P0171 And P0174

StillbetterthanchevyNew Member
Hey guys,
I am new to this site and I am having some trouble.

I have a 1998 mustang gt that I got about a month ago and am having problems with. When I got it, the AC compressor wasn't kicking on, the power steering pump was leaking at the shaft, the front bearings were bad and the starter was only kicking on intermittently. Also, coolant was leaking out the intake manifold. I replaced all these things and when I started driving it. I got 3 codes. Cylinder 4 misfire and bank 1/bank 2 system too lean. The misfire was because I got coolant down the spark plug tube, so I just replaced all the plugs (which was going to be done anyway) and that went away. But no matter what I do I can't get the lean codes to leave. I don't think it's the o2 sensors because I figured that it was not likely they would go out together.

I checked the fuel pressure and I was at 20 PS at the rails, so I changed the filter and was getting 30 PSi. I still thought that was low and so I changed the pump. Still reading 30 psi. I unplugged vacuum off the regulator and the readings didn't go up so I figured that was bad. Changed it, now that works, it went up to 38 psi with no vacuum. But Im still getting the codes! Is that normal pressure to have? Keep in mind that I do have a return. This whole time I've been monitoring my fuel trim and ltft is at 25% on both banks and stft is usually around 7.

The oxygen sensors are reading fine, it's just that occasionally the upstream sensors drop to 0V.

I put vice grips only return line to raise the pressure at the rails and that brought my fuel trim way down. Ltft was at 1ish% and stft was hovering around -10%.

I checked for vacuum leaks, over and over but couldn't find any. Clamped all my hoses anyway.

I also cleaned the maf with maf cleaner. Nothing.

I even warrantied the new intake manifold because I thought that it might be cracked, but the codes still came up! I'm at my wits end!

On an unrelated note, the AC was working fine, but I put some more oil in it because some spilled out while filling it and now the clutch won't engage. Low side is reading approximately 100 psi. Any ideas?

Any help is much appreciated. Sorry if I missed some info. If you need more info, just ask. Thanks so much for looking over this!
 
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The fuel pressure should be 40 PSI intake referenced. 38 PSI with the intake vacuum reference line disconnected is within 2 PSI.

At idle with the intake reference line connected the fuel pressure should be around 32 PSI. At 30 PSI it is off by the same 2 PSI. Are you sure the gauge is accurate? If so, it would seem that the fuel pressure regulator is slightly low.

What have you done to rule out vacuum leaks in other locations? For example the PCV system? Valve cover? Oil dip stick?

If the AC was working and then quit after performing work, recommend double checking for blown fuse and missed electrical connectors. If no problems found, do you have key on/AC on +12 volts at the AC low pressure switch?