P0171B + P1131 To everyone, especially Mr.Shorty

[andruxa]

I have a 97 4.0 SOHC with 110,000 miles on it. For last couple of years I have been having rough idle at different times (cold and hot engine), standing on the intersections, etc. Nothing that I was concerned about, since it only transferred a minor ammount of vibration to the steering wheel and only at idle (which was stable, btw).

It has been getting progressively worse for last couple of months, and I am now down im mpg (considerably), CEL is on. Pulled the CM codes last night (P0171B "Bank 1 System Too Lean" and P1131 "Mfr Ctrolled Fuel and Air Meetering").

MAF is the one that comes to my mind first - I have been cleaning it once a year, last time was in september. IAC was cleaned in september as well. I have never replaced my oxygen sensors, and before buying 4 (or even 2 for one side) I would like to see if anyone here has any thoughts on this code "combo" and what may be causing it.

Vacuum leak (headgaskets replaced) was fixed under warranty couple years ago. No sensors have been replaced in the truck as far as I know.

Does anyone have any thoughts where to begin?

 

[Runnin'OnEmpty]

Andruxa, my book says code P1131 is "Lack of upstream O2 switch, sensor indicates lean, bank 1."

That one effectively confirms the first code. I don't think it's caused by a faulty MAF. The lack of switching makes me suspect a bad O2 sensor (S1B1). You could also have another intake leak with the bank 1 gaskets.....

 

[andruxa]

Thanks for a prompt reply!
Could interchanging the o2 sensors (left and right side) be a logical next step? Btw, are you referring to upper or lower sensor? Are they even different on top and bottom?

Btw, which side is Bank1? I probably could test for vacuum leaks, but not sure how to do it.

 

[andruxa]

bump

 

[Runnin'OnEmpty]

The codes are referring to the first sensor after the exhaust manifold, on bank 1, which is the passenger side. If it were mine, I'd just replace the sensor if it's been in a while, rather than go to the trouble to change them from side to side. (They're about $50) The sensors are the same, but the wiring harness' may be a different length....? If you do decide to switch sides with the sensors, the code(s) will move to bank 2, IF the sensor is bad.

It may be easier to access the S1B1 sensor if you remove the passenger side wheel well.

That being said, you can also test the sensor with a multimeter if you wish. Just depends on how much time you want to spend on diagnostics. A search will turn up many threads on testing O2s.

Hope this helps some,
ROE

 

[Glacier991]

Ah the value of a scanner.

 

[ExplorerDMB]

Ah the value of a scanner.

Yup, today I had a f250 that had a "slow response" o2 sensors, and I was able to hook the Modis up, go through a few menus and bam -- 4 scope with all the sensors on it doing their thing. #1 sensor on both banks were slow as could be. Replaced both - done - MIL gone!

-Drew

 

[Runnin'OnEmpty]

Yep, scanners are great. Especially on the problems where there's no MIL nor a code set.

 

[andruxa]

Runnin'OnEmpty, followed your advice and placed a bid on fleabay for the one with longer harness (New Ford/Mazda Bosch 15718 O2 Oxygen Sensor (Sub 15717)). Up here in Canada they are over 100.00
Will replace that S1B1 o2 sensor, but meanwhile,

How can I check the engine for vacuum leaks?

 

[Glacier991]

Ahhh vacuum leaks. Common symptoms for those without CEL codes: engine idle rough and seem unstable with the idle speed constantly varying up and down. Common cause of Codes like PO0171. These pesky leaks can play havoc on engine performance and significantly affect their driveability. Whether the engine is carbureted or fuel injected, it's imperative to make sure that all incoming air passes through the throttle body ( or carburetor on one old enough.) A vacuum leak allows unmetered air into the engine and often causes problems, especially true on computer controlled engines that are measuring the intake air through the throttle body.

The best place to start checking for a vacuum leak is at the intake manifold. Begin looking at all of the ported vacuum sources around the throttle body (or carburetor.) Make sure they are all either connected or plugged with the proper fittings. Some vehicles feature a distribution block at the back of the engine near the firewall (octopus as it is often called) that should also be checked. Don't forget the vacuum line for the power brakes. If you think the leak is coming from the intake manifold gasket, try spraying it with something like WD-40. This penetrant will seep into the leak area, which will momentarily increase idle speed. Another method is to use an unlit propane torch around suspect leak areas, it will draw the propane fuel into the engine and you will hear a difference in idle speed. You can also check engine vacuum to confirm a leak exists, though this is not a foolproof method if the leaks are small and localized. Stock engines typically idle with 16 to 18 inches of manifold vacuum in Neutral and do not have significant changes in the needle reading, it should be steady. Of course this check does nothing to help you find the area of the leak.

That's a good start. If you had access to an ultrasonic leak detector that is about the easiest way of all to check - but it is not foolproof, and also few folks have those or will loan them out. Finally, probably the best way to check is with a smoke machine. Again those are not usually available to a DIY'er. Vacu-Tec makes one that I have been trying to get my hands on on E-bay but they always quickly get bid up out of my price range. Maybe someday (sigh).

So, back to WD-40 (or "fogging oil" with a nozzle on the spray button like the wd-40 can has is even better as it seals leaks even better as it is a little more viscous.) A WARNING!!! Anytime you are spraying a flammable substance near a hot manifiold, I RECOMMEND YOU KEEP A FIRE EXTINGUISHER HANDY. Very remote chance, but still.... better safe than sorry.

 

[andruxa]

Glacier991, thats an impressive post, thanks!

 

[andruxa]

Bump
Does anyone have thoughts on the trouble codes in the first post?

 

[andruxa]

Bump for Mr.Shorty

 

[andruxa]

Bump one more time...

 

[spindlecone]

As a compleate longshot ck the following, don't know if your SOHC is the same as the 01/03s, but the PO171 code is common and alot of the time means a vac leak.
One area that is notorious for cracking is the Rubber elbow as it enters the front of the Plenum Pass side (see Pic) the larger of the 3 hoses, hose comes from the PCV valve, back of valve cover drivers side.
It normally cracks on the bottom or inside edge where it is hard to see, remove and inspect it, maybe its worth a try, like I said is a common problem with that code and the newer SOHC V6s