JT-27

Truck is a 1997 F-150, 4.2 V6. I am starting to get stumped at this issue. First I replaced the plugs, then I replaced the IAC valve, and am now considering the MAF sensor or EGR valve next, but I am starting to wonder if it's just a vacuum leak. Before changing the IAC valve, my CEL was on, and the codes it threw were P0171 (Fuel trim bank one condition), P0174 (Fuel trim bank two condition), P0300 (Cylinder misfire, random cylinders), and P1506 (IAC). When the truck starts, it starts up instantly and idles great at 1400 RPM for a minute or two. Once warm, it adjusts down to 900 RPM and then it sounds like you are pumping the gas gently over and over, ever half second it's like its getting a small accelerator surge, obviously causing a bit of a rough idle.

Mike

Pcv valve and check the coil on the cylinder with a missfire

metropolis909

on the v6 i believe its one coil pack with wires... possibly bad coil pack... but im not familair with the v6 but idle control module would make it surge like u were pumping the gas. my truck does it now and then... im sure someone will fill in more than i can

JT-27

Thanks for the replies so far. The cylinder misfire was random, not one specific. I checked each coil individually (there is not a pack on this year, it's an individual coil per plug setup like the 8 cylinder has). I can try the PCV valve to see if it makes any difference, it's cheap

Crazy Cooter

If you are still having issues, all 4.2, 4.6, and 5.4 have a rubber air tube coming out of the back of the throttle body. They have a tendancy to get hot and melt, causing a hole to form which causes a massive air leak at the throttle body. This will make the engine run rough at idle like it has a cam, and will smooth out with throttle. As a Ford Master Tech, I have run into this issue several times at the dealership. It will also throw MAF codes and mulitple cylinder misfire codes. Hope this helps. By the way, the only way to get the correct replacement part is through the dealership.

ratatat

Did you get new plug wires?

JT-27

Thanks Cooter, I'll get back in there and see if I can find this tube. It sounds like it's going to be difficult to get to or see.

ratatat - I didn't change the wires, I may do that just to be safe considering the age of them now.

ratatat

Yea, I would double check all vaccum hoses and then get new plug wires. I think cooter may be talking about the infamous 90 degree air intake rubber elbow thingy. It's not hard to get to.

redman89

Got picks of this infamous rubber thingy? lol. My truck does exactly the same thing, but its a 4.6 V8. Whatever is wrong with it, I think it is also causing my gas mileage to suffer, or maybe it is a combination of things. I thought the high idle was just because of the cold, but it was mid 30's today and i figured it would idle normal but still did not. I have new plugs/wires/air filter. Mine makes a knocking noise for a bit after starting it up and driving it too. I assume its unrelated though.

redneckf1504x4

I am having the same problem with my truck 97 4x4 4.6. It only does it when I stop/ I have been noticing the same problem with quite a few trucks on here. I am about to change the idle arm hopefully that will correct the problem for me.