1996 LT auto 4x4 265000 km (165000 miles) Runs good, lots of power & fuel mileage has dropped. Has high tail pipe emissions barely passed inspection.
Scanning with Autoenginuity: DTC's P0171 & P0174
Replaced both pre cat O2 sensors with Denso's. Readings did not change much. Checked for gasket leaks and intake hose leaks many times with carb cleaner & propane. Disconnected power brake booster and accessory vacuum lines. Sure there is not any vacuum leaks.
At idle ST Trims bounce from negative to more positive.
At idle LT Trims B1 21.4 B2 21.1
Increasing idle slowly to 8-900 rpm, LT Trims drop suddenly to B1 11.0 and B2 9.80.
BARO, MAP, IAT, TPS, ECT & EGR live data look good.
MAF live data causes some questions:
Key On Engine Off 1.93 gm/sec
Idle 5.16 gm/sec
WOT 1st - 2nd shift 153gm/sec
Watching live data the MAF graphs smoothly from idle to full power and back. Pulling the PCV vent above throttle body drops to 3.1 gm/sec.
Should MAF read 'zero' with engine not running?
Full throttle 153 gm/sec seems low.
Autoenginuity does not have oscilloscope function so can't check MAF for clean signal or drop outs.
Misfire Live Data show low random misfires. These are the max misfires not all at the same time over couple minutes
Cyl 1: 0 Cyl 2: 3 Cyl 3:1 Cyl 4: 4 Cyl 5 :1 Cyl 6:0
Last full tune up approx. 25,000 miles ago. Do not know name brand of parts used.
If having mis fires would this not make O2 sensors show LT Trims rich?
Anyone have live data MAF stream they can post. The 1.93 gm/sec with engine stopped makes me suspicious.
Please chime in

 

just want to be sure..... are your fuel trims positive or negative?

 

Long Term trims are positive
Short Term trims switch rapidly approx. between -3.0 and +6.0
Unplugging MAF causes LT trims to drop B1 11.1 & B2 9.9.
Question: Are the unplugged MAF Trim values real or are they default MAP values?

 

Propane enrichment is an excellent way of locating vacuum leaks. However, the PCM reacts instantly and adjusts RPM & fuel trim if/when the engine injests propane, so you probably won't notice a change in RPM, O2 voltage, or fuel trim. The "work around" is to command the PCM into open loop with a capable scan tool. This will cause the PCM to ignore the O2 sensors. The IAC motor must be disabled, (unplugged) to prevent the PCM from making changes to RPM.

When the PCM commands a certain fuel mixture, it is immediately monitored by the O2 sensors. If the mixture is too rich, the PCM decreases injector pulse width to lean it out, (indicated by a negative trim number). Vice versa if the mixture is too lean, positive trim number. Long term fuel trim is used as an indicator of how much, and which direction, the PCM has to correct from its original command. Short term fuel trim is real time, live, data. Both O2 sensors must be supplying accurate data to the PCM for fuel trim data to be accurate. Engine RPM must be stable for an accurate short term reading. Ideal fuel trim is zero, but rarely happens Rule of thumb is that fuel trim should be ~5% or less.

"Question: Are the unplugged MAF Trim values real or are they default MAP values?"

They are live. Monitoring short term fuel trim will give you a more accurate indication of the change.

"MAF live data causes some questions:
Key On Engine Off 1.93 gm/sec
Idle 5.16 gm/sec
WOT 1st - 2nd shift 153gm/sec"

Over the years I have seen this several times. In theory yes, KOEO it should read zero. I've tried new MAF sensors, checked voltage and ground at the sensor, checked resistance on each wire from the MAF to the PCM, tried new PCM's, installed an additional ground at the sensor and PCM, none of which made a difference. If you find a cure, let me know The grams per second rate is fine. Possibly a tad high at idle, which may or may not explain your positive short term fuel trim.

I believe your Autoenginuity will access camshaft retard, might be a good idea to check it. Snap throttle above 2Krpm and back to idle: Spec is zero degrees (+ or - 2 degrees). Post the results of that. This could be what's causing the misfires.

 

Busy at work... Couldn't work on Blazer. Have not checked Cam Retard yet. Did manage to record 30 minutes or 1800 seconds of live data from cold start at work to home. This live data is with MAF DISCONNECTED. Have not calculated fuel mileage. Long term trims are much lower. Attached is a print screen (.doc) of the graph and the Autoenginuity file (.zip). if anyone has Autoenginuity. Have used Autoenguity extensively on 7.3 Powerstroke's, as this was one of the first PC programs to get into the proprietary systems of this engine and actuate its devices.

 

Autoenginuity showing Cam Retard is -8 from idle to 4000... Does not change except at engine start when it displays -7 momentarily.

Autoenginuity showing Idle spark advance is +22, increasing with rpm.

Have been reading that unburned fuel from bad spark or injector in most cases will NOT be detected by o2 sensors instead they detect the air portion of unburned mixture. This may cause ECM to believe engine is running lean, thereby adding fuel.
Cam Retard is adjusted by rotating the distributor? What is GM describing with this term. Is this the initial spark advance without the ECM adding advance?
Think next things to do:
Solve misfires. (ECM does not set DTC for misfires)
1. Set Cam Retard to 0.
2. New Plugs, Cap & Rotor.
3. Wires if necessary

 

The term "Camshaft retard" is misleading. A more accurate term would be camshaft position sensor alignment. When you turn the distributor to adjust camshaft retard, the camshaft position sensor also turns. The throttle must be snapped above 2K RPM and back to idle, after each adjustment, before an accurate reading is indicated on the scan tool. Turning the distributor does not change ignition timing.

"Have been reading that unburned fuel from bad spark or injector in most cases will NOT be detected by o2 sensors instead they detect the air portion of unburned mixture. This may cause ECM to believe engine is running lean, thereby adding fuel."

Unburned fuel in the exhaust, no matter what the cause, is reported as a rich mixture and the PCM will decrease injector pulse width to lean the mixture. If a sensor is lazy or dead, it will report a lean mixture and the PCM will increase injector pulse width to richen the mixture.

1. Set Cam Retard to 0.
2. New Plugs, Cap & Rotor.
3. Wires if necessary

1. Correct, your reading of -8 is substantially out of spec.
2. AC Delco 41-993 with a .060" gap, AC Delco cap & rotor.
3. AC Delco, Autolite professional series, Belden, (good quality stuff, no cheap junk).