Have 1996 Blazer LT 4x4 with 263,000 km (163,000) miles. Runs great fuel mileage has dropped over the last year or so. Long Term fuel trims are very lean at idle; B1S1 +21.74 & B2S1 +17.68. Even under fairly heavy acceleration they only drop to about +8.00 - +10.00.
Have checked for manifold & throttle vacuum leaks, power brake booster and accessory vacuum lines. Also checked hose and connecting plastic for cracks & leaks found none..
O2 sensors will peak close to 1 volt if I enrich fuel mixture adding carb cleaner. But the switching pattern looks rough & slow.
So am thinking the O2 sensors are going out?
The MAF looks OK, smooth change in-line with increased or decreased throttle(also cleaned it)
What do you guys think?

 

To answer the question in your thread title - AC/Delco.

 

I believe Bosch has made them longer than AC Delco, that's what I use and I haven't had a problem yet

 

AC Delco & Denso both have noticeably faster response and switching time than Bosch. The faster they switch, the better the fuel mileage. Bosch is a good third choice though

 

Thanks for the AC Delco / Denso advice. Will use them.
Am I on the right track believing the O2 sensors are providing a false lean signals to ECM, causing rich condition?
If engine had bad pressure regulator or faulty spider the long term trims would go negative??
Attached is last emission test, you can see its running rich not enough to fail but very close.

 

Wouldn't hurt to check fuel pressure and leakdown first. Low fuel pressure will cause a lean condition and the ECM will try to compensate for it by increasing injector pulse width. You are correct.... positive numbers indicate what percentage the ECM is adding fuel trying to satisfy the oxygen sensors. Your print out indicates a rich mixture, but the O2 sensors aren't recognizing it. The ECM is almost at the maximum correction of 25%. How many miles are on the fuel pump and oxygen sensors?

 

Learned my lesson the hard way. Put Airtex pump 3 yrs ago . The check valve failed in 2 years or so, but pressure was OK to start. Then the pressure eventually was not good enough to start.
Spent $425.00 plus tax and bought AC Delco pump and regulator last fall. Should have bought AC Delco in the 1st place. The first pump lasted 13 years. Its ironic that the Delco has 1 year warranty and the imitations have 2 or 3 years. Joke been that the Delco will probably run for years...
Back topic at hand.
KOEO pressure 67 lbs.
KOER 58 lbs.
Leak down 30 lbs after 10 minutes.
Leak down is not great but don't think its a problem yet. At least the spider or regulator are not causing this rich condition, i hope!

 

According to your print out, the engine is running rich, but the ECM is compensating for a lean condition, that's what the positive fuel trims mean. "Lazy" or "dead" oxygen sensors will cause this to happen. That's why fuel mileage takes a dump when the sensors get old.

Fuel pressure at 67psi is a tad high. 66psi is maximum allowable, usually it's around 62psi or 63psi, the regulator is probably the cause. This usually tends to make the fuel mixture rich and have negative fuel trims. The leakdown is a problem, it must remain above 55psi for at least 10 minutes after the pump shuts off. You might want to check the maximum pump output pressure and leakdown at the fuel filter. All fuel pressure and flow must end at the pressure tester. This is commonly referred to as "dead head pressure". It must be 73psi to 108psi while the pump is running, and it must remain above 55psi for at least 10 minutes after the pump shuts off.

The KOER, (key on engine running) readings are meaningless. GM doesn't publish any "engine running" fuel pressure specs for this engine. There are far too many variables involved that affect running pressure: engine load, engine condition, barometric pressure, ambient temperature, elevation ASL, etc, etc, etc.