I've just intalled a new O2 sensor in my car hoping it'll give me better mileage and I noticed this morning when I first took the car for a ride that the vacuum is different.

I've a permanently mounted vacuum gauge inside the car and it usually shows around 19-20 inches of vacuum at idle but the needle would always fluctuate slightly.

This morning I noticed that at idle, I'm getting what appears to be 21 inches of vacuum and the needle in the gauge is rock steady.

So, is it possible that a new oxygen sensor could affect the engine in this way, or is something else going on?

 

Unlikely but possible. Remember on cold startup your car is NOT looking at the O2- it's just reading off the fuel maps and sending it to the injectors.

What's learned during closed-loop only lasts until you turn off the engine and remove the key. Many people think what it learns in closed loop is "saved" from start to start. Spend some time with a scan tool and you'll see it isn't. They all get reset to 128 (baseline) every time you shut down and restart.

I'm sure it saves SOMETHING from start to start but fuel trim (block learns) is not one of those things. It's a "clean sheet of paper" on every restart. It could be one of these other "somethings" that is carrying over and helping improve cold start idle slightly. You'd need a smarter guy with ECMs than me to tell you for sure.

Regardless. SOMETHING is working better. Higher idle vacuum is always a good thing. Means everything is working jsut that much more efficiently and closer to optimum.

 

True, I'd forgotten about the car being in open loop on cold. Still, the car shows the same conditions when it's warm also. Seems like a happy camper. If the car is happy, I am too

My car isn't fuel injected, but I get what you mean anyway. The better flow of vacuum so far is staying consistent with the change of the sensor. I thought it also unlikely, but it seemed too coincidental to all of a sudden have better vacuum and not have the needle bouncing around anymore when I changed the sensor.

As you say, something is working better.

Thanks for replying.