I've got that problem for a while now but i don't know where i should start searching. When my car is warmed up it allways starts running a bit lean when it's idling. The problem is i can't adjust the idle mixture. I don't know why but it seems like the ecu is ignoring the fact that the engine is warmed up which means it also ignores the idle mixture resistor.

I guess my problem is a thermo sensor or a thermo switch but which one would it be? By the way the idle is set up correct and the car is idling at 750rpm and i also checked the tps. I even changed the tps for a know good one. I also searched for vacuum leaks. The engine got a new self made harness but i also had that problem with the old harness so i think that can't be the cause. What could my problem be?

 

If it's an S4, the "water temp switch" on the bottom of the radiator is used to determine whether to run a 'cold' or 'hot' fuel map. It should have continuity above 15-19 deg C. Open below that.
Usually when the switch goes bad, the car runs richer all the time.

S5's did not have the switch.

How lean are you running? Bad enough to cause problems or is it just something you noticed?

 

You also have two air intake temp sensors, one in the AFM and one in the Dynamic Chamber. You can test these at the ECU and compare to spec. Pin 2L is for the Dynamic Chamber and it should read 1 to 2 volts fully warmed ( 176 degrees Farenheit/ 20 degrees Celsius). The one in the AFM should read 2 to 3 volts at 68 degrees farenheit/ 20 degrees Celsius and it's found at pin 2J.

 

how do you know its lean? with the air pump hooked up and working properly, an O2 sensor should read lean.

 

I know it's running lean because the car doesn't have the air pump anymore and i'm using a wideband O2-sensor. The car is a S4 Turbo II. When it's warmed up it starts running at AFR 14-15 which makes it missfire a lot in idle.

 

I tested all the sensors today. They all seem to be ok.

I found out today that the ecu is trying to run the engine by using the O2-sensor. It's trying to idle at AFR 14.7. Might that be a grounding issue? I set the tps up right but maybe a bad ground connection is messing with the tps signal when the engine is running.

 

Today i wired an extra ground to the tps. It didn't really help.
My car still isn't reacting if i try to adjust the idle by using the variable resistor.

So far i checked the resistor (goes up and down from 0 to 5kOhm), the connection to the resistor (+5V, ground, signal to ecu), the tps (set correctly and grounded very good), the water temperature switch (it's connecting the two wires as soon as the radiator gets warm) and the intake air temperature sensor at the intake manifold (the signal wire is seeing round about +4V).

That +4V signal from the intake air temperature sensor seems to be a bit high but that could be because of the cold air. Round about -5°C (23°F) outside today. I'm going to change the water temperature sensor next week. It's still the 24 year old original sensor and a new one only costs 10€ for me so i don't even care if it's broken.

But what the hell could my mysterious problem be?

 

The Air Intake Sensor will read about 4 volts w/key to on when cold as mine does the exact same thing. The FSM spec indicates what it is supposed to be when the engine is fully warmed, which should be 1 to 2 volts.

The Variable Resistor will only have a minor affect on idle and again, the engine needs to be fully warmed and the FSM suggests jumpering the Initial Set Coupler.

And what does "I found out today that the ecu is trying to run the engine by using the O2-sensor. " mean with respect to the "trying to run the engine" part. Have you used the Green diagnostic connector w/four wires near the leading coil using the LED light to test when the Oxygen Sensor is blinking or not (closed loop/open loop)? Look at post #10.

https://www.rx7club.com/2nd-generati...ast-2k-856697/

 

The engine was warmed up when i tried to set the idle and the initial set coupler was jumpered. I know it doesn't make a huge difference but i should see the engine speed going up and down at least 100 rpm and i should see the AFR going up and down a bit but nothing happens. When i got the car it was still working but after fixing so many things last year it stopped working and i really don't know why.

I didn't know i could see the oxygen sensor working. I'm going to try that tomorrow.
But it really looks like the ecu is using it when the engine is idling because my wideband sensor shows that the AFR is jumping up and down between 14.2 and 14.9. Much too lean if you ask me.

Oh and i don't know if that means anything but as soon as my electric cooling fan comes on the engine runs even leaner.

 

The Resistor should range between 1 to 4 volts. Have you tried taking the reading at pin 2F of the ECU to make sure the ECU is receiving the proper signal?

 

Not yet. i'm going to test that tomorrow.

 

fixed.

 

Going out on a limb here, but here goes:
Doesn't the ecu try to idle at 14.7 @ idle or low load?

If the ecu is using the O2 sensor, it's expecting to be reading exhaust + fresh air from the air pump. Using made up numbers, let's say 13:1 AFR exhaust and a little pure air mixes to make 14.7 at the O2 sensor. Engine idles happy at 13:1.

Now remove the air pump and the ecu will try to lean out the mixture until it reads 14.7. Engine is now unhappy idling at 14.7. Maybe try unplugging the O2 sensor or adding in a pentiometer to get it to read/run correctly?

If that were really the issue, I'd expect to happen to a lot of people that remove the air pump, so maybe it's not a complete fix for you.

 

Sounds like it's at least worth a try.

 

Today i found out something interesting which might cause my problem. I had to drive with lights on because of a snow storm today the first time after i got the problem. I recognized that if i stop and let the car idle all the lights get darker.Even the turn signal relay starts working with a strange frequency. I think the voltage might drop low enough to cause the ecu to missread the sensors.

This only happens at idle speed and if i set the idle higher (1500rpm) my problem is gone.

 

check the voltage at the alternator (duh), and the battery. if the alternator is charging @13.6-13.9 volts but the battery is seeing less than that, there is a connection between the battery + wire and the alternator that gets funky.

i had that problem on my T2, new alternator, new battery, low charging voltage....

its funny all the car should need to idle is the RPM being under 1098 and the throttle being closed

 

Didn't do anything in the last months because it simply was too cold outside. Now the temperature is getting higher again and I can go on searching for the problem.

Today i checked the battery and the alternator. looks like they are ok. I get 14,3V from the alternator when the engine is idling but that stupid engine still isn't reacting if I try to adjust the idle mixture. I could try to set the idle lower because the engine is running at round about 1k rpm at idle. Could that change anything? That problem is getting on my nerves...

 

the idle "map" only goes to 1024rpm or so, over that and its out of the idle region.

 

I found my problem today. It was a broken signal wire from the variable resistor to the ecu right at the ecu connector. I needed about 5 months to find it.

 

Good deal!

 

lol, i would never have thought of that...

 

Ok, I have this EXACT same issue on my S5 1991 Turbo II, runs like a CHAMP before it warms up and soon as it hits closed loop. BAM missfire like you said.... S5's dont have that variable resister do they?