Fixing a Sluggish, Rough-Idling F-150

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Ford F-150

Is your older F-150 not quite running like it used to? Here are some member tips to make that old engine hum like new again.

There’s no denying that the Ford F-150 has quite the reputation for longevity. This is especially true with modern powerplants designed to last hundreds of thousands of miles with nary more than a tuneup. But eventually, as they always do, parts will go bad or break. And oftentimes, it’s difficult to figure out exactly where a problem might be coming from. But that’s why places like the F-150 Online forums exist.

So, when forum member darknessmatters began experiencing some issues with his 1997 Ford F-150, that’s exactly where he went for help.

Ford F-150

“I just bought a 1997 Ford F-150 XLT 4.6L. I did an oil change today and the old oil smelled of gasoline. It’s been having idle issues where it tries to idle lower than 800 rpm and then surges to 1500+. It stumbles all over the place. I am missing an elbow on the throttle body to a vacuum line that I can see, plain as day.

Was told by the previous owner that the O2 sensors were bad because he had a code come on with the check engine light. I was going to replace the upstream two, but now that it’s having more issues, I might just hold off. This is my first Ford truck. Any advice would be appreciated. Hoping it’s something small, but I can’t expect much after paying 400 bucks for the truck with 220k on the odometer.”

Ford F-150

Right off the bat, Roadie has a simple theory as to what’s causing the problem.

“I think that line connects to the intake before the throttle body, doesn’t it? There is another line that connects to the PCV on the passenger valve cover. It connects below the throttle body. That is the one that causes vacuum leaks most of the time. If the one you are talking about is missing, there would be air getting into the throttle body that isn’t being measured by the MAF and could cause issues.”

“If you have a vacuum leak, I bet the codes are P0171 and P0174,” adds glc. “Your O2 sensors are fine if that’s the case, they are doing their job reporting a lean condition.”

Turns out, the missing elbow connects the air intake to the crankcase ventilation on the driver side valve cover. Which is clearly not helping the situation, according to glc.

“That looks like it’s before the throttle plate, so it’s not a vacuum leak. However, it’s letting unmetered air in after the MAF, so the system will be confused and it will throw lean codes. See if it runs better if you cap off that port on the side of the intake.”

Ford F-150

After getting his hands on a scanner, the OP gets not one, but five different codes: PO174, P1151, P1131, PO161, and PO136. A little bit of Googling tells glc exactly what the problem is.

“Looks like you have problems with all four O2 sensors. The P0174 may or may not be a legitimate lean code because of that open hose.

Clear all the codes and see which ones come back AFTER you fix your vacuum and air leaks and get the plugs and wires changed. NOTE that the sensors themselves may be fine, something associated such as wiring may be faulty.”

In addition to those minor issues, the OP also finds that two of the spark plugs are stuck. Luckily, a local shop is willing to fix the problem for a reasonable $400. So it would appear that yet another mystery problem has been solved, thanks to the good folks in the forums. If you’ve got any advice to add for the OP, head over here and chime in!

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