[gb3347]

I've been battling the seemingly common loss of power in hot ambient temps (especially after car has been parked on short errands) since I bought the car 5 years ago. Better fuel and a do88 intercooler helped a bit but still haven't eliminated the problem. So I've been logging some OBDII data with Torque Pro and the one thing that jumped out at me was my O2 readings:


After car is warmed up, at ~900 rpm idle, the downstream O2 sensor (labeled Bank 1 Sensor 2) shows a perfectly consistent voltage. However, when I hold engine revs at 2000-2500 rpm, it briefly drops off every 5-7 seconds from about 0.7V to 0.1V. It doesn't fluctuate evenly, it can go even 10-12 seconds holding a steady 0.7V before again registering 0.1V for 1 second. Is this normal? And if not, does it point toward a bad sensor? Plugged cat? No CEL ever by the way and no symptoms aside from the loss of power when hot.


Thanks in advance for any help.

 

[FC Saab]

Downstream O2 is only reporting the efficiency of the catalyst and its ability to hold oxygen. Your catalyst is performing normally.

A failed catalyst will have O2 readings that directly mirror the response of the front O2.

 

[FC Saab]

The periodic dropping from .7v to .1v is an increase in oxygen in the exhaust. Whether it's an ignition misfire, fuel misfire, pirate air from a vacuum leak. It could be lots of things.

A lack of power and poor fuel economy would probably be a vacuum leak.

Look at your Long Term Fuel Trim. What is it at a warm idle? If its bigger than +10 you have a vacuum leak somewhere. If its -10 you probably have bad injectors.

 

[FC Saab]

One more thought. If you're having trouble starting and poor performance after its warm, I'd question the injectors. Take a resistance reading on each injector when cold, then again when Hot. Sometimes old injector's will act up when Hot and either leak bad because they stick open or the magnetic field weakens and the pintel never opens. The long term and short term fuel trims should tell all. Add the short term to the long term to get current trims. It's probably different at a cold start than a warm idle, though the Long Term doesnt care about temperature, it cares about rpm, throttle position and engine load.

 

[gb3347]

Looking through my logs, the LTFT is always between 1.5 and 2.5.


The STFT seems to be fine at idle hovering between 0 and 3, and it will go up to 6 at steady speeds. However, I noticed that on harder accelerations (above 50% throttle) it will hold somewhere between 6 and 10 which seems high.

 

[FC Saab]

Your fuel trims are normal. You can only really rely on fuel trim numbers at idle. The rest of the trims are written in the adaption table and are based on how you normally drive.

When trims get to be over plus or minus 10 there is usually a problem.