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A buddy of mine has a 1997 Ford F150 with the 4.2L V6 engine. After you drive it for a couple of minutes then stop at a stop-sign or red-light, the idle will “surge” — like it’s trying to stall out and then revs up to recover, over and over. Sometimes it will actually stall… And the fuel-mileage has become VERY poor since this started.
Scanned it with a scanner, no OBD-II codes, no freeze-frame data, no nothing. Ran a “live data” log while driving it for about 10 minutes… It *seemed* like the long-term fuel-trim was a bit high — but that was near the start of the drive, so maybe that was just while it was warming up? After a short while all the fuel-trims started shifting back and forth, just like you’d expect. And the O2 sensors were all shifting back-and-forth quickly, just like you’d expect a narrow-band O2 sensor to do. Engine coolant-temp, MAF-sensor, TPS-sensor — all the stuff I was monitoring looked completely normal to me… Yet it will continuously “surge” and nearly stall out every time you come to a stop. (If you put it in Park, the surge disappears, btw.)
So, what could possibly be causing this? I have no idea where to even begin with no OBD-II codes.
Thanks for any help you can give,
Laurence MacNeill
Mableton, Georgia, USA
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Checking for vacuum leaks is a good place to start.
These links have some good info.http://www.ericthecarguy.com/faq/finding-and-fixing-the-causes-of-poor-mpg
http://www.ericthecarguy.com/faq/solving-automotive-performance-issues
What values do your long term and short term fuel trims take while the engine is dying? If the problem disappears when the vehicle is in park and your long term is high that suggests a vacuum leak somewhere in the transmission actuator lines. Try recreating the fault and pinching off a line.
+1 on checking for vacuum leaks. What you describe are classic symptoms. More information at those article College man posted for you.
I feel kind of silly now — vacuum leaks should’ve been foremost on my mind, but I guess I was thrown by no codes showing up. Kind of expected a misfire or lean-condition code with a vacuum-leak — so with no codes, I just automatically dismissed a vacuum leak and started thinking of more complicated things. I guess the lesson is, don’t make stuff more complicated than it needs to be! 🙂
Now if I can just get my friend to bring his truck back over here, maybe I can help him check for leaks.
Thanks for all the replies.
L.
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