mixture adjustment as such

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rsanders's picture


My 74 450 sl is running very rich as shown by the plugs. Is there any way to adjust the mixture.none of my maint. manuals have any thing to sayon this matter, just the knob on the contrlol unit. Is there any sensor which controls the mixture.

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Jim Hansz's picture
Kalispell, MT
Joined: August 31st, 1998


1990 420SEL
1969 230


Before discussing adjustments, you need to understand that fuel metering for your engine is determined by engine vacuum signals. Hence, any small vacuum leak, or an accumulation of several tiny leaks can cause rich running. This is by far the most common cause of rich running in DJet engines. I recommend that you systematically replace all your underhood vacuum lines, i.e. the rubber ones that connect the plastic tubing. Carefully inspect all the plastic tubes for heat cracks and replace or splice in rubber tubing where needed to ensure a tight vacuum connection throughout. Don't obsess on the size of the rubber hose, just get some that fits nice and tight. Also, there is a vacuum reservoir under the PS front fender. Remember to check it for leaking hoses and tank leaks, although they are very robust. If you have any doubt about vacuum integrity of this tank or your door lock system in places not easily seen, get a Mity Vac unit, about $60 at Eastwood, and test the system for leaks that way.

If the vacuum does not hold after you pump it up, you have a leak; either fix or isolate it. Also, there is a large vacuum line from the rear of the intake manifold to the DS fender where it attaches to the MAP sensor. Check this line and if in doubt replace it and add a small hose clamp to each end for insurance. Once you have done this you may have solved your rich running problem. Try measuring your vacuum with a good gauge; it should register around 16-20 in/hg for a reasonably healthy engine in proper tune. If you cannot get near this reading, you need to look for other leaks, such as the manifold gaskets. Fix all the leaks first or you will never get it to run properly. There is no adjustment that will offset a vacuum leak. Period.

Put in a new set of plugs and try it for a few hundred miles. NGK BP6ES plugs are some of the best in my opinion. If there is no improvement and the plugs are still sooty, an adjustment may be needed. But you should first check other components. Check the cold start injector for leakage. Check fuel pressure to the rail, it should be about 29 -30 psi. And by all means get yourself a copy of the service manual. Paper is good but pricey, the CD ROM is surprisingly good and available here for only $20:

There are two ways to adjust the mixture on your 450SL; one easy and one not. The idle mixture can be adjusted via the knob on the control unit under the dash on the passenger side. It works like a volume control on a radio, turning CW increases fuel, CCW reduces fuel. The range of adjustment is about 10%. You need to adjust this with a CO meter to achieve the values shown on the radiator support data plate. I believe the range is 0.5% to 2.0%. You have to work hard to screw this up. The knob adjusts in clicks, so just count the clicks either direction from your start point and you can always return to the original setting.

The second adjustment is not for the faint of heart. If everything else checks AOK and adjusting the idle mixture shows no improvement, you may need to adjust the MAP sensor on the DS fender to achieve the proper AF ratio, again this involves an exhaust gas analyzer to measure the tailpipe emissions. Most folks don't have this, but a good one is also available here from Eastwood for about $220:

If any of this does not make sense then STOP... you need to do some basic research with the factory CD ROM manual and a good Bosch FI manual...I like this one because it has some good trouble shooting tips and explanations:

and also this one for more general information:

There is also some information online, but it can be a bit dense and not always helpful for anyone not familiar with the basics. One of the better sites is located here:

Once you ave digested some basic information and collected the tools, you can adjust the MAP sensor with a 6mm allen wrench after first carefully prying off the plastic end cap. There is a socket head screw under the cap. With the engine warm and gas tester installed on the tailpipe and calibrated, measure the AF ratio. Adjust the screw, if necessary, CW (in to lean the mixture) no more than 1/8 turn and then measure the AF ratio when the engine stabilizes. You should try for 13.5 to 14.0:1, but you may have to settle for something in the 12s. These DJets are not like the high tech FI modern wonders in your typical new car. They ran rich and gave good power because of that. Fuel was cheap and folks buying these cars had the $$ to pay for the fuel. Expect to get not much better than 13-15 mpg with your SL and learn to live with it. It won't be easy with $3 gas, but that's life.

Finally, make and keep notes on what you have done so you can refer to them later if you need to make another change to something.

Good luck with this...


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