I own a 1999 Chevrolet Suburban and recently started receiving OBD2 diagnostic codes P0172 (Rich Fuel Mix - Bank 2) and P0300 (Random/Multiple Cylinder Misfire). Previously, I had only seen a P0305 code. I replaced the spark plugs, ignition wires, rotor, distributor cap, and coil pack. After these replacements, the vehicle now shows both P0172 and P0300. What should I check next to resolve this issue?
What steps would you recommend to fix these codes?
I've already checked for vacuum leaks and confirmed I used AC Delco spark plugs. After driving approximately 60 miles, I inspected the plugs again — they are still wet and black, which suggests a fuel delivery issue.
Do I have fuel injectors in my 1999 Suburban?
A 1999 Chevrolet Suburban typically has a CSFI (Cylinder Sequential Fuel Injection) system — does your vehicle use this configuration? If not, what type of fuel delivery system do you have?
I was mistaken — I don't have a fuel rail in my model.
Perform both a fuel pressure test and a fuel pressure leak-down test. The wet, black spark plugs strongly suggest a leaking fuel injector or regulator component.
Does the throttle body contain fuel injectors? I have a 1999 Suburban LT — does this configuration affect how the codes are interpreted?
If your vehicle is stock, it likely has a CFI (Cylinder Fuel Injection) system — one of two 'spider injection' configurations. Identifying your engine series is key because component locations vary by model.
I replaced the spider injectors and the issue is now resolved. The codes have cleared, and the check engine light is off. Thank you all for the helpful insights!
No idea — I'm sorry I couldn't provide more help.
Did you install AC Delco spark plugs? This is important. P0172 indicates a rich fuel mixture — the engine is receiving too much fuel. Check for spark at all cylinders, especially on Bank 1 (driver's side). What did the old spark plugs look like? Wet, dry, black, or tan? Perform a fuel pressure leak-down test to check for leaks in the fuel system — including injectors, fuel lines, and regulator. Note: As a precaution, inspect engine oil for signs of gasoline contamination.