1963 230SL warm idle fluctuation

[perry113]

I'm helping a fellow forum member with some general service work on a very early 63 230SL manual transmssion car.

The car came in with an erratic idle fluctuation when warm and would experience a higher idle when applying the brakes.

I replaced the brake booster on the car which solved the high idle when brakes were applyied.

Now the car idles nicely when cold, but once fully warm the engine still hunts up and down at idle. It will start to level out a little after a minute at idle but once you give it a blip on the gas it will start it idle hunt (from 750 to 1100) when trying to find the idle. It acts like it has a vacuum leak, but I can't seem to trace one.

Any suggestions?

Peter Perry
1965 230SL Automatic

[merrill]

if you give the car a little gas when the idle is erratic does it smooth out?

you may have a mixture issue.

disconnect the cross linkage from the inj pump.  push down a little and let us know what happens.  does the car die or does the idle go up to 1500 rpm or so.  If it dies the car could be too lean.

If could be an vacuum leak, check the line from the intake to the booster, I found my hose had a leak and also check the line from the throttle body to the distro.  that line could also leak.

[ja17]

Hello,

Matt is on the right track. A slightly out of adjustment idle mixture will often cause this condition. Use the "Blacklick CO Meter" (linkage separation) to determine if our fuel mixture is  rich or lean then adjusty as needed.

[perry113]

if you give the car a little gas when the idle is erratic does it smooth out?

you may have a mixture issue.

disconnect the cross linkage from the inj pump.  push down a little and let us know what happens.  does the car die or does the idle go up to 1500 rpm or so.  If it dies the car could be too lean.

If could be an vacuum leak, check the line from the intake to the booster, I found my hose had a leak and also check the line from the throttle body to the distro.  that line could also leak.

I had the car back yesterday for host of other things to do. To answer your questions this is what I found.

Giving the car a little gas does set off the idle fuctuation.

With the car fully warm and the cross linkage disconnected I found when pushing down the engine idle speed to drop down as if you describe the car to be too lean.

There are no vacuum leaks from the distributor or from the brake booster. The ignition timing is right on.

My feeling is the injection pump is off.

Your thoughts?

Peter Perry
1965 230SL White Ivory
1972 911T Coupe Albert Blue

[jameshoward]

Hi Perry,

I had the same problem with my car. A little serching of the site might produce some info; I found quite a lot out there. Here's one thread: 

http://sl113.org/forums/index.php?topic=7460.15

You say describe a method that fixes the problem and mention that you think the FI pump is off. From my experience, I found that getting the linkage spot on held the solution. Joe's linkage tour is a godsend. I did it once and the problem remained. I removed the linkage rod to the pump and enriched the mixture manually and the problem went away. So I did the linkage tour again - in full. And again. On the third occassion I nailed it.

My point is that it's a art, not a science. There are a lot of things to get right when adjusting it, and it takes a while. I had my FI pump reset and it was too lean. I had to play with the idle mixture screw (back of the pump; spring loaded; never turn with engine running, etc) and the warm air idle screw at the manifold. By making tiny adjustments I hit the sweet spot. It took a while, but now it's perfect.

I'd recommend another go at the linkage, and then the odd turn of the FI idle mix screw (remembering how many turns, etc) and seeing how that goes. I find that by turning the warm air idle screw I can bring back the hunting idle problem you have now.

JH