Runs rich and high idle after restart

[mjr45]

If the potentiometer in the AFM is 12-15% out of whack, would the potentiometer in line with temp sensor be able to bring it back to spec? I realize I'm talking about 2 different parts of the EFI, and the CTS potentiometer will rich up or lean the mix by increasing/decreasing resistance and is unrelated to the AFM, but in my total ignorance of electrical stuff I may be asking dumbass questions.

Mike

Edited February 4, 2014 by mjr45

[FastWoman]

Mike, it's really very difficult to explain why, but almost every electronic control system is ultimately voltage based, even if it seems to be resistance based. The AFM itself has both input and output voltages, and it's the output voltage that the ECU monitors. It doesn't really care about resistance, except for the air temp sensor, I suppose.

The CTS, on the other hand, has a resistance, but that resistance is only meaningful when you send current through it and measure the voltage drop across it, which is what the ECU does.

So the way the AFM, if you were to change all the resistances in the AFM by 12-15% (excluding the air temp sensor, which works like the CTS), the output voltage would not be changed. However, if you were to do the same with the CTS, the current/voltage relationship would be changed, and the ECU would respond differently. That's oversimplified, BTW.

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Mike, it's really very difficult to explain why, but almost every electronic control system is ultimately voltage based, even if it seems to be resistance based. The AFM itself has both input and output voltages, and it's the output voltage that the ECU monitors. It doesn't really care about resistance, except for the air temp sensor, I suppose.

The CTS, on the other hand, has a resistance, but that resistance is only meaningful when you send current through it and measure the voltage drop across it, which is what the ECU does.

So the way the AFM, if you were to change all the resistances in the AFM by 12-15% (excluding the air temp sensor, which works like the CTS), the output voltage would not be changed. However, if you were to do the same with the CTS, the current/voltage relationship would be changed, and the ECU would respond differently. That's oversimplified, BTW.

[Zed Head]

One important fact here is that the potentiometer can only make the mixture richer. You can only add resistance to the CTS circuit. The only to reduce resistance would be to add a resistor in parallel and that would change the shape of the enrichment curve dramatically.

This thread started with the comment that "it still runs very rich". The actual parts being used haven't been fully described (in this thread, maybe in another). You said the injectors were replaced 500 miles ago. Did you replace them because of the rich-running or did the rich-running happen after replacement? Did the engine run okay before the work? What brand/model of injector are you using? Maybe someone gave you turbo injectors, which are of a higher rate.

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One important fact here is that the potentiometer can only make the mixture richer. You can only add resistance to the CTS circuit. The only to reduce resistance would be to add a resistor in parallel and that would change the shape of the enrichment curve dramatically.

This thread started with the comment that "it still runs very rich". The actual parts being used haven't been fully described (in this thread, maybe in another). You said the injectors were replaced 500 miles ago. Did you replace them because of the rich-running or did the rich-running happen after replacement? Did the engine run okay before the work? What brand/model of injector are you using? Maybe someone gave you turbo injectors, which are of a higher rate.

[mjr45]

Thanks Sarah, I'm thinking I may pull the AFM and try to calibrate it per Atlantic Z and see what happens. The plugs are very sooty and the exhaust has soot around the tip, but after it warms up there is no smoke from the exhaust, gas mileage is not great but not real bad either. Vacuum on this engine is at 14 but thats up here at 9000' so corrected for sea level is in the normal range, besides if there was a vacuum leak it should be running lean, not rich. I've adjusted the AFM spring by 8 teeth to lean side with only minor improvement, I suppose it could be running super lean, but the soot marks on my garage floor kinda are against that, they show up with every cold start up. I know that the mixture is richened up on start up but it seems way to rich. There is a new CSV, CTS, Thermotine, injectors, FPR, EFI connectors, fuel and vacuum lines, timing is at 11°. I've checked and adjusted the TPS and the fuel cutoff in the AFM. The car runs well, but kinda falls off at about 4200RPM. I think that about covers it, I really appreciate the help. Thanks.

Mike

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Edited February 4, 2014 by mjr45

[superlen]

Mike,

In the stock analog ECU there is a circuit that performs cranking enrichment as well as a small after start enrichment in addition to the external CSV. It's *possible* that this part of the ECU circuit has just drifted too far out of factory spec and causes your rich condition. You have already covered the most logical issues with the CSV system, so if it's still not perfect we need to look at the more esoteric possibilities. The AFM also could be a bit out as you noted as well.

Len

[Captain Obvious]

I've been through the START and AFTER START enrichment circuitry and I would be hard to convince that the problem lies there. That circuit's contribution decays to zero in 15-30 seconds after the key is released.

I would continue to concentrate efforts outside the ECU.

[mjr45]

I'm wondering if there is something in the thermotine circuit which may be contributing to the problem.

Edited February 5, 2014 by mjr45

[mjr45]

Zed, I don't recall the brand of injector, but I'm positive they were not higher flowing than stock per the vendor. I believe the engine was running rich prior to me starting the project, but I can't be absolutely positive since its been over 2 years ago and at the time I started I didn't really check. Since I've done the rebuild(everything except new rings and crank bearings) its run rich, however since I started the AFM adjustment it has improved slightly which again makes me think it has something to do with the AFM circuitry. I kinda thought that adding a pot to the CTS would enrich and not lean it out. As I've stated, it runs well, so maybe I should just leave it alone! Thanks.

Mike

[Captain Obvious]

Only link between rich running and the thermotime stuff is the cold start valve. But yes... Problems with the thermotime or cold start valve could certainly cause rich running problems.

I disconnected my cold start valve completely. Both fuel and electric connections. Only thing it's doing at this point is plugging the hole in the intake manifold, and I'll make a cap instead when I get a few spare minutes.

Can I notice a difference? Yes. When the engine is cold, it definitely cranks a second or two longer before it catches, and it sputters for a half second when it does catch.

Enough to be objectionable? No. I think the simplicity of getting rid of the thermotime, CSV, and all it's associated plumbing and wiring is a net gain.

Keep in mind, that I'm not driving my Z in the dead of winter, and I'm in the SE PA area, so if you're trying to start your Z on a very cold CO winter morning, then this may not be an option for you. You might NEED the extra fuel boost from the CSV just to get it to light off, so YMMV.

[mjr45]

Captain,

I don't drive it much in the winter, just an occasional start up and let it warm up, so I may disconnect the CSV and see what happens. Its a balmy -14 right now, so I won't be doing any work on it today, go figure..

[kacrow76]

Ok so i have been flowing this for some time. hopping you would find the trouble as my 76 is some what the same. new injectors new csv new plugs and wires ngk new.

mine starts right up when cold. but then is running very rich until it warms up it then is still rich i think just not as bad.the other day day i did find something interesting. right after start up when cold it i would remove the oil fill cap it would run better.after it was warm just the opposite. is there any thing to that?

[Zed Head]

You should start your own thread and put a link in it back to this thread. Otherwise you'll just make a mess of this one.