Rough Idle '79 450SL - PeachParts Mercedes-Benz Forum

Mark Herzig · #1 03-07-2002, 02:25 PM

I'd greatly appreciate any suggestions on what might be the cause of a rough idle when the car is fully warmed... The idle drops to around 600-650 RPMS. The oil pressure stays around the first bar which is within specs I believe.

Otherwise the car runs magnificantly. I recently had the injector seals replaced, new plugs, new chain tensioner. I also adjusted the idle screw 1/2 turn counter-clockwise. Also, motor mounts recently replaced (by prior owner).

I could not find anything specifically in the archives that addressed this problem and since I've dropped a lot of cash on this car the last 2 months, I'd like to try to fix this myself. Any suggestion would be GREATLY appreciated.

Also, what are the rules of thumb on adjusting the idle screw? How much and in what direction produces what result.

#2 03-07-2002, 04:15 PM

have you tried replacing the distributor cap and rotor?

Mark Herzig · #3 03-08-2002, 10:07 AM

No problems there... rotor recently replaced... are valve adjustments common/necessary with the '79?

#4 03-08-2002, 10:54 AM

With a 1979 car, you may want to see if you have any rusted fuel lines that are clogging up your screens in your fuel distributor.

Mark Herzig · #5 03-08-2002, 03:03 PM

I recently had the injector seals replaced and cleaned up as well as a new fuel filter... I'm thinking that's not the problem... Thanks for the suggestion though

stevebfl · #6 03-08-2002, 05:54 PM

The 450 motors with K-jet were the worst idling of all the V8s. You will probably want at least 8-900 rpm in neutral to get the best situation. The speed is adjusted by the large screw at the front center of the motor. Unfortunately proper setting requires adjusting the fuel mixture at the same time and that requires a gas analyzer as there is no electronic control on that motor.

one other expensive problem that takes about this amount of time to occur is the failure of the thermostaitcally controlled addition air valve. As it fails it requires higher temps to close fully. When this area reaches the normal range of the operating temp of the motor then the idle speed changes by how hot it is running at the moment.

Mark Herzig · #7 03-08-2002, 08:39 PM

wow.. thanks Steve... I'll look into this are report back...

Mark Herzig · #8 03-09-2002, 08:40 AM

Steve;

I've got my 107CD out and wondering if the air valve you referred to is the one in the Intake Manifold?

michael rybikowsky · #9 03-09-2002, 10:54 AM

put a couple of bottles of techron in it and see what happens

stevebfl · #10 03-09-2002, 08:23 PM

The add air valve sits at the front of the motor next to the air screw and has two large diameter (about 16-18mm ID) air hoses attached. It screws with two smal bolts into the water jacket that runs through the front of the intake.

Mark Herzig · #11 03-11-2002, 10:42 AM

gotta tell ya, i'm having a great time learning about the mixture preparation system on the sl... I'm moving to replace the supplementary/addional air valve.

I'd also like to learn to set the mixture setting but need a CO measurement instrument. Is there something a DIY'er can buy/rent or rig in order to accomplish this?

the specification is .5 - 2.0 for the '79 450SL. I'm thinking I'd like to get it as close to the 2.0 as possible thinking i'll get edgier performance... thoughts?

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