Engine LTFT at 1.24% at idle, throwing a stored P0174 code

zack2001

Member
Dec 11, 2019
20
1
13
Lansing, MI
2005 Mustang 4.0 V6

For awhile now my engine has been running lean, even on the stock tune. I replaced both PCV hoses with new ones (both of the old ones broke somehow and got stuck to the intake tubes they connected to, it was a pain to get them off) hoping that was the cause of my likely vacuum leak. After resetting my ECU by disconnecting the battery and then test driving around, my long term fuel trim is still high. Even though there's no CEL, there's a stored code for P0174 (Bank 2 too lean) but not P0171, leading me to believe there's a vacuum leak somewhere, but I'm not sure where to look exactly.

I should also add that my car had thrown code P2196 and code P2198 (not at the same time though), but after clearing them they haven't appeared since. One day while I was hoping to find the cause of the potential vacuum leak, I was tinkering around with the PCV hose that is connected from the intake manifold to the passenger side valve cover and the tab that you push with your finger to remove it was broken, which I thought was causing the vacuum leak since there was an air gap. I managed to get the hose off the intake manifold, but when I went to put it back on, it slid back on and off easily so I had to push the black hose while holding the green tab steady so it will get stuck again, which it did, but it was still broken. Next morning when I started the car, my CEL turned on immediately and I got hit with 5 codes - P0300 (Random or multiple cylinder misfire), P0316 (Misfire detected within first 1000 revolutions), P2198 (O2 sensor rich [But my car is running lean???]), and P0174 and P0171 (Bank 2 and Bank 1 too lean).

When my foot is on the gas at 80 mph the LTFT is around +1.22% to +1.25%, but when I lift my foot off the pedal, the LTFT drops down to around +1.03% to +1.08% (Does this mean anything?) I saw in a YouTube video today that if you rev the car while it's idling in park and the Long Term Fuel Trim goes down to where it should actually be at, it means there's a vacuum leak somewhere, but I haven't tested that yet.

I replaced the broken PCV hoses afterwards and there haven't been any more misfire or O2 sensor codes.

I also cleaned the MAF sensor and I'm yet to go out for a test drive to see if the LTFT will go down. I fired it up and let the car idle for a bit, but the LTFT still hasn't changed. It idles rather low on hot starts, idling at 1000-1250 rpm for about a minute and then dropping gradually to 650-800 rpm.

Are there any spots notorious for causing vacuum leaks on a 4.0? I'm really starting to get worried about my lean condition since I don't want my engine to get damaged. I also understand that dirty/failing fuel system parts can cause a lean condition as well, but wouldn't that trigger both P0171 and P0174?
 
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