TEST 1: Fuel Trim Check
The very first thing you need to do and the most important, after pulling the diagnostic trouble codes, is to check the Fuel Trim values with your scan tool in Live Data mode.
The reason for this is to see if a continuous and unadjustable Rich Condition is truly present.
If the extreme Rich Condition that's causing the PCM to cut fuel to its maximum limit is present, you'll see the Long Term Fuel Trim values of one or both banks stuck somewhere around -20%.
Remember, an uncontrollable Rich Condition is the one that causes P0172 and/or P0175 DTCs to set.
OK, this is what you need to do:
Start the vehicle to get it warmed up (let it run for about 15 minutes) and connect your scan tool to the OBD II Diagnostic connector (Don't have a scan tool? Need a scan tool? Check out my recommendation: Actron CP9580 Scan Tool).
Once connected and powered up, get to Live Data mode and then scroll down to the PID that's labeled: LT FTRM1 and LT FTRM2
LT FTRM1 reads the Fuel Trim values for Bank 1 of the engine and LT FTRM2 reads the values for Bank 2 of the engine.
If the Rich Condition is present, that's causing the problem Fuel Trim Too Lean code, the LT FTRM1 and/or LT FTRM2 will be at -20%.
You may or may not see them both at -20% and this is OK. Why? Well, it all depends if one or both banks are being affected.
Let's take a look at your test results:
CASE 1: Your scan tool showed one or both LT FTRMs at -20% This tells you that there truly is a problem causing the PCM to cut as much fuel possible to that bank or those Banks.
CASE 2: Your scan tool showed one or both LT FTRMs at between 10% and -10% These are the normal values that a vehicle without any Fuel Trim problems should have.
This also means that whatever is setting these codes is failing intermittently and intermittent failures can not be diagnosed as long as the problem is not present.
Rich Condition Is Confirmed
OK, after confirming that you do have a bona-fide Rich Condition on one or both banks, you can start doing some specific tests to find out what's causing it.
What you're looking for is anything that would add fuel to the air/fuel mixture beyond the control of the PCM. The most common problem areas are:
Ignition System Issues
Verify that you do not have a misfire condition. A misfire condition will cause the air/fuel mixture to go Rich. This will have a direct and negative impact on the PCM's fine-tuning of the fuel injected into the engine.
The most common component failures are:
- Bad spark plugs.
- Bad spark plug wires.
- Distributor cap.
- You can find the ignition system Tests here: How To Test A Misfire / No Spark-No Start Condition (4.3L, 5.0L, 5.7L 96-04) (at: easyautodiagnostics.com).
Fuel System Issues
- Dirty or clogged fuel injector(s).
- To be more specific: If the fuel that is being sprayed into the cylinder is not atomized correctly... the fuel will not burn completely and this will cause a Rich Condition that will force the PCM to trim fuel to the Bank to which this injector belongs to.
- Leaking fuel injector.
- Leaking fuel pressure regulator.
- The ‘Spider’ fuel injector assembly used on these engines are infamous for leaking fuel pressure regulators. What usually happens is the rubber diaphragm (inside the fuel pressure regulator) tears, leaking fuel into the intake manifold and immediately causes the air/fuel mixture to go Rich.
- Failed pressure regulator causing excessive fuel pressure.
- You can find the ‘Spider’ fuel injector and fuel pressure regulator Tests here: How To Test The ‘Spider’ Fuel Injector Assembly (4.3L, 5.0L, 5.7L).
Cooling System Issues
- The thermostat is missing.
- Incorrect temperature thermostat is being used. In other words, you're using a lower temperature rated thermostat instead of the stock 195° F rated thermostat.
- Engine coolant temperature (ECT) sensor is not reporting the correct temperature. More specifically, it's reporting a cooler temperature than normal. The cooler the temperature reported, the more fuel the PCM dumps.