Need your opinion vacuum leak or not


Jun 30, 2021
Hi everyone,

I'm pretty new to mustangs (as an owner, loved them since forever) and I would consider myself a basic hobbyist mechanic. That mean I now master the art of doing my own oil change and changing my rotors and pads, but that was it. I just bought a New Edge 2004 V6 3.9L Mustang so I can have some fun driving a Stang (have to start somewhere) but also to get better at mechanics. I plan to restore thy car and keep it for a long time. That been said, I now have an issue where my sensor is throwing nonstop codes like p0171 and p2195 so basically, this is a "System too lean" issue in bank 1 which appear to be on the passenger side. I know this code can be easy to fix or painful as hell since there are a zillion reasons that might trigger them.

The car came with a CAI (SR performance) and I know this mod can throw check engine lights on S197 if you don't tune it but sn95 shouldn’t be affected tuned or not. I don't have any idea how I can check if the car is stock or tuned as we speak.

What I did so far (I'm checking for vacuum leak):
  • Did a smoke test. It was leaking in the CAI where you have to plug the hole for the IAT that is not present on my 2004 (merged with MAS). I sealed the leak with black RTV and now it's not leaking anymore. Cleared the check engine and it came back after maybe 50 miles. Second smoke test didn't reveal a leak.
  • Changed PCV and breather hoses (they were dry rotten so I had to do it no matter what)
  • Test the brake booster and EGR valve, they hold a vacuum
  • Clean the MAS with MAS cleaner
  • recheck every join on the CAI for leak
  • Did a vacuum test (see at the end)
  • Checked all vacuum lines

What I still need to do
  • Test MAS resistance with multimeter.
  • Do a fuel pressure test on bank 1
  • clean my fuel injector
  • Change fuel pump and fuel filter (car have 150k)
  • Change O2 sensor, but I want to do toward the end. It's looking impossible to do under the car in my driveway, particularly on the passenger side.
  • didn't redo intake manifold nor throttle body gasket ( will do this at the very end if I can't sol the issue)

Result of vacuum test: steady at 16 In/HG on the vacuum test. Ford Service Manual for the 2003 (cannot find the 2004) state that normal vacuum is between 15 & 22, so basically, 16 is fine but I like to be closer than the middle. What do you think ? Can I have a leak or it might be more on the fuel/sensor side ? I did blow really hard in the hoses for the smoke test so if it was an intake manifold gasket issue, I think I would have seen smoke like I did with the CAI.

What would you recommend? Do you think it still might be a vacuum leak based on my results ? I'm pretty a noob on the subject so I really need a nudge in the right direction here. Three weeks ago I didn’t know anything about brake booster, PVC/EGR valves so I’m more than willing to learn.

Thank you very much.
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Hi COramprat,

You were right, I did plug my shopvack on my exhaut and used soapy watter on the exhaust manifold. RH one was bubbling like hell. I did try exhaust putty but it was still leaking at two different places so I ended up changing it and now after 200 miles, everything looks fine.

Thank you again.
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