Bad O2 Sensor Performance Loss

[Fullthrottle7]

Hey, guys, I've been looking at these cars for about 2 years now and was finally able to purchase one, a 1995 Talon TSi AWD, last Monday. I got it from a guy that had just gotten it rebuilt, and the engine has about 1000 miles since that rebuild. I don't exactly know what killed the engine, because the car is in excellent shape and shows no signs of abuse. The man I bought it from got the car with the blown engine, paid for the rebuild, and drove it 400 miles before deciding that it wasn't the car for him. Anyways, the check engine light is on due to a bad o2 sensor. The car runs smoothly when first started, idling right at 1,000 rpm. After driving for a few minutes, the car will change its idle to 1,500 rpms, which I'm assuming is due to the o2 sensor. I've been trying to take it easy on the car due to the rebuild and sensor, but I finally couldn't take it any more and got on it from a stop until about 95 today. The car felt fast, but not near the claimed 6.4 0-60 that I've read everywhere. I would like to know how much a bad o2 sensor will affect performance. I might just think the car is slower than it is because I hadn't really been in a turbo car before and it accelerates far differently than a n/a car. If anyone could try to explain the performance loss from an o2 sensor, it would be greatly appreciated, because if there is more wrong than just that, I'll need to get it into the shop sooner than planned.

 

[96TSi_Chris]

Well is it your upstream or downstream o2. Upstream is in the manifold and is controlling your fuel trims. Downstream is just basically telling the car if you have a functioning cat. Downstream is going to affect gas mileage more then anything, whereas the upstream will affect gas mileage, but its also affecting your fuel trims.

Once the engine is up to operating temp, the o2 gets a voltage of the air being brought in and sends a signal to the ECU. The ECU transfers these voltages and decides whether to add fuel. So in short, yes a faulty o2 sensor can lead to a loss in performance.

 

[leakyfaucet]

The computer doesn't look at the o2 sensor signal at WOT, so no, it won't affect performance. At least not at WOT.

 

[Fullthrottle7]

All right, so if I'm flooring it, there will be no change in performance? And as for the front or rear sensor, I don't know for sure, I was just told that the o2 sensor was the reason for the CEL. I'm kind of worried now, it doesn't sound like the sensor really does much to performance. Is my idle inconsistency due to another problem most likely, or would an upstream o2 cause the 500 rpm jump? If it helps, after driving the car somewhat hard for a while (10 minutes maybe), the car will drop to 1000 rpm when the clutch is pressed in or the car is out of gear for coasting and then slowly climbs to 1500 and then drops again to 1000. This happens until I slow down to about 5 mph before stopping. Does that problem sound like something an intake o2 sensor could cause?

 

[96TSi_Chris]

All right, so if I'm flooring it, there will be no change in performance? And as for the front or rear sensor, I don't know for sure, I was just told that the o2 sensor was the reason for the CEL. I'm kind of worried now, it doesn't sound like the sensor really does much to performance. Is my idle inconsistency due to another problem most likely, or would an upstream o2 cause the 500 rpm jump? If it helps, after driving the car somewhat hard for a while (10 minutes maybe), the car will drop to 1000 rpm when the clutch is pressed in or the car is out of gear for coasting and then slowly climbs to 1500 and then drops again to 1000. This happens until I slow down to about 5 mph before stopping. Does that problem sound like something an intake o2 sensor could cause?

That could be a number of things, ranging from a boost leak (which might be a factor in your performance) your ISC, your FIAV, dirty throttle body.

If you search, there are quite a few threads on each of these and how to built a leak tester to find any boost leaks. As far as the ISC, depending on how many orginal miles are on the car, it could be going bad. 2gs ISCs usually go out from 80k and up. An easy way to check it is to pull the connector. The car should die. If the idle stays the exact same, take a resistance check of the connector. Like i said, there are tons of threads if you wanna take some time and just do a few searchs

 

[oldman]

I don't know if bad o2 would cause it or not but I do think you should check your ECU for leaks. You can still save it by changing the caps and clean up the leak. I waited too long and the whole thing fried. Did you change the plugs and wire? Boost leak? If you just got the car, you should do all these maintenace anyway. Don't beat on it until you clean it up. Check out my best friend www.vfaq.com. Congrates on the car.

 

[96TSi_Chris]

I don't know if bad o2 would cause it or not but I do think you should check your ECU for leaks. You can still save it by changing the caps and clean up the leak. I waited too long and the whole thing fried. Did you change the plugs and wire? Boost leak? If you just got the car, you should do all these maintenace anyway. Don't beat on it until you clean it up. Check out my best friend www.vfaq.com. Congrates on the car.

Its a 2g, the ECUs rarely go bad. But i did forget to mention vfaq. That site is a MUST read.

 

[oldman]

Its a 2g, the ECUs rarely go bad. But i did forget to mention vfaq. That site is a MUST read.

Damn you 2g's

 

[Defiant]

The man I bought it from got the car with the blown engine, paid for the rebuild, and drove it 400 miles before deciding that it wasn't the car for him.

That sounds fishy as hell.

 

[leakyfaucet]

That sounds fishy as hell.

Yeah, best case scenario is that he had been having tons of problems, blew an engine, and knew he couldn't get anything for a DSM without an engine, so he fixed the engine and proceeded to sell it before something else broke.