CIS Experts, Advice Needed

[wswilburn]

I have a subtle CIS problem with a 1982 SC and could use some advice. If I set the idle mixture properly, the mixture is rich at higher RPMs. Fuel pressures are OK The lambda box seems to be working fine. I spliced some wires into its connector so that I can watch the frequency valve duty cycle while I drive. Operates at 50% until the O2 sensor warms up, then about 53% at idle. At higher RPM's the duty cyclle drops into the 20-35% range, indicating the feedback has to lean the mixture. The only time it goes above 50% in feedback (other than idle) is when letting off the throttle, presumably because of the extra air from the decel valve. It goes to 65% at WOT, as it should. Here are the possibilities I have thought of:

1. Small unmetered air leak, just enough to lean mixture at idle, but becomes insignificant when throttle is open. Could be a leak from the outside, or through one of the devices that adds extra air.

2. Incorrect air sensor plate height - screws up air metering at idle.

Questions for the experts:

i. Are there more possibilities?

ii. Any suggestions on diagnosing? Finding a small air leak is not easy.

I think I can find a reasonable compromise between idle and open throttle, but I'd rather fix the problem.

Thanks,
Scott

[Rondinone]

Are you sure that this is not the correct behavior? The dwell on the frequency does not indicate richness. It indicates how hard the lambda system is working to keep the mixture correct (in essence, correcting for deficiencies in the CIS). As long as it's not ever 0 or 100%, you're OK. The signal from your o2 sensor indicates richness. You can splice in with a GOOD digital multimeter (one with a high input impedence) and make sure that it's around .5V, on average.

[wswilburn]

No, I'm not sure this is not the correct behavior. I understand your point about the frequency not indicating mixture. It does tell what the mixture would be without feedback. I would have expected better tracking that what I see, but perhaps not. Anyone else here looked at the frequency valve under load? Where this really matters is for the WOT rich setting. What i would really like to do is set the idle mixture so that the engine has max. power for the 65% duty cycle in the 3000-6000 RPM range. If the lambda system can then keep the mix at stochiometric for other conditions, I'm happy. I don't know how to do this. I would assume the default setup (50% corresponding to stochiometric at idle) is close to this, but if something is screwing up my mix at idle, I'll have something other than than ideal for WOT.

[wswilburn]

I may be closer to my goal than I thought. I ran the idle speed up to about 2000 rpm and connected the lead going to the 15 C sensor to ground to force it into enrichment mode (65% duty cycle). I then tried pushing and pulling on the air sensor plate as described in another thread. As the engine speed dropped either way, it seemed to be optimum mixture. Now, 2000 rpm at idle is not the same as 3000-6000 rpm under load ( a lot less air flow!), but my mixture is closer to that condition at a 2000 rpm idle than at 900 rpm idle, from how the frequeny valve behaves in closed loop. Runs very well like this.

Other info to answer questions that might come up:

Yes, I did set the idle speed back after I was through (just a test).

I have checked the O2 sensor voltage - it behaves sensibly.

I have checked the CO. It confirms the lambda box is doing the right thing. I thought it might be a little awkward to have a hose running out of the tailpipe and flapping in the wind a speed, so I opted for watching the frequency valve in closed loop during driving and inferring the open loop behavior.

[vash]

scott, can you take a picture of your splice? the rest of this thread is a mystery to me, way over my head. but i read that you can hook a dwell meter on to act as a makeshift sensor to set mixture.

cliff

[wswilburn]

Cliff,

I'll post a picture later today. It's pretty simple, just gives you the same signal as the three-pin test connector in the engine bay, except inside the car so you can watch it while driving.

As to how it tells you something about mixture, that is not simple. I'll try to explain.

First, let's agree that we are only going to worry about normal operation here. Engine warmed up and not at WOT. I'll mention WOT later.

Second, this assumes the O2 sensor system (lambda system) is working properly and you're just trying to adjust the mixture. Diagnosing problems is more complicated.

The frequency valve affects the operation of the fuel distributor by changing the pressure in the lower chamber. The frequency valve is constantly opening and closing at a fixed frequency. When it opens, it dumps fuel from the lower chamber to the fuel tank. This changes the behavior of the fuel distributor such that it makes the mixture more rich. The frequency valve is controlled by the O2 sensor system. With no signal from the O2 system, it is open half the time and closed half the time (50% duty cycle).

