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Discussion Starter #1
After my mileage dropped from 21 city to 14, I decided it was time to tackle the list of Check Engine codes. There were four, so I changed the fuel filter, upstream 02 sensor and DPFE sensor. I also found that the PVC hose had become disconnected down below, so I hooked it back up.

I cleared the codes and drove Monday through Thursday night with NO check engine light and a smooth running engine at idle. It was a great feeling. But then, at a stoplight, the engine suddenly started running rough and the CEL came back on. The P0171 was back.

What would cause this sudden change after three days of smooth running? I suspect that I need to change the PCV hose, since it looked pretty rough, but if that was the cause, why would it run smoothly for a while?
 

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Your question is aimed at God once you begin to suspect chaosian theory like that. Why do the cars even break at all if they were bought running fine?

See how that works?
 

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Discussion Starter #4
I replaced the PCV tube, but the rough idle has only gotten worse. It surges at idle and dies if I don't push the gas pedal. What next? Idle air bypass valve?
 

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The PCV rubber hoses are well known for bubblegumming up from fuel exposure to then suck a hole to screw the idle up, then it can reseal to run fine. Chased one for months before I finally found it. Do NOT use the hose if the rubber has lost its' body.

Do NOT buy a new IAC unless the problem continues with new rubber hose. Most IACs get bought for no reason at all, they are another trick buzzword part of the day like ATX solenoids that commonly fix nothing. IACs don't trip lean codes either unless they are vacuum enhanced ones.
 

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Discussion Starter #6
I replaced the PCV hose with a new one last week. I don't think the old one had collapsed, but it was getting gummy on the inside and didn't fit the valve cover very tightly any longer.
 

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I see no engine type or year of car, if this is early zetec then there are TWO pieces of hose.
 

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2004 Focus Wagon, Zetec DOHC, Auto
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didn't fit the valve cover very tightly any longer.
The hose on the valve cover on the Zetec DOHC goes over to the air filter box. It does not fail very often, and even if it did, it would not cause idle issues or throw codes.
As AMC said, there are two rubber hoses for the PCV valve. One at the front of the engine that goes to a metal tube under the ignition coil. Then a second rubber hose from that tube around the back of the engine under the throttle body. That is the one that fails most often.
 

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Discussion Starter #11
The rear PCV hose was completely collapsed, even when the car wasn't running. I replaced the PCV valve, elbow and rear hose. Presto! Engine running smoothly, CEL off. Thanks everyone.
 
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