Questionhave a 99 ford ranger XLT 4.0 engine about a month ago the check engine light came on took
it to the local auto zone they hooked up there meter and the code read PO171 @ PO174 so I
replaced the Oxygen sensor's there's three in line and also replace the fuel pressure
regulator that work'ed for about a day so the next week I took and put it in the shop he
checked out to see if there was any vacume leak's none to be found he ask'ed if the fuel
filter had been changed I said no so he chang'ed that it work'ed for about four day's and
then the light came back on any idea's that might help??????
AnswerDouglas,
Both your engine codes indicate a lean fuel mixture. On your Ranger there are a few things that can cause this. The first is a dirty mass air flow sensor. You can remove the hot wire sensor from the mass air meter and inspect it for debris or dirt stuck to the wire. Clean it only with compressed air or electronics cleaner as it is fragile.
Second check your fuel pressure. Low fuel pressure will cause this also. With a fuel pressure gauge hooked up you should have 35psi running and it should increase slightly when you stab the throttle. Under acceleration it should never drop out.
Next, have your mechanic check the DPFE sensor. This monitors the EGR gas flow and is famous on Fords for breaking. It should be below 1 volt at idle and increase as the EGR opens during part throttle. If it isn't replace it. They are now available after market much cheaper than a dealer.
Good Luck!!!
