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Discussion Starter · #1 ·
Ok,

I've had an issue with low MPG in my '95 saab 900s. If I drive more aggressive, my check engine light would come on. My MPG would drop to 15 MPG. Now Previously, I found a problem with the Vacuum hoses. No problem with the IAC. This car has 185K miles. I had a leaky vacuum hose and connectors that just broke off to the Valve cover vent. I replaced the plastic part hose and grommet. THat fixed the problem.

I had a rough idle at times. But once the vacuum leaks were fixed, MPG went straight up to 30MPG or better highway.

Then after the car sat a while, I had the check engine light come on and my MPG dropped to 14 - 18 MPG. I had a rough Idle.

I found oil leaking in the distributor cap. Replaced the expensive distributor to prevent the oil from leaking inside the cap and fouling spark.

Then, I found the vacuum hose to the SAI(Secondary Air Injector) was rotted. Replaced that. Found out that I think my Pump is not working. What benefit/loss is there to have the SAI pump working? My check valve is ok and not leaking.

Next, Usually I would drive a few miles and the problem would go away, but this time it didn't. So, Now I refer to the possibility of the O2 Sensor being the problem. I avoided this since in the Past I would fix the other small problems, Vacuum, Etc and after driving the car the problem would go away.

Now, I will admit, I did have a rough Idle at times and an oscillating Idle which many have indicated is the sign of the O2 Sensor weakening and going bad. But with the terrible gas mileage, the rough Idle(Hunting/oscillating) went away.

So Here's what I did. I checked the Air intake sensor by unplugging it while the engine was running, the car ran rough and almost stalled and I plugged it back in and figured that sensor is working fine. So then I wanted to see if I unplugged the O2 sensor would the engine performance change. So I unplugged it, checked the voltages and resistance and no affect to the engine, so I suspect it's the O2 Sensor. What should be the resistance readings on the O2 sensor? I have a 3 wire sensor and I suspect wire 1 is voltage supply the middle is the common and the third is the sensor, is this correct?

Now, I decided to leave the O2 sensor unplugged and immediately my MPG started to climb back up to 23-25 MPG!!!! This is not what I expected and was wondering if anyone here would know why? I thought that with the O2 sensor unplugged the MPG would stay low or drop.

Now you ask about the check engine light, My car has a problem. The electronics work and the check engine light works to tell you there is a problem, but there is a problem with the cable to the connector in that many mechanics have been unable to read the OBD II connector. So, I can't read my OBD status.

Does my car have a rear O2 sensor? I couldn't see it.

I didn't check the grounding of the O2 sensor to the exhaust.

One other variable is I did spray cleaner on the grounding lugs next to the intake manifold and wires.
 

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If I had to guess, I'd say that you've got a problem with the front (or only) O2 sensor.

With the sensor unplugged from the car, the check engine light will come on, but the ECU will maintain the engine in it's "Open Loop" mode, simply using pre-defined fueling maps based on manifold pressure, engine temp and throttle position.

If you plug the O2 sensor back in, the system is probably going into it's "Closed Loop" mode and using data from the sensor to manage the fuel.

If the O2 sensor is incorrectly reporting a lean condition, the ECU will over fuel the engine trying to get the O2 sensor to report a correct fuel mixture.

Probably time to replace that sensor and clear the codes.
 

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Discussion Starter · #5 ·
If I had to guess, I'd say that you've got a problem with the front (or only) O2 sensor.

With the sensor unplugged from the car, the check engine light will come on, but the ECU will maintain the engine in it's "Open Loop" mode, simply using pre-defined fueling maps based on manifold pressure, engine temp and throttle position.

If you plug the O2 sensor back in, the system is probably going into it's "Closed Loop" mode and using data from the sensor to manage the fuel.

If the O2 sensor is incorrectly reporting a lean condition, the ECU will over fuel the engine trying to get the O2 sensor to report a correct fuel mixture.

Probably time to replace that sensor and clear the codes.
That is a very clear explanation of "open loop" vs "closed loop". That explains why I now get much better gas mileage than before. Also it explains why I'm also not seeing the mid to high 20's on gas milage. I'm hovering around 22-24 at the moment.

