Talked about this a couple weeks ago. Merc. Grand Marquis, 75 K miles, changed all spark plugs, replaced 2 Ignition coils, MAF sensor, Changed all 4 O2 sensors, replaced Intake manifold due to crack in Thermostate housing. Used small hose to check for Vacum leaks (no hissing sound found) around Intake Man. and throttle body. Replace 1 vac. hose. I still get codes 171 & 174 on read out. Car now is bucking when at a stop light, I also hear a ckicking sound like some thing is switching on and off. Engine runs fine w/ AC on. Totally lost now on Ideas?
Code P0171 : System too lean (Bank 1)
Code P0174 : System too lean (Bank 2)
This is caused by either a vacuum leak allowing too much air in, or a fuel problem, not getting enough fuel. If your sure there is no vacuum leak, best checked with a vacuum gauge, then have the fuel pressure checked.
Air leaks in the exhaust system before the O2 sensors can cause these codes also.
The “Car now is bucking when at a stop light” is a symptom of a brake booster leaking vacuum. Or, a torque converter lockup not releasing …which it should when speed goes below 30 mph.
Idle the car. Press on the brake pedal. Did the idle change? If the engine ran rougher, that indicates that the brake power booster is leaking vacuum.
Vacuum test gauges are very cheap, and not difficult to use. It’s one of the most effective tools you can use.
Indications with a vacuum test gauge: http://www.international-auto.com/fiat-lancia-tips-on-reading-gauges/tips-on-reading-gauges-vacuum-gauges.cfm
Agree with Cougar’s quick assessment, because of symptoms. An OBD II tester is a big help in these kinds of cases. However, considering all the work you did (at one time?) it’s possible it could be in anything you worked on, OR, something which was affected near the things you worked on. Let us kn ow what happenned, okay?