Low Idle

[n/a]

I know there has been a lot of discussion regarding low idle but I can't seem to find anything in the knowledge base that fits my problem. And I should say it's not a problem all of the time, it's more of a hassle.

During cold starts the car starts right up and idles around 900 - 1100 rpms but once it reaches 35C in temp (when the thermo warmup-switch closes) the rpm's drop off. Oil pressure will fall slightly (to around 2.5 from 3) and the red voltage regulator light comes on and off and the car wants to cut out but typically doesn't. However when I give the car gas, there's no problems, the car drives great. Then when I come to stop signs the car's rpms drop again - most of the time, at times it doesn't at all. And at times the rpms will drop then pick up and return to 900-1100 by itself. It almost seems like the vacuum venturi system is sticking but I'm not sure. Does anyone have any suggestions of what and how to adjust/fix it?

Thanks,

Scott

Scott
1966, Signal Red, 230SL

[Tom]

Scott,

Many are tempted to start adjusting the idle air screw and idle fuel screw (on the back of the FI pump) to fix this problem. It would be a mistake to try to fix your low idle by adjusting the fuel and air mixture screws without first going through the steps outlined in order listed below.

First place to start is to ensure all of your linkages are correct, which is critical to a correctly running engine.   Disconnect and lubricate each of the ball joint ends.

Starting on the passenger side of the engine, check that the throttle plate (venturi butterfly valve at the air intake manifold) closes tight without binding.  Adjust the length of the throttle rod to the main regulating rod (big rod that goes across the valve cover) so that the ball connection on the main regulating rod is centered with the hole in the support bracket.

Now move to the driver's side of the engine.  Adjust the linkage from the main regulating rod to the injection pump so that the injector is against its stop when the linkage is tight.  From center of the ball end to the center of the ball end on this linkage should measure 233 MM.  The idle stop on the other side of the engine should also be tight against the idle stop screw.

Next step is to check the timing to ensure that it is set properly, gaps in points are correct and points are not pitted.  Check spark plugs for signs of too rich or too lean running.  

Once you get the linkages and basic tuning down, only then start adjusting the idle air screw and the FI pump idle fuel mixture.  You can loosen the linkage at the FI pump and press it down while the car is running but since it is disconnected from the rest of the linkage, pressing the linkage down will tell you if the car is running lean (as you are adding fuel without any air).  If the car runs better with the linkage pressed *slightly* down, the car is running lean (you gave it more gas and it performed better.)  If the car stumbles with this procedure, the car is not getting too much air.

Now hook this linkage back up and go to the other side (passenger side) of the engine and disconnect the linkage from the venturi air intake to the main regulating rod.  Do the same trick on this.  If the engine accelerates by more than 300 RPM before it stumbles, then there is too much fuel.  Less than 200 rpm acceleration before it stumbles could indicate lean condition.

Once you determine if the fuel/air settings are lean or rich, you can adjust the idle air screw or the idle fuel screw (on the FI pump) to get the mix right.  This is done with a vacuum gauge.  The ideal vacuum at idle is 16-18 at 800 rpm.  Turn air screw clockwise until the vacuum reaches it's max before it falls and then turn it back a notch. You would then adjust the FI pump idle fuel screw (with engine off) to until you get the vacuum to a max of 16-18 on the vacuum gauge.  Check the Big Blue Book but I think clockwise adds fuel to mix-turn it 4 indents at a time and then recheck.  Let me repeat-only adjust the FI idle screw with the engine off.

It would be a mistake to try to fix your low idle by adjusting the fuel and air mixture screws without first going through the steps outlined in order listed above.

Hope this helps,

Tom  

[ja17]

Well done Tom!
This was a nice summary. Scott, also be sure that the injection pump air filter is not clogged and check the distributor which can have a broken advance spring.

Joe Alexander
Blacklick, Ohio

Joe Alexander
Blacklick, Ohio

[Tom]

Thanks Joe.

You mention another area to focus on-that being the distributor.  Additional areas to focus on to get everthing right include lubricating the distributor (felt pad inside and the external oil port at the distributor), check to ensure the cold start valve is not leaking (letting in fuel that will disrupt the correct fuel/air mixture), ensure the FI pump thermostat is working by testing for suction when the air filter on the FI pump is removed-suction should be there on start and as the car reaches temperature, the suction should go away.

Other areas to check include valve clearances, vacuum leaks (while the small vacuum hoses are very durable, they do get brittle over time).  I had a poor performing engine and my wife noticed the vacuum hose from the air intake manifold to the brake booster was cracked.  I sheepishly thanked her, replaced the hose and wow!!!  Off to the races!!!

Best,

Tom

[n/a]

Thanks for everyone's suggestions.  

