← Back to list

1998 Chevy C2500 2WD 5.7L Engine P0174 Code: Rough Idle on Highway with Black Soot on O2 Sensors and Plugs

Model: 1998 Chevy 2500 2wd Fault Code: P0174 Posted: 2014-06-26 16:22

I own a 1998 Chevrolet C2500 2WD with a 5.7L V8 engine. I've been consistently receiving the P0174 diagnostic trouble code, indicating a lean condition in Bank 2 of the engine. I’ve already performed extensive troubleshooting: - Conducted thorough vacuum leak checks — no leaks detected. - Replaced both oxygen sensors (O2) twice. - Installed new fuel filters and air filter. - Cleaned the Mass Air Flow (MAF) sensor. - Replaced engine temperature, coolant temperature, and crankshaft position sensors. After these repairs, the truck ran smoothly for about two months under normal local driving conditions. However, when driven on the highway for 10+ miles, it begins to run roughly at idle upon exiting. Recently, I removed both Bank 2 O2 sensors and found they were heavily coated with black soot — a sign of excessive fuel burning due to an incorrect air-fuel mixture. My assumption is that the ECU detects a lean condition and responds by injecting more fuel, which leads to unburned fuel being deposited on the sensor surfaces. The truck performs fine in town but fails under highway conditions. This behavior has persisted for over two years with no resolution from mechanics or the Chevrolet dealership. I suspect the catalytic converter (CAT) might be interfering — perhaps it's not heating up properly and skewing O2 readings? I'm seeking confirmation on this theory. Any insights into potential root causes, especially related to fuel delivery, intake system leaks, or sensor calibration would be greatly appreciated. I’m open to testing fuel pressure, vacuum levels, and live data using a scanner.

Related fault codes

P0174

Comments (7)

Anonymous 2014-06-26 18:36

Perform a fuel pressure test and a fuel pressure leak-down test. Check the spark plugs — look for black, wet, or dry conditions. A backpressure tester (a tool that screws into an O2 sensor bung) can help assess exhaust system function. If available, try borrowing one. Also, use your scanner to monitor live data such as fuel trims and engine parameters.

Anonymous 2014-06-27 05:31

Thanks for the tip! I do have a diagnostic scanner, but it wasn’t in the truck during the initial observation. I’ll now replicate the highway-to-idle scenario, capture live data, and post it here for review. The Bank 2 spark plugs are all dry and covered in black soot — same as the O2 sensors. Should I test fuel pressure at the throttle body?

Anonymous 2014-06-27 08:47

Is this a CSFI (Continuous Fuel Injection) system? There’s a fuel gauge attachment point on the top rear of the intake manifold. Since Bank 2 is showing black soot, that may be the source issue — also consider checking the Positive Crankcase Ventilation (PCV) system.

Anonymous 2014-06-27 09:19

I’ve already replaced the PCV valve along with all other components. The engine uses a non-multiport throttle body, which is part of the first-year CSFI setup. What should the fuel pressure be at idle and under light throttle?

Anonymous 2014-06-27 11:28

Yes, this is a CSFI system — all injectors and the fuel pressure regulator are located behind the intake manifold and not easily accessible. A key diagnostic step: measure fuel pressure with the engine running at idle. Also test when the key is on but the engine is off — note how quickly pressure drops after the pump shuts down (this indicates fuel system integrity). After replacing the PCV, check for any signs of oil contamination — though this is unlikely, it’s easy to overlook. You have a scanner — what does B2S1 look like? Is it fixed or fluctuating? What are the Long-Term Fuel Trims (LTFT)? At 2000 RPM, how do the Short-Term Fuel Trims (STFT) appear? What is the Engine Coolant Temperature (ECT)? Try this: a vacuum gauge can help — but if you have MAP readings, record them at 1500 RPM and hold steady. Watch for any drop in pressure, which may indicate intake leaks or system issues.

Anonymous 2014-06-27 12:13

Sent you an email with additional data.

Anonymous 2014-07-15 15:57

It’s been a while — I was away with family. MAF readings: Idle = 0.93, 1800 RPM = 2.82 LT RM Bank 1: Idle = 3.9, 1800 = 3.1; LT RM Bank 2: Idle = 2.3, 1800 = 3.1 ST FT Bank 1: Idle = -1.6, 1800 = 1.6; ST FT Bank 2: Idle = -0.8, 1800 = 0.08 ECT: 193–197°F (constant) B2S2 shows variable readings. When the P0174 code is active and the engine is hot, B2S1 fuel trim consistently reads 50%. New observation: Yesterday I noticed a sweet smell from the exhaust — no white smoke. I checked coolant level and it was low by about 3 inches. After refilling, the smell returned today. Coolant level dropped again to 3 inches with no visible leaks. I haven’t done a pressure test yet. I believe these engines are prone to intake manifold gasket failures around coolant passages — is this accurate? If so, could that be the root cause of my issue?