Codes 174, 301 & 306 | Ford Explorer Forums - Serious Explorations

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Codes 174, 301 & 306


February 6, 2010
Reaction score
City, State
Denver, CO
Year, Model & Trim Level
97 Explorer EB
My 1997 Ford Explorer, SOHC V6 has been posting the same codes for months. I clear the 174 (System to Lean Bank 2), 301 (Cyl 1 Misfire) and 306 (Cyl 6 Misfire) and they return within hours. I have replaced some parts but to no avail... all spark plugs, all plug wires and the ignition coil. So it's not electrical I believe and now I feel that the misfires are related to the air/fuel mixture or a vacuum issue. I have sprayed Brake Cleaner all over the top of the engine with no noticeable change in the idling. The engine starts fine but the idle varies and my gas mileage has gone down a few mpg. So sluggish idle and worse mpg are the issues. I have researched this a bit and have a list of suspects... PCV Valve, Vacuum hoses, MAF Sensor, Intake Gaskets/o-rings, EGR, Fuel Injector....
Any one ever seen these 3 codes consistently come up? In summary, I have a misfire on Bank 1 (Cyl 1) and a misfire on Bank 2 (Cyl 6) with a Bank 2 System to Lean being detected by the O2 sensor (which I replaced on Bank 2 on the engine side of the cat converter with no improvement. Chasing a ghost? THanks.

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lean condition

I agree that engine misfire codes on both banks with a lean on the driver side typically are due to air/fuel mixture problems. Both banks may be lean but the limits have only been reached on Bank 2. That could be confirmed if you can read the long term fuel trims. Have you measured your fuel pressure? Your fuel filter could be clogged, your fuel pump could be weak or your fuel pressure regulator diaphragm could be ruptured. Have you measured the compression? Your camshaft timing chain may have slipped. Have you added fuel injector cleaner to your fuel tank? Your fuel injectors may be clogged. Have you checked the hoses to the VIS components? This list of intake manifold vacuum connections may help: Intake manifold vacuum lines It doesn't show the VIS vacuum regulator and vacuum motor which is only on the 1997 & 1998 models.

Thanks 2000StreetRod. The fuel filter was replaced this year so I will focus on the fuel pump pressure and regulator diaphragm along with the vacuum components starting at the MAF and working to the intake manifold - appreciate the link to the photos. As I go through a set of checks I will post results. Hopefully I can avoid pulling apart the top of the engine. This is a good time to clean the top.

Cracked Insulator on Cyl 6 Spark Plug

I must have cracked the insulator when I put in the new plugs months ago. I replaced the plug and the codes are gone and the engine is running like a top.

misfire codes

I must have cracked the insulator when I put in the new plugs months ago. I replaced the plug and the codes are gone and the engine is running like a top.

I'm surprised that a bad plug in cylinder #6 would result in misfires for both cylinders #1 and #6 . I'm glad that your engine is running good and thanks for posting the solution.

I agree. I first pulled the #6 plug and found the insulator cracked on it. I have deep plug wells, so I'm sure there was sparking to the engine block...I should have run my car in a dark garage. After I replaced the #6 plug, the engine ran like a top. I went ahead and replace #1 plug anyways since I bought two new plugs. The original (4) are Motorcraft Platinum and the replacements (2) were Autolite Double Platinum.

It has been 3 weeks and no more CEL codes!
I did learn a lot about the intake manifold and all the vacuum users. THanks 200StreetRod!