Idle mixture adjustment

[Johnny]

What is the procedure for adjusting the idle-mixture for an N9TEA-engine?

I know where the adjustment-screw is, but how do I know when the mixture is right?

[august]

What is the procedure for adjusting the idle-mixture for an N9TEA-engine?

I know where the adjustment-screw is, but how do I know when the mixture is right?

there should be a single female plug that's sticking out of the wiring harness near the air box (could be anywhere from the front of the airbox to the strut). using a dvom, probe that with one lead, and attach the other lead to a good ground. Run the car through 3 fan cycles. Using the adjustment screw on the AFM, adjust it so the voltage is between 7.0 and 7.4 volts (ideally 7.2)

let us know how it goes.

[Johnny]

That seems to do the trick, I got rid of the "misfiring", probably just uneven running.

The voltage changes alot, but I got it to stay pretty much in the range 7-7,4V.

I have a feeling that my lambda-probe isn't too healthy though.

I keep burning the fuse to the pre-heating, and when I disconnect one of the connectors to the probe, the fuse holds.

It could be that it's just the pre-heating of the probe that's broken.

So now I think it's just a matter of tweaking everything so I get a stable idle and mixture and a working aux-valve.

I look forward to it, I can't do it now because it's in the middle of the night here and we live in a pair-house where garage is really close to our neighbor's bedroom...

[august]

good to hear.

it's interesting hearing about the progress of your project. I look forward to you getting it on the road and running well.

[Johnny]

I look forward to getting it on the road too, just gotta wait for spring...

Well well, I ran into a problem today.

I got the idle pretty stable, it pretty much sits at 900 +-10 rpm now.

I did a little trick with the ignition, which solved the unstable rpm.

When I hook up the dvom to the test-cable (after 3 fan cycles) then the voltage is bounces around alot, but after a while it stabilises at about 7.5x V.

No matter how much I turn the screw on the afm, It doesn't move. or well, it does sometimes and falls to about 4-5V.

When I turn the screw the opposite way then it slowly rises again (propotional to the screw-movement) and when I get close to 7V, it jumps to 7.5x V and sits there, just as it did before.

Is the ECU running closed-loop on lambda or what?

[august]

hmm, i'm honestly not sure johnny. have you tried using another afm?

the voltage bounces around a bit, but usually only .10v or less (from my experience).

[Johnny]

I don't think it's the afm, I've looked inside it and the carbon-track looked pretty much fine, but I don't really know.

Seems unlikely though that an bad afm would cause the voltage to stabilise?

Often when the reading drops, the rpm drops and vice versa , so I guess they has to do with each other.

The question is, why is the rpm dropping?

I think it's because the ignition-timing "wanders" a bit at idle, when checking with a strobo-light.

I can also hear a knocking sound at idle which disappears when the rpm is raised.

I think it's the cam-chain that rattles, causing the timing to wander.

I've walked around the problem, by raising the timing when rpm sinks below 900, so I think that's why I got the idle pretty stable now.

But the only thing I can't get a grip on is why the voltage stabilises at 7.5x V....

The voltage is very stable before the engine is warm, and sits around 6.8V I think.

And when the voltage bounces, it's maybe 0.4V.

[Johnny]

I found the procedure described in a workshop cd:

Idle mixture & CO-level

Turbo:

1) Engine should be at normal operating temperature. Ensure

fan clutch is released, air filter is clean, and there are no air or

vacuum leaks in the air intake system.

2) Ensure throttle plate is not completely closed or jammed

in its bore. Ignition timing should be properly adjusted. Ensure A/C

and all electrical accessories are off.

3) Ensure idle speed is adjusted corrected. Disconnect wires

for 140°F (60°C) thermoswitch. See Fig. 8. Connect wires together.

Disconnect PCV hose at valve cover and plug.

Fig. 8.

4) Remove tamper-proof plug covering idle mixture screw,

located in airflow sensor. Drill center of plug with an 11/64" drill

bit to depth of 13/64". Screw sheet metal screw into plug. Remove plug

using pliers.

5) Ensure engine is warmed to operating temperature. Connect

voltmeter to test lead on right side fender skirt. See Fig. 4.

Fig. 4.

6) Voltage at test lead should be 7-7.4 volts. Also measure

CO level from tap in catalytic converter. CO level should be to 1.3

percent at proper idle speed.

7) Turn idle mixture screw in airflow meter to adjust CO

mixture. To lower voltage (richen mixture), turn screw clockwise. To

raise voltage (lean mixture), turn screw counterclockwise.

8) After adjusting idle mixture, reset idle speed. If voltage

changes, reset idle mixture. Reconnect PCV and thermoswitch wiring.

[Guest adegnes]

Great article Johnny!

[Johnny]

Thanks Adagnes!

I've done some general testing of the sensors, and I found that the NTC-sensor has a resistance of about 7.1kOhm @ 14degC measured directly on the sensor.

From what I can gather, the resistance for that sensor should be 2.5Kohm @ 20degC, so it should be a bit higher than that when it's colder than 20deg, but 7.1@14degC sounds a bit high considering it should be 9.6@-10degC.

The strange thing is when I measure the resistance at the ECU-connector, then the resistance is 3.6Kohm....

I also have two NTC's from an N9TE-engine and they have 3.3 and 3.5kohms when they are laying on the inlet, those sensors give ~2.5Kohms @ 20degC so they should be ok.

Are the NTC/resistance different for the N9TE and the N9TEA engines?

Are my test-results reasonable?