I have a 1999 f150 XLT 5.4 and thought I had a leaky gas cap. Replaced it and thats not whats wrong. I had the autoparts store scan the computer and it indicates both banks running lean. The motor runs perfect and no side effects are noticed. There are 4 O/2 sensors. (How would you determine which one?) The scan suggests other things like injectors, but it all runs perfect so thats why I stuck with the O/2 sensors. Are there any other thoughts on this. The vehicle has about 170k miles on it.

 

It's unlikely that both O2 sensors went bad at the same time. One place to start looking is the PCV system; a broken hose can cause a massive vacuum leak, which would cause that code.

 

standing at the front of the motor.

4 8
3 7
2 6
1 5
Front
Bank 1 is passenger side the front will be senor 1.
Bank 2 is driver side the front will be senor 1 again.

the front 2 senors tell the computer the A/F ratio and if they go bad short out or become lazy you will get a code and bad gas mileage.

the rear 2 are not as impornant they just tell the computer the Cats. are still working.

Do you have the codes???

Also lean could mean a vacuum leak or some leak past the MAF.

if you go to autozone the code on the scanner will say something like.

Bank 1 senor 1 lean.
bank 2 senor 1 lean.

im not sure on the codes thoe.

 

Sensors are fine, doing their job. You have a vacuum leak. The PCV elbow behind the intake manifold is the prime cause.

 

Yep, PCV system most likely when both banks show positive for lean. This is a 98, - the 99 is a little different, but this PCV line that connects in back of the Throttle body elbow or manifold is usually the culprit. - I believe the 99 connects to the elbow. Reach your hand back there while she's running and move that line around. It's probably get bad enough ,- you may be able to hear it leaking along with your idle deflecting while fumbling around with it.

The 98 -



BTW- Disconnect the battery before you fix the leaky hose. Reconnect when finished, - Positive first/ then neg.

 

Sorry everybody. There is more. When I first had it scanned at a shop and I didnt see any readout but he told me both banks were indicating lean. Today Autozone scanned and gave me a printout. It just says code po174 for bank 2 that says ECM has detected a rich or lean air/fuel ratio condition on engine bank 2. Then it lists several different causes.

The other printout says code # p0171 and then just repeats the above.

I will start checking hoses tomorrow. Already replaced the gascap.
Thanks for all your help .......

 

jbrew..... Thanks. Ive replaced the idle valve so i know just where your talking about.

 

The technical cause is too much unmetered air entering the intake. A bad gas cap will trip an evap code, not a lean code.

 

X2

check the PCV as stated and if its really bad it could be the intake at the throttle body but you said your truck runs fine so its prob. in the PCV or some place small like that.

i replaced my PCV with 2 little filters on the valve covers and just blocked off the vacuum holes on the intake. runs so much better and keeps the Throttle body from getting all nasty and gumed up i havnt had to clean it since i took the PCV's and the EGR out of the intake loop

 

Shifty85 ...... This sounds like a great mod. Do u have any pics of this?

 

humm let me look nashR i think i do.

on the left side by the oil cap. its the lil tiny blue filter *Yes i know the filter is nasty i just got done washin the engine bay after muddin*


Thought i had another shot you can buy these at any autoparts store i wanna say there 5-10 bucks each i forget. but yeah i put one on the passenger side. and i have one on the driver side but i left that hose on that went to the intake so the filter is sitting right by my coil bucket. and i just pluged the hole in the back of the Thottle body. ill snap some more pictures 2 morro.

 

On the circled part, what is this thing called? I'm having a hard time trying to look up this part.

 

PVC throttle body elbow. from rock auto.. it about $7.50
DORMAN Part # 46017 More Information About this Part {#F75Z6C324JA, F85Z6C324AA}

 

I'd rather clean the intake every 5 years than replace the engine when it fails from oil sludge. PCV is there for a reason.

 

Jethat got it. Ford might want to sell you the entire PCV system, which isn't a bad idea to replace. I think it's between $39 -$59. Here's a pick of just an elbow, - DON'T refer to this part # below for your model. Some use different parts.