Hey guys, I have recently acquired a 1999 F-150 4.2L auto tranny and I did the plugs and wires etc..

I am averaging 15.2 MPG which is ok by my standards but I keep seeing 17-19 and even 21 mpg posts from some people. I do mostly highway driving and 15 seems a little low for that. My dad, (who never replaced anything unless it broke or caused problems) has original o2 sensors in there. Would replacing both up and downstream sensors have a drastic affect on mpg? I have no issue dropping 80 bucks on a set if they pay off in the long run. The plugs have already. There is no CEL and just a slight hesitation on accel. That is why I am asking the question. Do you guys think that's it?

Also... To assist you guys here's what happens-
It makes a sort of rattley noise when cold (disappears when warm) and also when cold or warm it seems to hesitate for just a second then launch forward. (not smooth) it does this more so now than before as it was an intermittent thing.

The truck has 158,000km or just shy of 100,000km. Pretty low miles for a 99

Sorry if this makes no sense or you need more details. I am exhausted as hell from studying this week..

Regards,
Phil

 

Dont replace the down streams EVER unless they give a code. The upstreams DO affect performance along with several other things go to rock auto and buy the Motorcraft ones. At Rock Auto there only 40 bucks.

 

Ok thanks! Yeah I saw those but the shipping to Canada is always 30+ dollars... Usually ship to the boarder

Would that he a cause for rattling though? Lean or something?

 

Is the Check Engine light on?

 

Just re read no CEL so from what I read the first question is have you replaced the fuel filter lately? if not start with that..

 

The rattling may be due to the wrong oil and filter. At 100k miles it's due for spark plugs, and after 13 years it may need new plug wires. A thorough intake cleaning and a new fuel filter sure wouldn't hurt.

 

Ok. I will put a motocraft filter in next time. You think that would cause it? I have 5w20 in there for the winter time and use 5w30 in the summer. -- I replaced the oil 3000km ago so I will be replacing it sometime in december. (Along with a seafoam job in the crank case) The fuel filter was done then too -- I just put in new plugs (NGK) I know now that I should have put in motocraft ones but it runs fine. The rattle was there even with the motocraft ones so i know its not the new plugs. Also new bosch plug wires.

Thanks for all your guys help!

Edit- I will try to take a video of the sound to better explain what is happening.

 

NOT a smart move.

Also not ideal. I'd replace both the plugs and wires with Motorcraft.

If you have a Fram filter on there, get it out of there. I use 5W20 year round.

Rattling could be an exhaust heat shield or a cat/muffler starting to come apart internally. It can also be a loose engine lifting hook. If a cat is starting to clog that will kill your mileage.

 

Ok, thanks for the tips. My next set will be solely motocraft. I did manage to found out what it was... The IAC was very dirty and after a nice cleaning she runs like new! Thanks for the help guys. (I used sea foam deep creep if anyone wants to know)

 

most guys use throttle body cleaner on the IAC and WD-40 to lube and clean the IAC. I alway just replace mine when its dirty..

 

That's what happens when a spot weld comes apart at the converters heat shield. Yea, it gets worse alright. Soon it will rattle after operating temp as well..