1973 4.5 fuel injection adjustment - Mercedes-Benz Forum

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post #1 of 4 (permalink) Old 08-01-2006, 07:57 AM Thread Starter
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Date registered: Jul 2006
Vehicle: 1973 280SE 4.5
Location: Reno, NV
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1973 4.5 fuel injection adjustment

My newly-acquired 1973 280 SE 4.5 will not pass Washoe County, NV smog tests. The hydrocarbon emissions are well within limits, but the CO is over limit on both the idle and the 2500 rpm tests (idle 4.23, limit 3.50; 2500 rpm 5.27, limit 3.50). The car came from Palm Desert, CA, which is at sea level, and Reno, where it is now is at 4500 ft. elevation. The car is obviously running too rich, but I don't know how to adjust the fuel injection. Can any of you out there give me any advice on how to do this?
Thanks in advance,
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post #2 of 4 (permalink) Old 08-01-2006, 08:16 AM
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To adjust the CO you will need to access the ECU and turn the thumbscrew (detent potentiometer)on the side.Without a CO probe in the tail pipe,this will be difficult to get exact but the CO% should be 0.5 to 2.0 .Do adjustment only when the engine is fully warmed up!

Are you sure you don't have leaking injectors or vacuum leaks?

Also the pressure sensor contains an altitude compensating aneroid in it that adjusts automatically for changes in altitude.


Last edited by Nutz 4 Benz; 08-01-2006 at 08:23 AM.
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post #3 of 4 (permalink) Old 08-01-2006, 09:26 AM Thread Starter
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Vehicle: 1973 280SE 4.5
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I would think that if I had leaking injectors, that the hydrocarbon levels in the exhaust would be elevated as well, due to unburned fuel in the exhaust. I could be wrong about that, but it makes sense to me. As far as vacuum leaks, I don't think I have any --- all the important vacuum hoses and clamps seem OK, and the entire peripheral vacuum system (door locks, trunk lock, gas cap lock) functions perfectly. I don't know for sure about the vacuum advance to the distributor, but it seems fine as far as looking at it and feeling it. I hate to fool with the ECU myself, as I really don't have a lot of specialized tools for Mercedes, although I used to be a decent mechanic in my younger days when cars were simpler. Thanks for the input.
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post #4 of 4 (permalink) Old 08-01-2006, 11:55 AM
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Vehicle: 73 280SE 4.5, 71 300SEL, 03 BMW Z4, 72 Fiat 850, 80 Triumph TR7, 85 Porsche 911
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My 108 wouldn't pass Colorado emmissions (it passed California's just fine though, 6 months later), so I took it for a high speed run on a long straightaway, in 2nd gear, and immediately re-did the test. I passed (though barely) with a pretty hot engine.

Last edited by Grubeguy; 08-14-2006 at 07:23 PM.
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