I have a 2008 F-150 4.2 V6 w/manual transmission. I wanted a V8. Not offered. The 4.6 is a great motor and at only 4 more liters, why not! But anyway I settled for what they had because I like the style and the little extra room with access door behind the seat in the regular cab.
This V6 doesnt idle right. I can barely eek-out 20mpg on the highway. The idle is rough and changing speed. The dealer says that this is normal for this engine and that the device that they plug-in to the dash records 24mpg on the highway at 55mph. Well balogna is good with mustard too but overall it gets 19mpg. The V8 would probably get better. My GM 4.3 got 24mph highway. My Mercury Marquis dual exhaust 4.6 V8 gets 27mph highway. Runs/idles very smoothe. Wish that they would do a recall and give me a 4.6 V8 in place of this quirky V6 motor.
Anyone out there having funny idle with this 4.2 V6 motor and if so what the fix?

 

Go test drive a brand new 4.2 and see if it idles the same, when it idles better show the service manager that the new truck does not idle bad why does yours. If its inherant to all the 4.2s (which it isn't) then the new one should be doing it too. As for the fuel mileage, thats actually good for the v-6

 

Have an 02 4.2, don't think I ever seen 19MPG. Averaged 17.1 for the first 100K then quit logging. Have 182K now, been a great truck and engine. Agreed I am an easy driver and have absolutely nothing negative to say about the truck. New tires every 60K, new plugs at 95K, changed out DPFE sensor at 50K, 2 sets of front pads, backs went to 155K. Battery changed Jan 08, intakes seals and bolts with new green grommets last April. This truck is just a pile of good news.

 

My 01 4.2 had a very unstable idle that fluctuated alot it was a vac. leak somewhere, I do not remember where though that was a long time ago.

 

Hey dude, tell that service mgr. He's full of crap man. If it does'nt idle right, something is obviously wrong. He's just trying to get in your pocket bud. My '01 4.2 Had the same Problem, A new set of FELPRO upper AND lower intake gaskets. And a good look for any vacume line leaks fxed 'er right up. I was getting 13 mpg before the fix, now im up to 17mpg. And shes idles smooth, and runs like a top.

 

I also have an 08 w/ 4.2L V6 & manual transmission. Until the last 2 tanks, I was averaging 18-19 mpgs with a decent mix of highway and city driving. Pretty sure winter (letting it idle while I scrape the frost/snow/ice off), more city driving, and putting on the Flowmaster exhaust (getting on it a bit more to hear the rumble ) all led to the mpgs falling. I'm hopefully taking a small road trip this weekend, so we'll see how it does.

As far as the idle, I have noticed mine seeming a bit rough at times. Not constantly, but every now and then...I plan to take it over to the dealer and have it checked out, just haven't done it yet. I'll let you know if I find out anything.

 

My '08 4.2 manual tranny runs great. No rough idle. Something is amiss on your vehicle to run rough. If it's still under warranty, make them fix it. Might try another dealership for better service rep. Good luck, gary.

 

Hey, I get 20-22 mpg regularly in my 00 ext cab 4.2 with the 5 speed. I drive it easy and the only conversion is a cherry bomb in place of the regular muffler. The truck has 145XXX miles and runs great.

 

My 2002 is a 4.2 and it idles fine as long as it's running fine. Sounds like somethings a miss in yours.

As far as gas mileage goes...4.2's don't get great gas mileage. If you're getting close to 20 mpg then when they fix whatever is wrong then you'll get a tick more. 20 mpg isn't bad from that motor, I've never seen it in mine!

For the record...the first mistake you made was, you settled. Never do that in a purchase as large as a new automobile. 9 out of 10 times you'll never be happy with it in the end.