Hi everyone just recently purchased a 04 str with 173k on it. ( yea i know not the smartest buy but i needed a vehicle and im in it about $4500 so far) Figured it had a vacuum leak and would be an easy fix. Boy was i wrong....

Codes P0171/P0174.

First I started with smoke testing from a couple different locations of the vacuum system to make sure i was not missing anything. Also watched fuel trims as i sprayed brake cleaner over the common leak points with no change.

Replaced maf with no difference

So driving around town it will throw a restricted performance notification and cel. When i get on the freeway and start to cruise over 45-50 the restricted performance goes away and cel remains

Fuel Trims at idle (630rpms)
STFT 25%
LTFT 19.5%

@ 1500rpms
STFT 8%
LTFT 19.5%

@ 2500rpms
STFT -7%
LTFT 19.5%

@ 3500rpms
STFT -10
LTFT 19.5%

Fuel Pressure at idle 53-57
All of those readings are in park but similar while under load


few things to note.
Has a reman throttle body on it and the connecter on the driver side is somewhat melted. i dont think it is melted to the point where it is causing issues but i could not remove it.
The transmission is slipping occasionally, planning to do new pan and fluid just wanted to take care of the codes first. wanted to mention in case they might correlate.
also the battery looks old but always holds a charge. doubt it would cause lean codes.

Any help would be much appreciated at this point im ready to drop it off at a jag specialist. I work on chevys and fords for a living but this is beyond me.

 

Your doing it right by measure fuel trims and checking for trouble codes. The old way of spraying cleaner around does not work. The electronic engine controls are just too fast reacting.
Now one well know vacuum leaker if the PB vacuum connector on the SC elbow. It has an internal O-ring that gets hard and cracks. Especially if you have been in there moving stuff around!

I got bit on that one and we have a number of threads on the repair.

It's pretty buried but follow the vacuum hose from the brake booster back to the SC elbow to trace it out.
If you have any DIY skills I would NOT take that car to a commercial mechanic!! They won't know a thing about that car and all the problems have long since been figured out. They are all on this forum.

From your posting I think you do have ability to fix everything yourself. No one cares more about your car than you!
Do get the JEPC-Parts Catalog
and the JTIS-Shop manual. Both are free on this forum but it does take a bit of work to get them installed and operating correctly.
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RP going off when driving is a vacuum leak 99% of the time. brake booster leak you will see on short trims right away when you spray down where the line hooks up

if you suspect fuel pressure just unplug the FRP. just like an old ford, they do skew and they will mess with your trims while reporting perfect data. unplug it and you get guaranteed 55 psi at the rail if your pumps work right

 

thanks for the reply. Wouldn’t that leak be seen with a smoke tester ? Or not necessarily

 

sorry not familiar. What is the frp ?

 

fuel rail pressure sensor

 

It might show up with smoke? I know it took me a while to find it.

Xalty is right when I had a FPR failure I finally unhooked the sensor completely. The system then defaults to a constant fuel pressure. On my 2005 it defaulted to 70 psi. Normally it varies with manifold pressure/vacuum. But the car will run fine with it unhooked.

Do you know where the FPR is?
Here is my thread and it has a diagram from the JTIS showing the Schrader test port and the FPR. One bit of trivia is 2006 up STR's apparently do not have a Schrader test port?
2005 STR FPR Failure

On my 2005 the part number was #AJ87977. But as always be aware of the identical Ford part number because of the Lincoln LS. This can be a life saver!
Ford 3R3E-9F972-AA about $45 from many places. The identical Jaguar part is well over $150!!!
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Big +1 on the brake booster hose going underneath the throttle body. Very hard to see on a smoke test. It's well hidden. I changed out the o ring - what a royal PITA.

Another area is the EGR tube. Again, the slightest pin hole, cracked tube or gasket will throw your fuel trims off kilter.

I would start in those areas.

Search P0171 and P0174 here on the forum. Lots of info on lean codes, including my adventure.

If you see P0171 most likely either PCV, brake booster hose (o ring) or EGR leak.

If you see both P0171 and P0174 - the leak is in an area that feeds both banks. Air tube, filter, MAF sensor or FPS.

If you go down the road of replacing the MAF or FPS - get original Delphi/Ford Motorcraft or Jag parts - stay away from aftermarket.

Good Luck