# What controls fuel volume? Not pressure



## ps2cho (Apr 16, 2009)

I've got an 87 Mercedes with the KE-Jet system...

Got massive overfueling "possibly"....My acceleration is simply aweful. I pulled the O2 sensor and it looks like this:










Running heavily rich wouldn't you agree? (The sensor is BRAND NEW, less than 1k mi on it, so its not old or anything)

I was getting ~150cm^3 of fuel at WOT out of the injector lines (injectors not attached) instead of the MB spec of 100cm^3. Maybe I am just simply running heavily, heavily rich.

Now the question -- if this is so, _what_ in the CIS-E system controls fuel VOLUME to the injectors? The pressure is within spec, so what controls volume? Is that simply a fuel distributor deal?

Any thoughts from you guys? Just looking for fresh thoughts away from the MB forums.


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## ps2375 (Aug 13, 2003)

If I understand the system correctly, the voltage at the DPR controls the volume based upon the position of the meter arm. The DPR in the CIS-e acts the same as the CPR aka WUR in the CIS basic system. They are what determine "control pressure" within the fuel dist. and determine how much resistance the air meter arm has to the airflow in the venturi and thus determine the mixture. A Bently Service manual should give instructions on how to measure DPR current to get a decent baseline setting.


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## WaterWheels (Aug 14, 2005)

Flow, or better put maybe the volume of fuel is a combination of two things, the pressure pushing on the fuel and the size of the opening which it is allowed to pass through. 

CIS and CIS-e (KE-Jetronic) although they look almost the same operate using different principles. The WUR (control pressure regulator) of basic CIS is nothing like the function of the DPR in the CIS-e systems. The WUR controls fuel pressure at the top of the plunger in the fuel distributor. As the engine heats up it increases this pressure so that the air sensor lifts less than when it did when the engine was cold. This gives a cold running mixture and once warmed up, the “final” operating pressure. With CIS-e there is no pressure on top of the plunger which changes to adjust the air sensor lift. The DPR, via the signal from the ECU, constantly alters the lower chamber pressure in the fuel distributor to which deflects the thin metal “valve” to allow more or less fuel into the injector lines. Both use a plunger operated by the air sensor plate to open and close (allow more or less) of the slits in that plunger to let fuel pass into the lines.

Although they do work different, both rely on the same thing for the volume of fuel, presure and “orfice” size. So for your engine to be running as rich as it seems to be there could be a few things going on, poor ignition of the mixture in the combustion chamber, cold start valve leaking, mixture set too rich, poor extraction of the exhaust gasses or malfunction causing the ECU to signal for more fuel.

"I was getting ~150cm^3 of fuel at WOT out of the injector lines (injectors not attached) instead of the MB spec of 100cm^3" . Are you sure that this is not a “minimum” requirement that the manual shows? The injectors will flow as much as is allowed based on their opening size and the “pressure” being applied behind the fuel (assuming there is ample volume). That the open line flows more volume by itself really is unimportant. Kind of like a fuel pump, it will flow much more volume than the engine can use if measured via an open main feed line, but the complete system narrows down just how much reaches the cylinders.


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## ps2375 (Aug 13, 2003)

I stand corrected. :beer:


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## ps2cho (Apr 16, 2009)

The manual does not specify which is frustrating. It only talks about flow differences between cylinders and my spread is perfect, less than 2%. I had contamination in the fuel lines and and I can't get the car to run like it did prior. Power sucks.

I replaced both fuel pumps, tank strainer, filter, rebuilt distrib from Larry @ CISflowtech, new eha, new injectors...

Like I said, it was running great before and all my ignition system was replaced too when I bought the car 10k ago.

Something weird is goin on here.

Edit: oh and my pressures are dead on spec FYI.


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