Hi there, new guy here. Is this where I post a problem and hopefully someone will offer a solution? Here goes:

I have a 2006 F150 4.2 litre running rough under load. No codes. My LTFT is hanging around 10-13% at idle. To the best if my knowledge that suggests a vacuum leak. I have replaced both O2 sensors and both upper and lower intake manifold gaskets. Anyone have any ideas?

 

Welcome! Nothing wrong with posting it here but I think you may get more help in the V6 forum, so I am taking the liberty to move it. If you don't get what you need, send me a PM and I will move it back.

 

Thanks Bluejay, can't seem to find the V6 forum but I'll keep looking. Forgot to mention that those numbers are for Bank 1 only. Bank 2 seems okay.

 

Do you have an exhaust leak?

 

Nope, no exhaust leak.

 

I've checked vacuum hoses from rear of the upper intake (plenum) to fuel rail regulator and EGR/BARO/MAP sensor as well. Compression is good. I'm really stumped.

 

Something inexpensive to try - put a bottle of Techron in the gas. A dirty/clogged fuel injector can cause issues.

If you had a vacuum leak, you would probably have lean codes.

How many miles on your spark plugs?

 

Plugs are brand new (Autolite) as well as wires. I switched injectors from Bank 1 to Bank 2 and vice versa, didn't change the numbers.

 

Incidentally I did have lean codes before I relaxed the intake manifold gaskets.

 

WRONG plugs. Replace them with Motorcrafts. What brand wires did you use?

 

glc is absolutely correct. Use only Motorcrafts plugs in these engines and make sure you triple check the gap on them them, and adjust as necessary. This particular engine for some reason runs much better on the Motorcrafts versus other brand plugs.

I even recommend Motorcraft wires too.

As far as the long term fuel trim being above +10%, as your rpm increases you could be having a large amount of unmetered air in your engine, thus creating a leaned out condition, upping the number.

If your long term fuel trim is still above hovering at +10% at low RPM, like idle, but decreases as engine revs increase, a smaller vacuum leak could be the issue.

Now how does the spread between your LTFT and your STFT look? Do they even out? If you have a +15% LTFT and a -15% STFT, you have a 15 point spread, which still isn't good. Meaning that your computer is trying to compensate for a MAF or a MAP issue.

A LTFT of 4% and a STFT of 2% would give a total trim of 6% which is within the acceptable values and is ideal. Notice there isn't a huge spread between the values.

 

GLC is correct. I had a 97 F150 with the 4.2 and installed Autolite plugs. That poor engine ran like 3 plug wires were disconnected. After installing new MC plugs it ran smooth as silk. The 4.2 is very finicky about spark plugs, other than MC brand.
Have you tried the old mechanics trick? Spray carb cleaner around where you suspect any leaks.

 

Yep, and they aren't that expensive if you mail order them - about 40 bucks from rockauto.com. Duracraps at Autozone are $30 and BWD's at O'Reilly are $40.

 

MC's eh? Well this problem 2006 is a coworkers truck. My own truck is a 2004 4.2 with the same plugs and it works great. But I'll try the MC's, can't hurt. Maybe I have a couple of bad ones. I did try the spray, no difference. I'm getting 20 in Hg at idle on the plenum port just behind the throttle body. I just spoke to a friend of mine that tells me sometimes the valves in the intake manifold get gummed up and don't close properly so I'll try cleaning those up as well. Thanks for your tips guys, I'll keep you posted.

 

If the IMRC's are sticky, you should get a CEL with codes.

I would STILL try the Techron in the gas.