I have a 99 Grand Marquis, I drive all week long in the city (check engine light dose not come on) but soon as I get on the interstate on the weekend the check engine light come on (codes 0171 @ 0174 ) . I also hear engine knock at certain times when the motor is under certain accel. conditions. Any suggestions from experience out there.
Ed
The two codes indicate a lean condition (too much air/not enough fuel). P0171 is bank 1 and P0174 is bank 2. So whatever it is, it is likely to be before the actual intake/throttle body. This does mean that if anyone tells you about replacing oxygen sensors you should be skeptical since both sensors suddenly throwing a code at the same time would be too much of a coincidence for me. (The lean report comes from the O2 sensors).
The first thing to do is clean the mass airflow (MAF) sensor, clear the codes & see what happens. The MAF will be somewhere after the air filter leading up to the big snorkel. It probably has two tiny little wires in it. You spray those down with a MAF cleaner which you can pick up at any auto parts store.
If you clean the MAF & they come back the next thing to do is check for vacuum leaks. In fact, when you pull the intake snorkel apart to clean the MAF you should just check it carefully for cracks/splits etc. Other than that check vacuum lines, the intake manifold, the throttle body gasket, etc.
Thanks cigroller, I will do that, forgot to state I also cleaned the throttle body last week but did not think of the MAF. Wil also check for vac leaks.
Ed
Did you clean the throttle body on or off the car? Perhaps you removed it & the gasket didn’t quite seal up properly when you replaced it? Or perhaps you just forgot to tighten down the snorkel clamp?
I cleaned it in place. and I have cleaned the MAF sensor yesterday. Checked the Vac. hoses everything seems OK. Thanks for the info.