Connecting and disconnecting the O2 sensor and then just driving around is a good way to test the O2 sensor and watch the gas mileage on the car without having to replace the O2 sensor outright. I didn't think about testing open loop versus closed loop driving and fuel economy/performance until now.

Now, if someone could explain in more detail the "Value" of the SAI (Secondary Air Injector). Does injecting air into the exhaust affect fuel economy? Since air is added to help unburnt fuel does that affect the O2 sensor readings? How do you know when the blower is working or not? When does the motor run? My check valves are working fine but I was trying to determine how this SAI circuit worked with the vacuum lines after replacing the rotted vacuum hose from the manifold to the switch.
 

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The purpose of the SAI pump is to inject air into the exhaust system when the car is cold since it speeds up the heating up of the cat and o2 sensors. They both work optimally when hot, since the o2 sensor readings aren't used until they heat up, the extra air from the SAI pump as no bearing on o2 sensor readings. SAI can also be used to better assist the cat.
 

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Discussion Starter · #7 ·
The purpose of the SAI pump is to inject air into the exhaust system when the car is cold since it speeds up the heating up of the cat and o2 sensors. They both work optimally when hot, since the o2 sensor readings aren't used until they heat up, the extra air from the SAI pump as no bearing on o2 sensor readings. SAI can also be used to better assist the cat.
So, are you saying that this pump only operates when the engine is cold and during warmup to operating temperature or does it also operate while we drive at normal operating temperature? I don't think my pump works, it's very expensive to replace and so what benefit does it really have? I don't have any check engine light to this. So I figured, if it doesn't affect performance, then why replace it?
 

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Discussion Starter · #8 ·
Got a 4 wire OEM O2 Sensor where my '95 SAAB 2.3 non turbo used a 3-wire in the cable even though the connector was 4 wire. The car just didn't used the 4h ground wire next to the heater connections. Had my mechanic jack the car up and replace it. He has a lot of experience with SAAB and said that the 4-wire connector to a 3-wire car doesn't matter and is a non-issue. It will still work fine.

Verdict, It did. No having to rewire anything. He also says if you order the correct Universal BOSCH oxygen sensor for your car, it should work fine. It just may take you 30 minutes or longer to prepare the universal connector to fit your car. BOSCH Guarantees these O2 Sensors and the mechanic hasn't seen any problems with them.

He had installed a Universal BOSCH in a Mercedes and then a Mercedes dealer wanted to charge thousands of dollars to replace the O2 Sensors as NON-OEM in a recall notice on the car. A Call from BOSCH Shut the dealer up and prevented them from charging to replace the Universal Sensors since they didn't throw a code.

Also, the mechanic says in my SAAB the Secondary Air Injector is a complete waste. Only operates on a cold start and shouldn't throw any codes if just the air blower motor is not functioning. But, some cars(VW) will throw a code if the blower motor doesn't work and will fail emissions.
 

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Discussion Starter · #9 ·
Ok,

My MPG is up to normal. But, my check engine light turns on after I"m driving a few minutes and usually during an acceleration. I turn the car off. When I turn it back on, the check engine light is off. Idle and all is good. I drive at slow speeds and all is good. But, after a few minutes of driving, no specific time, on an acceleration, the check engine light turns on. I don't want to have to go to the dealer to have them scan my 95 saab since these are the only persons I know who can scan it.

I am thinking it may be my Purge valve and during a purge valve test while driving, it fails. So, is there a way to test the purge valve? I ohmed it out at 20 ohms, so I would think that's ok. versus if it was an open connection. Any ideas with the car running? Can I pull the hose on the canister side and feel for a vacuum or when does that vacuum occur? I know the tank valve, if there is one, is closing because I had a pressure release when I opened the gas cap on the last refill.
 

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Discussion Starter · #10 ·
ok, purge valve is fine. i hooked a hose blew through it and its closed. then connected 12 volts to the lead you hear it click then air passes when you blow through it.

so i go to the saab dealer to ask about reading the codes. the dealer says they're too busy and i have to make an appointment 5 days from now. very rude. its too much trouble to pull a mechanic for 5 minutes of work.
 
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