99 Mazda Protege with the code "P0171 System too lean" since Nov/Dec.

Question

I have a 99 Mazda Protege that has been coming up with the code "P0171 System too lean" since Nov/Dec. In January I replaced a cracked exhaust manifold and put in a new battery. This month I replaced a cracked air intake tube and gasket and put in new spark plugs. Now while sitting at a red light the idle will be sitting just below 1000 rpm and then fluctuate and jump just above 1000 rpm and go back down, sometimes it'll jump up to around 1500 rpm instead. I'm beginning to think that it is a fuel related problem. I'm ready for this problem to be fixed! Any ideas of what it could be?

Answer 1

This code can be caused by a vacuum leak at the intake manifold, the most common on these is the hose for the pcv valve. It can also be fuel pressure related, you need a fuel pressure gage to be sure.

With the engine running spray some carb cleaner along all the vacuum lines and the intake manifold, if the idle stops searching then you will locate the vacuum leak.

Answer 2

It ended up being a crack in the vacuum line to brake booster. =)