I never even considered the brake vaccum line, now that I think about it, I really have to push down on the breaks while at a stop sign idling.  If that's it I can fix two problems at once.  I'll check it out.

Thanks again

Scott
1966, Signal Red, 230SL

[n/a]

Joe/All:

I am still having major problems with the idle of my SL.  This weekend I confirmed the linkage is in tolerance and tweaked the idle air and idle fuel, which worked for a while but then reverted to the same rough, idle.  

Joe you had mentioned that the advance spring may be broken, but I'm having difficulty in locating it.  Can you describe its location or better yet can you reference a page number in the Haynes or BBB manuals.  

I've hooked up a gauge to the vacuum line coming off the manifold and it's all over the place - surging from 0 - 15 Hg and of course the rpms follow.  The only other conclusion that I can come to is that I need another tune-up (just did one a couple of months back) or the entire vacuum advance system needs to be replaced.   Is there anything else I should check?  

Appreciate the help.


Scott
1966, Signal Red, 230SL

[n/a]

Surging Idle continues, however, I have been able to stabilize the vacuum advance.  Not sure what I did, but now I get a consistent 13 Hg vacuum.  The only thing I can think that I did to effect the vacuum was that I bumped the points and checked the Dwell.  Points are pitted, again.  Thinking that I should go with an electronic system instead of continuing to change points.  Could that be the problem with the advance?  Dwell was 38/39, according to the Haynes manual that's within tolerance for the 250s and 280s but can't find the Dwell in the BBB for the 230SL.  Does anyone know the correct angle for a 230SL?

RPMs have also slightly stabilized, but still fluctuate between 650 - 750 rpm.  When the rpms drop to 650 the oil pressure also drops from 3 to around 2.5 then returns to 3.  It's like something it turning on putting load on the engine then immediately turning off. It does this every couple of seconds.  Any thoughts?

Also would warm weather have any effect on the rpms/advance?  The other evening it was 85 Degrees here in Hershey when I experienced all of the problems, last night it was 40 degrees less and the car was 100% better.  Could just be a coincidence.


Scott
1966, Signal Red, 230SL

[ja17]

Hello Scott,
The distributor advance springs are below the breaker plate in the distributor. They hold two flyweights in place. You cannot see them unless the distributor is disassembled. However you can check their action simply by turning the distributor rotor clockwise the springs will return the rotor when you let go.
A searching and low idle can be caused by too lean a mixture. Make sure the fuel filter is clean and a good supply fuel (correct pressure and quantity)is being delivered to the injection pump. The injection system leans down the fuel mixture as the engine warms up. It sounds like your engine is asking for more fuel, especially when warm. If you gun the warmed up engine when in park and it "coughs" back through the intake this is a sure sign of "not enough fuel". Try a couple of clicks richer at the injection pump adjustment after everything else checks out.

Joe Alexander
Blacklick, Ohio

[n/a]

I know I've said this before, but this time I think I got it fixed!!! Had to have my girlfriend’s father help me - just needed a little fine tuning and education on how to use the tuning equipment.

First off he gave me a lesson in how to use that new stroboscope/timing light and how to properly set the timing. Then we worked on figuring why the idle would die out after warm up. The reason was, I had turned back the idle fuel so far that it was running out of fuel. The lack of fuel was the reason the vacuum advance was not retarding (increasing rpms) but dropping/stalling the car when I would remove the line from the manifold. Also, the lack of fuel was the reason why the car was hesitant while cruising at 2500 rpms. Once we had a decent mixture, we then could time the ignition.

I was previously trying to set the timing with the graduation scale on the crankshaft pulley only and not using the advance setting on the stroboscope/timing light to assist with the BTDC setting. With the vacuum line removed and attached manifold vacuum connection plugged with a golf tee I dialed the stroboscope up until the graduated scale read 0 (TDC), BTDC reading on the stroboscope was 27 Degrees. From there we rotated the distributor until the stroboscope was set at 8 degrees - checked the rpms, they were at 1000, connected the vacuum line, the rpms dropped to 700.  Next, we turned the car off clicked up the idle fuel up two clicks, restated, tweaked the idle air and the rpms were 800 steady

The final test was removing the vacuum connection, to gauge the vacuum - the rpms rose to 1150, then dropped back to 800 when gauge connection was made, and vacuum was 18 Hgs. Oh, we also checked the dwell as we increased the rpms - from 800 to 4000 the dwell stayed within the tolerance range 38 - 41.

In the end, I should not have screwed around with the idle fuel/air setting. She only needed to have the linkages calibrated to their correct length and the ignition timed. But that's one of the reasons I got the car in the first place - to enjoy driving her, learn how to work on her, and to restore her.

Thanks for everyone's help - until the next challenge!



Scott
1966, Signal Red, 230SL