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Check Engine Light On

I have 99 Merc. Grnd. Marq., code 0174 know showing up on the scanner, I also had 0171. I have done a vacum test and holding 21.5 Hg, I have changed all rubber vacum hoses even if they did not need changed. I even changed the gromet for the PVC Vale on the right valve cover. Checked the injectors and all are clicking. I had changed spark plugs earlier and show a normal firing color. I had changed the fuel filter(last week) and MAF sensor a month ago. do I need to change the MAF sensor again. Lost?

its common to both banks,PCV hose ,intake leak at the split plenum,these could be the cause,even though the vac reading is ok.

is the PCV valve a bobo model or the OEM model?

good luck.

You have a V6 engine which is showing a lean condition on both cylinder banks with DTCs P0171 and P0174.
We need to know what a LEAN condition is before we can determine what has caused it. What is it? A lean condition is when there is more air in the air/fuel mixture than the ideal A/F ratio of 14.7 parts air to 1 part of fuel.
A lean condition may not be an excess of air, it may be a deficit of fuel. Fuel can be less than it should be because, the fuel flow is less than commanded by the engine computer; the command may be for too little; there may be a fuel flow restriction; the fuel pressure may be less than specified.
Check fuel pressure. Use a Throttle Body Spray Cleaner to clean the throttle plate and throttle bore. You should consider a professional “fuel system cleaning” to clean the fuel injectors, intake tract, and cylinders. It should cost under $100.
If you have one of the more comprehensive scan tools, as contrasted to a “code reader”, you could look at the MAF flow, and STFT (Short Term Fuel Trim) and LTFT (Long Term Fuel Trim).
There is other stuff to check; but the DTC charts for the “fuel control” system are your best bet. You can get the DTC Charts at places like

There is a Technical Service Bulletin, dated 10-15-2001, Article No. 01-20-5 “Driveability-DTC P0171 and/or DTC P0174-Detecting Vacuum Leaks”. It instructs you to use a Rotunda Smoke Machine to do a smoke test on the intake and engine. Not everyone has a Rotunda Smoke Machine; so, I have an alternate method that may work: acquire smoke bombs from a novelty store, use the smoke bomb (your choice of colors) to do the test. The specified “smoke” pressure is only 1/2 psi, so a smoke bomb should be enough. Try it. It can’t hurt. Might yield usable results.

It’s a 4.6L V8. The forward oxygen sensor is generating the “too lean” code. Replace the sensor.

which one? caddyman

fore there are two.

take a guess?


original and working and I cleaned it.

I HAVE A 4.6 LITRE V8, 75 K miles.

More info on howto use the smoke bomb?

I had changed it right before Xmas? I changed all four, the conectors where a pain - unscrewing the sensors was no problem.( used a candle on the threading after I ran the engine for 2 minutes - unscrewing was effortless… The right front manifold O2 sensor was less time consomming hat the drivers side the one just in front of the cats where a piece of cake

never had thoes codes come up?

No V6’s on a Grand Marquis!
It a 4.6L V8.

We see people all the time who try to change enough parts to solve a problem. It seems that we can never persuade them that their “method” is very expensive and is unlikely to fix it. They continue to push for that one “sure-fire part”. It never comes.
This problem needs another, less expensive, more thorough approach to solve it. Are you willing to try a different approach.? If not, you need to take it to a shop.

what is the different approach