Assuming too rich under load conditions, you would reduce the fuel pressure (but not below 2kg/cm2). If at idle, you would adjust the ECU screw first.

The idle and load mixtures are controlled separately. As soon as the throttle plate moves, you switch from the idle mixture circuit to the load mixture circuit.

You need an exhaust analyzer or wideband AFR gauge to make the adjustments.

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Graham
85 300D,72 350SL, 98 E320, Outback 2.5

That's pretty much what I figured. Thanks.

TBH I never touched fuel pressure on my 4.5. I left it right around 29-30 the whole time, and made adjustments via the idle speed air screw and ECU for idle, and MAP for load. I would advise against using fuel pressure to adjust mixture. The D-Jet system is supposed to be the same pressure regardless of condition. If you're too rich under all conditions, MAP. If you're too rich at idle, ECU knob and/or idle air screw. If you're too lean under load or high throttle, the fuel filter or tank screen is clogged.

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Current:
2021 Charger Scat Pack Widebody "Sinabee"
2018 Durango R/T

Previous:
1972 280SE 4.5
2014 Jeep Grand Cherokee Limited "Hefe", 1992 Jeep Cherokee Laredo "Jeepy", 2006 Charger R/T "Hemi"
1999 Chrysler 300M - RIP @ 221k

I don't think you could use the idle screw for mixture adjustment. But, you should set it to the specified number for your engine (700-800 rpm for 117), and then adjust idle mixture with the ECU knob. This requires a way of measuring %CO in exhaust.

For load mixture, MB Technical Data Manual says to adjust mixture using fuel pressure. I didn't know that until recently, but if they say so, then I wouldn't question it! They probably assume the MPS is new, has been factory set and sealed and should not be touched. But we buy old cars where someone has likely already messed with the MPS

There is no question though, that as you increase fuel pressure, slightly more fuel will pass through the injector at each pulse and you will have a richer mixture. And vice versa. Adjustments should be in very small increments using a gauge that has reasonably small graduations.

I have adjusted my own mixture a little differently. I set the fuel pressure to 32 psig (**) as a good number to reduce vaporization and flashing across the injectors. I then used the MPS to adjust load mixture so that it was close to the MB specified figures . (Having an installed wideband AFR gauge allowed this to be checked while driving)

The emission specs for the 117 engine according to the MB Technical Data Manual are below

** This was not done arbitrarily, but as a Chem Eng, I looked at true vapour pressure of present day summer/winter fuels (and those from earlier days), and chose a pressure that would inhibit vaporization at expected underhood/engine temperatures.

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Graham
85 300D,72 350SL, 98 E320, Outback 2.5

How do you adjust the MPS? I'm familiar with everything else but have never touched one of those or made any adjustments.

There is an adjustment screw on one end of the MPS. Originally, the factory sealed them, probably so owners/shops would not mess with them. The sealant is an epoxy like material that can be removed, if it is still there. Then you adjust with a 4mm Allen key (on some cars, the screw is slotted). (There is actually an inner and an outer adjustment, but usually just the centre adjustment screw is used.)

I forget offhand which way is rich and which way is lean, but changes should be made in very small increments - say 1/6th of a turn max. Then run engine for a while on road before rechecking %CO in exhaust.

This link has detailed explanation, but here is a poor pic of adjustment screw

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Graham
85 300D,72 350SL, 98 E320, Outback 2.5

You're hired. You can start on Monday.

Thanks! Glad to hear I passed the test.

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Graham
85 300D,72 350SL, 98 E320, Outback 2.5

Now that made me laugh. At least Graham now has a fall back job.

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Paul S.

2001 E430, Bourdeaux Red, Oyster interior.
79,200 miles.

1973 280SE 4.5, 170,000 miles. 568 Signal Red, Black MB Tex. "The Red Baron".