4.5 keeps going lean!!? - Mercedes-Benz Forum

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post #1 of 14 (permalink) Old 05-16-2010, 08:30 AM Thread Starter
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4.5 keeps going lean!!?

Ive got about 80 hours in my 1972 280sel 4.5,,, Fully rebuilding the entire car. I bought it from the original owner who had it garaged kept but it needed a lot of love. (Maintaince). Ive got it very fine tuned and it drives like a brand new car for a bit!! The engine control unit was at full rich and I was adjusting the mixture with only the MAP sensor. Now that I discovered there is an adjustment know on the computer I revised my thinking. The spec for co% is .5 to 2.0 WAY TO LOW if you ask me. So basically what ive done is richened the mixture via the MAP sensor and leaned it out with the computer to get the knob arund the center of its adjusment range. I set the co% to around 4% idle at 800rpm and ive got my HC down to around 150. SO IT RUNS GREAT. for a couple of days. then when I cold start. theres a hesitation on acceleration. (lean misfire backfiring through the exhaust) I reconnect to annalizyer and low and behold i'm lean again under 1%co. WTF.
Only thing I can think of is my MAP sensor going out. I adjust the map to the right mixgture and it screwes up again. PLEASE HELP.
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2011 ML350
2000 C230 sport (modded)
1972 SEL 280 4.5 (sold)
2010 E350 (sold)
2009 E550 (sold)
2007 E350 (sold)
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post #2 of 14 (permalink) Old 05-16-2010, 08:35 AM Thread Starter
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I took apart the engine control unit. Found nothing fully cleaned it and checked all the solder joints. All was good.! I know these can go bad but it looked ok.
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2011 ML350
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1972 SEL 280 4.5 (sold)
2010 E350 (sold)
2009 E550 (sold)
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post #3 of 14 (permalink) Old 05-16-2010, 10:35 AM
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If the spec calls for certain gas percentages, I'd stay with that. If you go too rich, you'll end up fouling your plugs, or worse, washing down your cylinders. If you intentionally go too lean, you risk overheating (I've done both). How are the connections from your other sensors?
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post #4 of 14 (permalink) Old 05-16-2010, 11:11 AM
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You are on the right track...
I would first start with checking ALL the ground connections (the main from the battery and the ones under the battery tray) ,and voltage stability from the alternator and regulator. 13.7-14.1 Vdc
They are grounding everything, even every injector, because of voltage stability importance. Gently pull every connector and clean, check for oxidization.

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post #5 of 14 (permalink) Old 05-16-2010, 11:49 AM
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Amazing photos.

Former MB: "Miss Juliana" - 1972 280SEL 4.5
Current MB: 2013 C250
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post #6 of 14 (permalink) Old 05-16-2010, 05:53 PM
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The Mixture at idle will not relate to the mixture under load. You would need the car on a chassis dyno. There is a procedure for testing/adjusting the pressure sensor here:

The Manifold Pressure Sensor

Nice looking car!

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post #7 of 14 (permalink) Old 05-16-2010, 06:04 PM Thread Starter
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Hey guys. Thanks for the responses. Im actually a Master Certified Mercedes Benz Technician. I'm all about going by the specs -grubeguy!! On the other hand in 1972 the government was just starting to CRACK down on emission regulations, in one way by leaning out the mixture. Unfortunatley theres no feedback system to the Engine control unit. (JUST A bit BEFORE LAMBDA). These engines run SO much better with a richer mixture. Not necessairly an OVER rich mixture. Over rich would be like above 15% which would wash out the cylinders. So that i'm not worried about. Ive had the car for about a month, and one of the first things ive done is remove all the grounds and clean them, remove all connections for sensors and clean therm. cleaned all fuse connections. Also under the washer tank is a 4 prong power distribution block which I removed all of and cleaned as well. My vacuum to the map sensor at idle is at 17 hpa. so the source is good. fuel pressure is at 32psi at idle! My idle voltage supply is 13.7v to the map sensor and ME. so basically as a techinician in a benz dealer I dont work on hardley any old cars. The oldes car ive seen in the shop was an 85 (which I worked on.) I'm putting out my feelers to people who love these cars like I do and may have seen something like this. My educated guess is the map sensor. ANY ONE HAVE ANY FOR SALE? (the pics were from my I phone : )

2011 ML350
2000 C230 sport (modded)
1972 SEL 280 4.5 (sold)
2010 E350 (sold)
2009 E550 (sold)
2007 E350 (sold)
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post #8 of 14 (permalink) Old 05-16-2010, 06:59 PM Thread Starter
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WOW 3.5 tony is that your website? The info there is incredible. Very informative!!! I continue to learn more and more about these cars. I love merecedes benz and decided to venture into my 72. So far ive read (what manuels we have at work) on this car front to back,,, but the best info I get is from people like you. THANKS A LOT : )

2011 ML350
2000 C230 sport (modded)
1972 SEL 280 4.5 (sold)
2010 E350 (sold)
2009 E550 (sold)
2007 E350 (sold)
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post #9 of 14 (permalink) Old 05-17-2010, 12:24 AM
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I can't take credit for tha info-I just found it poking around on the web. If one is inclined the information should help maintain the D-jet system. It's pretty technical but if one is going to repair the D-jet they need to be informed.
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Originally Posted by 72benzyacht View Post
WOW 3.5 tony is that your website? The info there is incredible. Very informative!!! I continue to learn more and more about these cars. I love merecedes benz and decided to venture into my 72. So far ive read (what manuels we have at work) on this car front to back,,, but the best info I get is from people like you. THANKS A LOT : )

Tony
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Stick shift
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post #10 of 14 (permalink) Old 05-17-2010, 05:49 AM
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I had the same problem with my 72 350SL with a 4.5. The problem was a small crack in the vacuum hose that goes to the map. Sometimes it would leak more than other times effecting the fuel mixture.
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