failed emissions test in GA - 1994 Loyale

[solakian]

It took 5 visits to the emissions test to get a passing grade on my 1994 Loyale. Every failure was due to high HC at idle. The test is not done on the dyno. I replaced the injector, spark plugs, PCV valve, and O2 sensor. tested the throttle pos switch, MAF sensor, EGR valve and solenoid, carbon cannistor and solenoid, checked for vacuum leaks, ran a compression test, adjusted the idle speed and ignition timing to spec, and checked for diagnostic codes. Everything was ok.

 

What finally got the car to pass was to turn the idle stop screw in until the throttle switch (part of the throttle position sensor) opened. I noticed that when the throttle was properly adjusted and idleing at 700 rpm with the idle swith closed, the O2 sensor was barely active. When the idle was about 1200 with the idle switch open the 02 sensor was very active.

 

At 1200 rpm idle, HC was 161ppm. At 700 rpm idle, HC was 320 to 420 ppm. In GA, max HC at idle is 220, max speed at idle is 1250.

 

As an added bonus, the throttle lag that this car has always had off the line and between shifts is gone with the higher idle setting.

 

How does the engine control stratey change when the idle switch closes?

 

The HC at 2500 rpm was 35 to 40ppm throughout all 5 tests.

[naru]

 

How does the engine control stratey change when the idle switch closes?

 

The HC at 2500 rpm was 35 to 40ppm throughout all 5 tests.

 

Richer.

Possibibly an ignition timing change too.Not sure.

[MilesFox]

have you looked at the coolant temp sensor?

[solakian]

I replaced the 2 wire coolant temp sensor (input to the ECM) about 6 months ago due to physical damage. I replaced the one wire coolant temp sensor (gauge sensor) about 1 month ago after a radiator hose blew, along with a new 190 tstat.

[WJM]

There's no telling what kind of logic there is behind the idle switch. Even on the newer cars there's nothing thats 100% positive what all it does. However, when its not working, the ECU doesnt like it and it drives badly.

 

Your converter is probably not up to par either.

[solakian]

I suspect that when the idle switch is closed, the ECM puts the car in open loop. It only has a one wire O2 sensor and the potential of the sensor cooling down at idle is high. If I'm correct, the ECM would then use the MAF, TPS, and Coolant temp to determine the correct mixture. If my assumption is correct, preventing the idle switch from closing, as I did, keeps the engine in closed loop. It runs much better and the test shows that it's cleaner too.

 

I own the factory manual but it does not go into specifics about what happens when the idle switch closes. I was hoping to find a knowledgeable, Subaru trained, performance / emission tech who could offer a solid explanation.

[WJM]

I was hoping to find a knowledgeable, Subaru trained, performance / emission tech who could offer a solid explanation.

 

SUBARU doesnt tell us too much about it. Just that it must be working and what data points to look at when its working or not working.

 

Pretty much the same about everything else.

 

If SUBARU would go into more technical details about how things ACTUALLY worked when they trained the techs then we wouldnt look like idiots when something like this comes into the dealer.

 

Sadly, if one wants to know, one must dig deeper into aftermarket documentation and play with a vehicle on your own time in order to find out how the cars really tick.

 

There's a lot of aftermarket resources that actually explain things better than subaru ever wants us "factory trained" techs to know. But, be careful about what you read, some of it is plain wrong.

 

 

From what I have discovered about "idle switch"

 

When "on":

1. Smooths the transition from open throttle to fully closed throttle (in conjunction with the neutral position and clutch switch on vehicles so equipped)

2. Engagement of idle logic control.

3. Fueling and Timing changes occur (unknown exacting details)

4. Transmission logic changes (unknown exacting details)

and others

 

When "off":

1. Smooths the transmission from closed throttle to open throttle (in conjunction with the neutral position and clutch switch on vehicles so equipped)

and others

 

Under both conditions the vehicle should be in closed loop.

However, when on throttle, there are several condition levels which determine open or closed loop. Idle is almost always closed loop MAF+TPS control.

When O2 sensor readings are stabilized, MAF+TPS+O2 closed loop occur as long as the conditions are met.

Single and double wire O2 sensors may or may not be active because they have to be at operating temp. Idle conditions may not be enough temperature to have it at operational temp.

 

With heated O2 sensors, the O2 sensor is part of the closed loop control at idle once it is up to temp.

Edited March 20, 2011 by WJM

[solakian]

Here's my latest theory. The TPS is out of adjustment. When the throttle is closed, the air flow is so low that the MAF isn't as good a measure of air flow as the TPS, hence the idle switch (it tells the ECM to set fuel delivery by TPS, ignoring the MAF). The Subaru manual describes setting the TPS according to resistance at position, and to check the idle switch by inserting feeler gauges between the stop screw and throttle lever. Problem is..... I can't find a procedure to correctly set the throttle stop screw (the one that sets the physical stop to position the butterfly in the throttle body).

 

The idle speed adjustment procedure adjusts the IAS screw that is mounted just below the air horn on near the IAC solenoid. But again, no mention of the throttle stop screw adjustment.

 

Can anyone provide a procedure to adjust the throttle stop screw?

 

If the stop screw is wrong and the TPS is adjusted the mixture will be wrong at idle which may lead to a failed emissions test and a lag as the ECM transistions from idle to accelerate (off-idle). My car has both symptoms. By adjusting the idle stop screw in, until the idle switch was open, I belive I caused the ECM to depend entirely upon the MAF... and... I am keeping the O2 sensor hot enough to stay very active. It elimitated the symptoms but it's not right and the engine idles between 1150 and 1300 rpm.

[WJM]

There is plenty of airflow by the MAF to calculate the fuel required. The MAF scaling is VERY close together at lower air flow levels.

 

SUBARU tells their techs that if the throttle plate stop screw is adjusted, replace the throttle body assembly.

 

It is to be set at the factory and once it is adjusted, there is no going back.

 

Seriously. That's what they tell us.

 

 

Realistically, what you need to do is find another engine that has not been adjusted and take some measurements and make yours the same.

[ruparts]

hi,

the throttle switch ( position sensor ) is adjustable separate from the throttle plate stop set screw , which should NOT be moved from its orig setting , in fact they seal that setting with a paint like marker on the threads.

[naru]

 

How does the engine control stratey change when the idle switch closes?

 

From a Porsche site.Yours will be similar if not identical.

 

"At idle, the idle switch is closed, from just off idle it is open. This tells the ECU that the throttle is closed, so that the ECU can change the mixture for best idle, retard the ignition timing by about 10 deg, and cut the fuel flow off at above 1200 rpm.

The Wide Open Throttle (WOT) switch is open until close to full throttle, at which point it closes. This tells the ECU that the throttle is open, so that it can richen the mixture for best power. If this switch fails, the ECU will compensate anyway at very high air flow rates, but not at low air flow rates."

 

Resetting the throttle stop screw is no big deal.

You want the throttle plate just barely open.

This is merely to prevent binding w/wear.

[solakian]

I understand that Subaru states that the throttle body must be replaced if the stop screw is adjusted. I am hoping that someone has had some success adjusting the screw despite Subaru's recomendation. I bought the car with 200,000 miles on it and the paint on the screw was long gone. I believe the screw had been adjusted prior to my purchase.

[naru]

Subaru is full of shyte.

 

Lots of peple(including me) have adjusted it.

Not rocket science.

[WJM]

Subaru is full of shyte.

 

Lots of peple(including me) have adjusted it.

Not rocket science.

 

Exactly.