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CTFordTech, Ford Technician
Category: Ford
Satisfied Customers: 4976
Experience:  Ford Master School Certified, Kia Elite School Certified, ASE Certified, A/C 609 Certified
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98 Ford Expedition: P0171..Error codes..the MAF + oxy sensors

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98 Ford Expedition - P0171, P0174, P1131, and P1151 Error codes with a P0102. I just changed the MAF + oxy sensors and to no avail. What else?

Hello and Thank you for choosing Just Answer.


More than likely you have a vacuum leak somewhere on the motor causing all the codes you have and the rough running motor. Most common is the PCV hose becoming pinched and breaking or leaking intake manifold gaskets.

Customer: replied 7 years ago.


Replaced the PCV hose. Looked for vaccum leaks, found none. The garage said he saw a P0304 code in the history, but I do not get one now. All wires and plugs are new. I would think I would get a 304 code as it feels like 7 cyls, but no 304 code shows up. How else can I find vaccum leaks? I can spray gumout carb cleaner around the intake manifold. What about fuel injectors, can I remove and clean them?

Yes, spraying carb cleaner around the manifold would be a good way to check for a vacuum leak but a scan tool would be needed and the fuel trims will need to be watched to check for a change when spraying the intake. You can remove the injectors and clean the tips but the o-rings should be replaced if they are removed. It is very rare for them to go bad on this motor.
Customer: replied 7 years ago.

Are bad intake manafold gaskets common here?


Would a vaccum leak on #4 cyl from the intake gasket create a P0304?

Yes, intake gaskets are common to fail on the 4.6L or 5.4L motor.


Vacuum leaks would be rare to cause a specific cylinder misfire. A cylinder 4 misfire I would suspect a bad ignition coil as they are also very common to go bad on this motor.

Customer: replied 7 years ago.
.... but you are confident about a vaccum leak, vs injectors, other sensors etc...?
I am 99% positive you have a vacuum leak. Just be sure the oxygen sensors are are plugged in properly.
Customer: replied 7 years ago.


I will need to look further into this. Can you hold out till this weekend, as it's now 10:30 eastern and my wife's birthday tomorrow. I would like to keep the conversation going as I find new info. I do have a ACTRON scanner #CP9175, I believe i can monitor the fuel trims, but I'm not sure yet... Any other advice with the monitor I'm using would be real helpful.


Thanks and talk to you later...

Short term fuel trim really should not go over positive/negative 5%. also if you can watch the voltage from the downstream oxygen sensors, the voltage should be close to 0, if that voltage changes when spraying you found your leak
Customer: replied 7 years ago.

Just got a new code PO307. pooped out of nowhere. I had misfire on #7 which I corrected. Interesting enough I found water in the plug hole. Plug was covered with water. I'm scratching my head on that one. When I corrected the misfire and erased all codes, they do not show up anymore. So far so good. Doesn't look like a vaccum leak as mentioned. However water in the plug hole - would that point ot a manifold crack or gasket problem? So far it is running great...


With the PO304 the garage said it saw in the history, that goes back to last year's problem. Would that still stay in the PCM memory that long?

Yes, the water or coolant in the coil hole is a sign of either a cracked manifold or blown intake gasket which could also cause the lean codes you had in the memory. A p0304 code can stay in the memory until it is erased.
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