How can adjust Idle speed?

Hello,

My 1972 280se 4.5 idle speed is to high. How can I adjust the speed?
Please could you tell me the exact procedure on doing that?

Thank You.

cK

[240 Ed]

I don't think it IS adjustable.

You more than likely have a vacuum leak somewhere, which is causing a "false air" condition.
Check all hoses, clamps on the air box, etc.

You can do a search for fast idle. There are people who are experts on this forum who have already explored this in detail.

[Tomguy]

There is a unit with a very large (slotted) screw in it. I believe the hose from the driver's side valve cover goes to or near it. That is the idle speed adjustment screw. Also, make sure your vacuum advance is working properly or it will affect the idle (and power!)

[psfred]

If the vac retard diaphram is gone, or the hose leaking or loose, the idle speed will also increase. Won't run badly, just faster.

To set the idle speed, get the engine to operating temp, attach a tach to the coil (electronic one, that is!), and adjust screw to 800 rpm.

Peter

[240 Ed]

A point must be made here.

If your car has idled normally for however how long, and the idle suddenly changes--you will have a vacuum leak.

The idle adjustment is there to set up the injection system, then after that you don't really mess with it.

Don't start messing with idle adjustment, or mixture adjustment until you cure the leak.

When you cure the leak, you will have maintained the correct settings for the injection (because you DIDN"T mess with it!)

Wouldn' t you agree, Peter?

Ed

[Tomguy]

Ed: Although that logic makes sense, it doesn't necessarily mean he does have a leak. I would imagine that setting the timing, adjusting the mixture and other things would affect the idle speed.

[240 Ed]

Yes and No.

If you set the timing and mixture correctly--you will end up with the correct idle speed.

The only caveat here is that if the engine is worn, blow-by, leaky valves, etc.--then your idle will be LOWER than normal.

There are many sources of a leak, including the elusive ventilation controls (lessee, what car are we talking about?) and transmission vacuum, etc.

[psfred]

There are several things that can make the D-Jetronic idle fast -- loss of vac retard, which increases the idle timing by 12 degrees, stuck aux air regulator (aka fast idle valve), which can be cured sometimes by a sharp tap to the top of it, vacuum leaks in the tranny modulator, door lock system, vac retard line, or the seals between the manifold halves (eight round rubber rings).

I'd make sure everything else was OK before just adjusting the idle speed, it should never change much except for fast idle on start. I don't get fast idle unless the car is still slightly warm from a previous run, but I don't care since it runs fine otherwise.

Peter

[Tomguy]

Leaks in the DOOR LOCK SYSTEM? But my car idles fine! Speaking of which, does anyone know where to get either new rubber for the trunk lock unit or a new unit itself? I've looked at FastLane and other places and couldn't find it listed even.

[psfred]

Try Rusty at mbz.org or a dealer, or someone who can order from WorldPac, they might have one. Different than the door lock diaphrams, alas.

You can bet a pretty good vac leak if the grommet for the vac tank, or the tank itself, leaks!

Peter

[Tomguy]

I looked at Rusty already. He doesnt have it listed either. But FastLane AND Rusty AND oemparts all have them listed (but not in stock) under a different chassis (I think I saw them under the 220). I'm trying to silicone it first (Ultra Blue is great usually)

[psfred]

I've not looked at mine, but it isn't the same as the one in the W115, I'm sure -- the cover is too small for the diaphram from the W115 to fit, whatever is in there.

You can also try some of the places that advertize in Hemmings Motor News -- that is the route we are going to new diaphrams for the 75 300D

Peter