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Old 01-03-2014, 04:11 PM   #28
ehunter
Senior Member
ehunter has a spectacular aura aboutehunter has a spectacular aura about
 
Join Date: Nov 2010
Location: Pacifica by San Francisco 5sp Engine rebuild with the help of people here at 177K due to 500mi/qt oil consumption. After 2yrs I am back to 40mpg+ after discovering the refirb head I used was bad. Car runs better than a top. Now at 214k.
Posts: 1,459

1996 SL2
Default Re: short term fuel trim

You can chase this issue for quite a long time and never find why your idle has the occasional hic-ups. My car used to do the same thing and I never quite pinpointed why. Possibilities are:
1) O2 sensor does not switch fast enough. I had a Bosch sensor that switched from high to low and back every 1.5 seconds. I can see how the inaccuracies of a slow sensor might periodically cause a hic-up. The Denso I have now switches much less than 1 second.
2) Vacuum leak causing the O2 sensor to periodically fuel starve the motor. The leak can be anywhere.
3) Fuel filter becoming clogged.

Note: the ST and LT information that has been posted up I see little value to. The numbers are just snapshots that change wildly and very quickly so you really cant deduce anything from them. Even the LT values can be inconsistent from starting and stopping the engine for a re-measurement. The LTs are relative measurements that may not be repeatable from one engine start time to another. The LTs are fuel delivery stair-step-ups or step-downs when the ST corrections fall out of min/max boundaries. But in general, the LTs should be fairly stable in a controlled driveway idle.

If you want to be able to utilize the ST/LT information in a more meaningful way, you will have to watch the behavior, data point by data point. (tenths of a second apart is best) Your goal is to validate the average of all STs is zero with a controlled idle in the driveway. Trying to do this while driving introduces too many moving influences.

Most likely your ST average is going to be negative (running rich). Not often is it too lean. Can you say what your fuel economy is at this point with new plugs and wires?

So in summary, make sure you have a good quality O2 (Denso/NGK) and start looking for vacuum leaks. Also you should be able to validate your old PCV runs with the same results to rule out the fixed orifice is not an issue.

The fixed PCV helps to adjust the crankcase pressure to either lower or higher pressures. If it brings the pressure down, less oil can get pushed out of the engine(leakage). If higher pressure is created, more leakage can occur.
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