I can't find anywhere on this site to post questions about a 1972 280SE so I'll do it here.
4.5 L injected engine: When I first start the car, the RPM is around 2100. As the car warms, the RPM degrades to an acceptable level. My question is how do I slow the start-up rpm and yet have it hover around the 800 mark as it warms?
Heater controls: The heater has several "disks" to which are attached small tabs which control the heat. All of the tabs are broken. Does anyone know what procedure I must use to remove/replace the disks?

You can adjust the idle probably through the throttle linkage which should have an idle screw (Tomguy will know). I wouldn't mess with the map sensor.

Replacing the heater levers are a big pain but you can buy aftermarket ones for a lot less that go over the originals and hence you don't need to take the dash apart. Those can be bought on this site.

All the best

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With best regards

Al

You do not adjust the idle speed with the "idle screw" on the throttle linkage...there is none. The screw on the throttle plate under the air filter is something you don't mess with either. It is used to adjust the throttle position sensor...leave it alone.

Adjust the idle speed with the big brass screw adjacent the auxiliary air valve at the front of the engine. The thermostatic AAV valve is what makes the idle higher when cold and lower when the coolant warms up. If the car races to 2100 rpm, then you AAV is really working well...maybe too well or your basic idle adjustment is off. Cold idle speed should settle around 1200 to 1300 rpm and then fall even more as the coolant reaches about 70C where it should settle down to about 700 -800 rpm. When the engine is thoroughly warmed up try adjusting the brass idle screw to set the speed to the value shown on the emissions tag attached to the radator support. You may find it can be screwed down quite a bit before rpm drops much. If so, you may then see the cold idle more near to the 1200 to 1300 rpm range next time you cold start the engine.

All this assumes you do not have some other issue influencing idle speed, like timing or a vacuum leak.

I believe you can search for heater lever kits with Google, there are several vendors out there. These are a common problem, and you can search this site for references and repair tips.

230/8

If your cold idle is above 1500 RPM then the throttle position switch is not properly adjusted.

If it were then the fuel cutoff at 1500 RPM would kick in and your car would "surge" up to 1500 then the cutoff would kick in until it dropped and then it would rev back up again.

If the position switch is not set right then the cutoff won't work because it thinks the gas pedal is being pressed and not at idle.

There's a good description of how to set the throttle position switch here at post #7:

My 72 sl surges when the engine is cold

First perform this adjustment, then adjust the idle as above.

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Marty D.

2013 C300 4Matic
1984 BMW 733i
2013 Lincoln MKz

Do those work worth a hoot?

I've seen them and I thought I had heard someone say that they were really not usable. Have you (or anyone else) tried them?

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Pxland

2001 Honda Accord

1995 Jeep Cherokee

1973 MB 280SEL 4.5

Ive seen them too and they are very flimsy. I bought a whole heater assembly from Germany last year but have not installed yet.

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With best regards

Al

Tom: Welcome to the forum! Your 4.5 may have several issues. Got a pen and paper handy?

1) Check and verify your air temperature sensor wire (on the air cleaner assembly) is functioning properly. Since your mixture is rich as is during startup, if this wire is disconnected it will be even richer and your idle at startup will be through the roof.
2) Verify your vacuum retard is working. If not, your idle will be very high.

Now, you say that it's a high idle as in 2100 RPMs, but you did not say anywhere that it "surges" or "searches". If your throttle were properly set, it would fluctuate - as the idle hits 1200+ (if memory serves, but below 2k) what happens is the ECU senses the throttle is in a "rest" position. Since the pedal is not being depressed, it assumes you're decelerating from speed and actually cuts fuel to the engine until about 1000 RPMs. It will do this regardless of being in park or drive or any gear. Therefore, your throttle position switch and/or plate are definetly NOT adjusted properly. A non-DJet-familiar mechanic (or non-EFI familiar one) may use the throttle plate "adjustment" screw to raise idle. This is the WRONG way to do so.

Disconnect the linkage and spring on the throttle plate. You may want to remove the throttle body. What you need to do is ensure the plate closes COMPLETELY without binding - a little stick may seem OK but with the vacuum pressure you'll have in your manifold it may result in a sticky pedal. Mine is like this but I find it perfect. Lock down the set screw. Now, this next part (adjusting the throttle position sensor) is also much easier with the throttle body in your hand. With the throttle closed, turn the TPS SLOWLY until pins 12 and 17 measure closed. Tighten one of the secure bolts, move the throttle from closed to open and then back to closed. Re-measure 12 and 17. If they don't measure closed, adjust the TPS further. If they do, open the throttle and watch where the switch measures open - it should be immediatley. Then open it a few degrees and close it slowly and see where the plate is when it measures closed again. It should be open no more than 2 degrees. If you're unsure, turn the TPS 1 notch (1 notch = 2°).

Now, once you are SURE the TPS is functioning properly (Simpe spot test: Check 12 and 17 while still on your vehicle, they should be closed), you can adjust the idle misture screw directly on the ECU. But this screw will have NO effect on your engine's speed if the TPS is maladjusted and these 2 pins aren't closed - telling the ECU to send the "Idle mixture" to the engine. Sometimes I pull the plug to do work on the injectors and forget to plug it in, and my engine idles quite high when cold when I do.

When you check these, I can provide you with issues 3 - 1000000000.

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