The O2 sensor detects oxygen in the exhaust, indicating a richer than stochiometric mixture. Stochiometric means you have exactly the right proportions of fuel and air to burn the fuel completely with no oxygen or fuel left over. It is important to remember the O2 sensor does not tell you how much O2 is in the exhaust, just that there is some. Therefore, you can't use the O2 sensor directly to tell you the mixture. If the O2 sensor voltage is high (about 1 V) the mixture is richer than stochiometric. If the O2 sensor voltage is low (about 0.1 V) the mixture is leaner than stochiometric. Again, from the O2 sensor, you just know rich or lean, but not how much.

Without the O2 system, the mixture control system runs 'open loop', that is with no feedback. The air meter and fuel distributor add fuel to match the amount of air, but don't check to see how well they are doing. You can set the mixture at idle, but have to count on the mechanical precision of the unit to keep the same mixture for all air flow rates.

The O2 sensor system adds feedback or 'closed loop' operation. It looks at the signal from the O2 system to tell if the mixture is too rich or lean and then makes small adjustments by changing the duty cycle of the frequency valve (ratio of time open to time closed) until stochiometric is reached (O2 sensor says you just changed from rich to lean or lean to rich).

The O2 system can only the change the mixture by so much. The limits are the frequency can either be open all the time (100% duty cycle, max. richening) or closed all the time (0% duty cycle, max. leaning). In fact the limits of the control circuit may even be less than these extremes for example 10-90%, but I don't know. If the open loop mixture is too rich or too lean, the O2 system can't change it enough to reach stochiometric. You want to set the idle mixture so that for all air flows, the open loop mixture is within the range of the O2 system to correct. Setting the mixture so that the frequency valve is running at 50% at idle is a good place to start.

Now let's think about what watching the frequency valve duty cycle while driving tells us. If we run in open loop (disconnect the O2 sensor lead and the frequency valve stays at 50%), it tells us nothing. The mixture is changing with conditions due to the air metering not being perfect, but the frequency valve always reads 50%. Now reconnect the O2 sensor for closed loop operation. The mixture alays stays at stochiometric because of feedback from the O2 system, but now the frequency valve duty cycle varies. How much it varies tells you how hard it is working to adjust the mixture, and therefore how far off the mixture is in open loop operation. For example, if the duty cycle is below 50%, the O2 sensor is leaning the mixture to achieve stociometric: in open loop it would be rich.

Now, if you have adjusted the idle mixture so that the frequency valve can always achieve stochiometric within its limits, all is good and the exact setting shouldn't matter, right? Almost. As pointed out in many threads on this BBS, stochiometric mixture is great for producing minimum pollution and giving good fuel economy, but it doesn't produce maximum power. For reasons I don't understand (maybe someone else can tell us), maximum power occurs at some amount of richness. That is why many people disconnect the O2 sensor and set the idle mixture rich. Porsche understood this too and included a throttle sensor to tell the O2 sensor system when the car is at WOT. In this case, the system goes to open loop operation and the frequency valve is set to a constant 65% duty cycle (rich). Since you only need max. power at WOT, this gives you the best of both worlds.

The ideal setup is to tune the idle mixture so that 65% duty cycle corresponds to max. power. Since the air metering system is not perfect, this is max. power only over some range of air flows, and where you really care is at high air flow, far from the air flow at idle. Then as long as the O2 sensor system can adjust to stochiometric for other conditions, life is good. I don't know how to do this exactly. You can tune the idle mixture to give max. power at idle for 65% duty cycle, but this is likely to be wrong for higher air flows. Note that you can force the sytem into the 65% mode by removing the wire to the 15 C sensor on the right chain cover and connecting the wire to ground.

What I did was to set the idle mixture so that the frequency valve ran at about 50% at idle. Ithen connected a dwell meter to my splice and watched it as I drove around under different conditions. I found that the frequency valve ran well below 50% for almost all conditions except idle. This told me that open loop, high air flow conditions were rich compared to idle. I leaned out the mixture a bit, so that the frequency valve has to run at about 53% at idle, richening the mixture a bit to achieve stochiometric. Now at higher airflows the frequency valve still runs below 50%, but not as much. If you read my previous post, I think this setting is getting me closer to max. power for 65% at high air flow. I would really like a way to measure this that doesn't involve a dyno or other expensive equipment.

One last note. If you use a dwell meter to measure the frequency valve duty factor, the dwell meter measures in degrees and you need to convert to percent. If the dwell meter is set to 6 cylinder operation, 60 degrees corresponds to 100%, so divide by 60, multiply by 100.

Sorry this was so long.

[wswilburn]

Here are the pictures I promised. I used coax cable, because I had some and it is neater, but not required. You need to splice a wire for the dwell meter to the green/white wire going to pin 17 on the connector (green/white). The dwell meter will also need a ground. You can splice into the brown wire going to pin 16.