Hot idle adjustment

[Chrisheg]

I have an issue with my '69 280SL. The car would start and run well until it reached temperature, then the idle became rough and the engine slowed down and stalled. It would stay running if I held the RPMs to about 1200. I was able to get it to stop stalling by screwing in the idle stop on the Venturi until the idle was at 1200, then adjusting the RPMs by screwing the idle air adjustment screw all the way in. The engine now idles pretty well at about 950. I know this is probably not the right way to do this. I am able to keep my 190SL running well but I have never worked on a fuel injection system before. How should I go about troubleshooting the hot idle?

Thanks

[ja17]

Hello,

A common mistake.  The venturi stop screw is used only to keep the butterfly from binding in the housing.  You will never get the engine set up correctly if you leave it out of wack. In addition, if you have an automatic transmission you will cause a variety of shifting problems by tampering with the setting. 

First, read the "Linkage Tour" and set all you engine linkages.  Use the air screw on the intake and the idle thumbscrew on the back of the injection pump (with engine off ) to make warm idle adjustments. The "split linkage CO test" will tell you the mixture conditions. GEt back with us with questions, we will walk you through the process.

[J. Huber]

Hi Chris. Of course, do everything as Joe suggests.

I only wanted to add that my car does something that may be similar or maybe not. For years, my car has run pretty well with only minor tweakings of the idle here and there. It always starts great without any throttle, runs well cold and hot, and idles when warm every time I go for a drive. However -- there is one scenario where this is not true. If I start the car and let it idle without ever actually driving it or hitting throttle, it will idle at about 800-900 and sound just fine. Then after about 7 or 8 minutes I'd say, it will start to stumble and then die (if I didn't rescue it). What's strange is if I do drive it for even a little bit, it will then stand at warm idle. Anyway I am watching this thread so keep us posted!

[jeffc280sl]

James,

I don't think our cars like to idle for long periods of time.  Over 7 or 8 minutes I think it gets rich.  When I was tuning my car I was told to clean that stuff by reving the engine some and to then adjust idle. 

Are you saying you car will idle for long periods (longer then 7 to 8 minutes) after to drive or rev the engine?

[J. Huber]

Hey Jeff. Well, this is by no means empirical. I do know that the car eventually sputters at idle when not driven. Its happened a couple time recently. The rich thing makes sense. As for when I drive it, I guess what I meant was, I can sit in traffic or a drive through and it idles just fine. Maybe if it was in park? it may eventually shut off as well? I can try it!

[jeffc280sl]

Hey James,

The idle issue from a cold start getting rough after 7to8 minutes may be the same issue as warm idle for a similar period of time.  Maybe there is something about the pump design or wear over time that causes this versus an out of adjustment situation.

Chris,

I agree with Joe on the linkage.  It should be very tight with no slack in the ball and sockets or bushings.  By that i mean if you move the fip lever even a little the venturi should move also.  If you have slack anywhere the fip and venturi will be out of sync just a little.  This will cause your afr to suffer.

[awolff280sl]

Just for the sake of completeness I can add that if I don't drive it enough once warmed up, it will idle at 350-400. On the other hand, if I drive it a few miles at more than about 40mph, it will then maintain a reliable 800 from then on. Normally I don't see this phenomenon because I've already driven enough by the time the WRD has finished doing its thing.

[J. Huber]

Well in the name of science (wink) I went out and experimented...

With the hood up, Ipod stopwatch engaged, and my tach gauge clipped on, I started the car by reaching in (no throttle) -- it fired up at about 720-730 RPM. Stayed at this range for 3.5 minutes, then RPMs climbed to 780-800. (by the way, switched the meter over to dwell since I was killing time: 42.5. A bit high but that's another thread)...

Anyway, at exactly 8:48 the car stumbled and RPMs dropped and fluctuated around 550-600... I expected it to die but guess what? It slowly recovered to around 740-750 and locked in pretty good. I pulled the plug at 15 minutes... So it never died. Just like when you go to doctor -- things work again...

Then I went for a nice little spin, pulled over and timed it again. The car idled at just over 800 and never wavered. 15 minutes and again I called it. In summary, that stumble at 8 mins or so did indicate there is a point were the pump and/or engine gets rich and winded but for today anyway, things were fine.

Now back to Chris! Let us know what transpires!

[Chrisheg]

Thanks for all of the good advice. I no longer have the problem. I used the excellent linkage tour and idle adjustment instructions on in the Tech Manual and it runs very well now.  This site is a great resource.

[Bonanza]

I have similar idle problems with my 66 230 SL automatic. The idle problems with the automatic are much worse than the standard because of the increased load on the engine at idle. I don't think the constant speed solenoid on the linkage is working properly, but I can not find a wiring schematic for the US export version to do any effective trouble shooting. Can any one tell me where I can get a copy of this diagram? Any help would be greatly appreciated!!