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Discussion Starter · #1 ·
Found another van and this one throws a P0171 along with a P0174. This means unmetered air or not enough fuel is getting to the engine. This one is also a '97.

Step 1: Look at your Freeze Frame. Here's the critical info // Load = 9.8% // ECT = 87C // Short Term Fuel Trim 1 = 15.6% // Long Term Fuel Trim 1 = 23.4% // Short Term Fuel Trim #2 = 17.2% // Long Term Fuel Trim #2 = 23.4% // MAP (kPa) = 67.0 // RPM = 1,327 // VSS = 65km/h // MAF (g/s) = 20.65 // TP = 9%

Step 2: Those Long term fuel trims can mean a vacuum leak. Visual inspection of vacuum hoses = OK.

Step 3: Ran a smoke test, No leaks

Step 4: Ran a fuel pressure test = Took about 5 key on cycles to build pressure for a startup.

Step 5: Replaced fuel Filter: Pressure builds normally (60 PSI) BUT it doesn't hold. Without starting the engine, the pressure slowly drops. Took about 3 minutes to dispose of some rags and checked fuel pressure = 45 PSI.

Inspection of gas cap = cracks on the seal. Something else in the fuel system is leaking. Will smoke test next time.
Drove the van for 2 miles (reset the PCM) . During the drive the long term fuel trims went from 0% and at the end they were already up to 18% Wow! The fuel pressure stayed a constant 50 PSI (Not sure if this is to spec.) Drives good. There's still plenty of items I haven't checked. (O2 Sensors, Purge Valve, exhaust system leak, the rest oft he fuel system). Let's solve this bugger! I bet AstroWill or Mmusicman would of enjoyed troubleshooting the van in real time.
 

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Discussion Starter · #2 ·
Happy Thanksgiving! Did some more diagnosing today. Smoke tested the engine through the check valve and PCV Hose. No Leaks. Smoke tested through the exhaust pipe....inconclusive test.

The Oxygen Sensors are quite suspicious. There's 3 upstream oxygen sensors sitting tight, 1 downstream o2 sitting tight. Here's a rough Diagram:

--------------
Front
-----------Bank 2 S1 ---Bank 1 S2----[CAT]----Bank 1 S3
of vehicle----------------------------------------
-------------- ------Bank1 S1

Here Bank1 S1 (sensor 1) is located on the driver's side (Right after the flange connected to the exhaust manifold). Bank 2 S1 is located on the passenger side after the flange connecting to the exhaust manifold. I confirmed these to be GM Genuine (AFS-105) [OEM part for model year]. The 2 oxygen sensors after that are unmarked.

Live scan tool data shows what I've learned as fuel trim "contribution."
Bank1S1 = 5.5%
Bank2S1 = 0%
Bank1S2 = 99%
Bank1S3 = 99%
What can we conclude from these numbers?
It means the front 2 oxygen sensors (Bank1 S1 and Bank 2 S1) are not contributing to the lean condition. These are also verified as OEM. B1S1 is telling the PCM to add 5% fuel, B2S1 is saying all is good OR it is still in (open loop) (Could be an issue...but it is reporting identical voltage to B1S1).

The rearmost sensors, one pre-cat (upstream) and one post-cat (downstream) are telling the PCM to add 99% more fuel. These are unmarked o2 sensors. Who knows if they are for this model year.

Do you see where we are headed with this info?
The sensors Bank2S1 and Bank1S2 are about 4 inches apart on the pipe....yet they are reporting a world of difference to fuel trims.

The fix? I'm suspecting the rearmost oxygen sensors are defective and sending incorrect info to the PCM OR they are incorrect for this model year. The '97 ONLY ACCEPTS 1996 or 1997 Oxygen Sensors to function properly. Either that or the CAT could be leaking and letting air in, the air would fool the sensors close to it as unmetered air.

Before I advise the owner of the fix, Id like a second opinion based on the info shown.
 

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Discussion Starter · #3 ·
Update: I need to go slap the sap out of the person that advised me about specific O2 fuel trim PIDs. Apparently these are meaningless and the whole 99% deal "should be ignored."
However, the 0% PID could mean that oxygen sensor is still operating in "open loop" meaning the heating element could be faulty. We went ahead and ordered 2 oxygen sensors will update in a week
 

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Discussion Starter · #4 ·
Smoke tested through the exhaust pipe....inconclusive test.
Update: I should have thoroughly smoke tested the exhaust system before ordering the oxygen sensors. I also did not monitor the oxygen sensor voltage for all 4 sensors on the scanner.

Today I thoroughly tested the exhaust system, entered from the oxygen sensor at the driver's side. Found a HUGE exhaust leak at the exhaust manifold/"Y pipe" flange. Looks like a new gasket is needed. Tested from a passenger side o2 sensor and all was sealed. Tested through rear again and found another leak at the cat/muffler flange. Yup, the first leak is the cause of our issues. The 3 flange bolts look to be in decent shape, I've enjoyed this process fellas!
 
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