thing about duty cycle and apparently get ignored as I watch people spend lots of money replacing parts in order to get the duty cycle exactly 50% under any and all conditions. This is just not necessary because as long as the duty cycle reading is SWITCHING the system is capable of properly adjusting the air/fuel mixture. Does not matter if it is 30% or 80%. Fact is these cars are no longer brand new and they now have some engine wear and are less than perfect. Duty cycle on these cars is NOT an indication of air/fuel ratio. It is more analogous to what is called "fuel trim" on late model cars. It is an indication of how far in the adjustment range (either rich or lean) the engine computer has to be in order to get the stoichiometric (ideal) air/fuel ratio. Unlike late model cars, these systems have an adjustment for air/fuel ratio, so it is possible to put it within range even on an improperly functioning system. So, yes, it is possible to have the ratio correct at idle, but not at speed or vice-versa. But if the duty cycle reading is CHANGING/SWITCHING within its limits over the engine speed range then the system is able to control air/fuel ratio properly. It does not have to be at exactly 50% (or whatever target value) to be classed as properly operating. IMO some people have the mistaken impression that duty cycle represents air/fuel ratio directly when I have explained above that it does not. Proper duty cycle readings do not, however, rule out problems with the fuel system or base engine, so it is still necessary to be sure that the engine is OK (good compression, valves sealing, ignition system OK) and that the FD and injectors are operating properly before reading too much into the duty cycle adjustment and readings.

As far as your car goes, it is possible to have the adjustment turned too far in the rich direction that it is holding the FD plunger off of the rubber seal when the engine is off. This will cause hard starting when hot. That is easy to check; push down on the sensor plate and there should be a small amount of movement before it contacts the plunger on the FD. Now, if you have the mixture screw adjusted correctly but still do not have the required free play, then it will be necessary to remove the FD and turn the threaded ring around the plunger CW so as to move the ring (and rubber seal) upward. Then reinstall the FD and recheck the free play.

Vacuum leaks are fairly easy to find when monitoring duty cycle (fuel trim) by spraying carb cleaner and watching for changes in the duty cycle reading (that is if the duty cycle reading is switching to begin with).
Erich Loepke
2010 Ford Focus
Currently Benz-less
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Old 06-21-2010, 12:18 PM
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Join Date: Jul 2008
Posts: 1,236
There is a lot less accusation in this post than the title would suggest. I think he is referring to his new level of understanding, and not something he actually faults Steve for. Perhaps this falls into the line of sarcasm that goes missed due to the lack of emoticons?

1990 300SE "Corinne"- 145k daily driver - street modified differential - PARTING OUT OR SELLING SOON - PORTLAND OR. AREA - PM ME FOR DETAILS
1988 560SEL "Gunther"- 190K passes anything except a gas station
1997 S420 - 265k just bought it with a rebuilt trans. Lovely condition
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Old 06-21-2010, 07:23 PM
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Join Date: Sep 2004
Location: Seattle
Posts: 1,961
Yes, confusing title.
1998 C230 330,000 miles (currently dead of second failed EIS, yours will fail too, turning you into the dealer's personal human cash machine)
1988 F150 144,000 miles (leaks all the colors of the rainbow)
Previous stars: 1981 Brava 210,000 miles, 1978 128 150,000 miles, 1977 B200 Van 175,000 miles, 1972 Vega (great, if rusty, car), 1972 Celica, 1986.5 Supra
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Old 07-07-2010, 12:40 AM
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Join Date: Jan 2010
Posts: 5
If you really want to get a better understanding of the JetTronic (CIS) and K-JetTronic (CIS-E) systems, you may want to invest a few $$$ in, even after working on CIS (and CIS-E) systems for decades, sometimes I find it good to review and get back to basics.

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