CIS Idle problem
 

79SC. Happened twice in the last 45 days. For no aparant reason when driving and coming to a stop idle would go to "0". Engine restart normal. I adjusted the idle screw ccw 1/2 turn and all ok. It happened again today. Weather foggy, 55 degrees, humidity high. Adjustment returned engine to normal operation. What could be causing this on a CIS w/o oxy sensor?

Note over time the fuel pump, accumulator, filter, injectors, coil, spark plugs and wires, several injector sleves have been replaced.

Try checking your decel valve by plugging the line on the top.

Also see if you are still running too rich by pulling down on your air sensor plate. If the revs rise you can go leaner.

It would also be a good idea to check your control pressures.

Use a 7mm allen wrench and turn the mixture 1/16th to 1/8th turn (no more than that) counter clockwise. Drive it for a couple of days you should notice a difference.

Be careful. You have already leaned it by 1/2 turn, which seems to be somewhere in the order of 4%.

My car has no O2 sensor either. When I am set up near to the limit of richness, I can get that propensity for the idle to drop when coming to a stop, when the air is muggy. Then I know that I can lean it slightly in 1/16 turn increments.

I am just wondering why your mixture was so rich to begin with.

If you try the leaning procedure, be mindful of any lean running characteristics, such as popping, a light tailpipe, or a lag under acceleration.

If in doubt, try the sensor plate thing, or have someone w/ an analyzer set it.

I think it's a 3mm.

Spend a few dollars with a shop with an exhuast anayzer. 15 minutes should be all you need to get it set correctly.

Ummm. It was the idle valve I adjusted, not the mixture. Big screw on the left side, not the little one on the distributor.

How does the deaclerator valve (or what ever its true name is_ work? What is inside it?

While I am not sure, can a loose fan belt cause this symptom?

Thanks for the leads so far!!!:)

Sorry for misreading your original post.

You do have one of the symptoms of having a rich mixture. Adjusting the idle speed will not affect this, as you have noted. A search will net you lots of info on making the adjustment and checking your setting. I would start w/ the mixture.

Use a 3mm allen wrench in the top of the fuel dist and do counterclockwise turns in 1/16 turn increments. If this does not help you should probably set it back and go on to other things, although that usually does it. If you are unsure, a good shop can set it for you quickly and cheaply.

Search the decel valve. I don't think that is the cause but it might be, if leaning the mixture doesn't fix it.

If you have a loose belt you could be losing battery power, although didn't you say the car start back up OK?

Let us know if leaning works.

I'm not sure we are still on the same page. Idle is good. Starts right up. Runs good. CO is 3.42, ppm 77, CO2 7.5, o2 6.0, AFR 17.98. What it did was quit when coming off the throttle. Now Rpm will ususally sweep to 450-600 rpm then return to normal 950 - 1000. Before I adjusted the idle it would drop through 400 rpm to 0 rpm. Why?

Note I tightened the fan belt today, one shim removed, and it doesn't squeel anymore but, I did not get a chance to run her around.

I hope what you mean is that one shim was moved. you need to always have a total of six shims either between the halves or outside them.

Try leaning just a little, like 1/32. That would be 1/4% or so. If that doesn't help try another 1/32. If a few of these don't help you can always put it back, since you know your starting CO is 3.42.

Without the O2 sensor you can't get fine automatic control of your mixture so you have to set it for your best compromise. You have to allow for changes in atmospheric conditions, like muggy or wet days when your mixture that was great on a clear day will suddenly be too rich.

You may also have to richen up just a tiny bit if youstart to encounter consistent lower ambient temps. I always have to do this w/ my car around this time of year.

You may also have WUR or AAV problems as was previously said but I think this is just mixture related.

The decel valve is just a diaphram that is spring loaded against the presense of vacum. Basically when you let off the throttle quickly during decelerated coasting, it by pass's the throttle with a sllight gulp of air to stabalize the meter plate. So how does it noticably affect the car's performane, basically if you disconnect and let off the throttle during shift, you'll notice that the revs drop much quicker. And during quick decel while coasting, you'll notice loud pops from the exhaust (or at least that's what i noticed). I had disconnected mine for a little while to see the difference it would make, but now i'm going to connect it back up because those pops next to walls are hella loud.

Thanks, I don't hear any pops. Maybe the 911 is like its owner. When winter begins to set in the creaks and groans start!

I hear you.

Seriosly, I think this is why we get a lot of posts from early CIS car guys that say they are getting popping in the fall (as temps go down) and other posts that say they are getting a hunting idle or stalling @ stop signs when it is hot & muggy in summer. Without O2 or DME you have to set the car up the best you can and vary it as the conditions change by season.