Help! 1999 Explorer Code Issue


My 1999 Ford Explorer 222,00 miles (I know don?t say it) has failed Calf emissions. I was advised to replace the intake manifold upper gaskets due the code readings. My nephew and I tackled the job and seven hours later she fired right up. We replace the upper gaskets as well as the injector o-rings.

Next day she failed emissions again, well sort of. All the emission test are well within range and pass. The issue is the check engine light and a code reading ?PO171 System too lean bank 1?. After much online research I replaced the ?Mass air flow sensor? also the PCV valve, checked for vacuum leaks (none found).

She seems to be running smother, not rough at idling speed however that dam check engine light is still on. The only thing I have not replaced is the o2 sensor. I did pull the battery cable to re-set the light. Is there more I need to do? On line research said that the 02 sensor is mostly working fine and reading too lean due to the bad Mass air flow sensor. Thanks for helping if you can

At 222K miles, I would replace ALL the oxygen sensors (if I lived in California).

You said you reset the light…If it stays off for a couple of days, get that puppy re-tested!

If living in CA, I’d get an OBD II scan tool (about $60), and save myself a bunch (money, not bananas). Then, erase the DTC (Diagnostic Trouble Code), and wait for the check engine light to return, or not.
After the trouble codes are erased, the monitors have to get ready. On the scan tool, the monitors which are not ready, will flash their appropriate icon. It will take several “drive trips” (drive cycle) for the monitors to get ready. When all are ready, NO icons will flash on the scan tool.

It only stays off for about 15 miles. Tthe first re-test it was off and then went back on durring test, failed that one. Next test I re-set and took it right in. Then the computer had no information in it so I failed that one as well.

Dirty fuel injectors are seldom the cause of these kind of problems, but if you have a couple partially plugged up, that could cause the lean condition. The BEST product for this is BG-44K. Wait until your fuel tank is below 1/4 to put it in. Chevron’s Techron works ALMOST as well.

So how did you check for a vacuum leak? A visual inspection is not a very good way of doing this and the only surefire method is with a vacuum gauge.

Since a vacuum leak could cause this problem and you have made references to what is apparently a “smoothER” running engine as opposed to a “smooth” running engine it sounds like it’s still not as smooth as it should be. Is this the case?

You also state that intake upper gaskets were replaced due to code readings; plural. What were those codes?

Its something to try without trowing more money into a part. Ive been thinking about trying something with the injectors. Thanks

Yes its still a little rough. Also somewhat sluggish, but with 222k on her she?s not whole shotting. It?s fluctuating at idle and idle is little high just over 1000 and dips down to about 600-800 after a long warm up. It used to idle lower almost from the get go before.

I checked the vacuum leaks with a visual inspection. Also spayed carburetor cleaner on the hoses and manifold while at idle. Did not get any reaction from the motor so I felt all was well there.

The codes from the very first test, before any work on the engine were ?.PO171 System too lean Bank 1 and PO174 System too lean bank 2. That last one went away after changing the intake gaskets.

It sounds to me like you still have a vacuum leak. Vacuum gauges are cheap and a good way to figure it out. You can also take an unlit propane torch & feed propane around everywhere listening for engine surges. There is a vacuum schematic under the hood to help you make sure you hit on it all.

In the midst of all of this has anyone cleaned the throttle body?

Tahnk you for the tips, Ill give this a shot. As far as the throttle body goes, yes I cleaned it durring the gasket change out, removed build up with brake cleaner and brush then blew it out. Its not like new but cleaner than before. Do you have any thoughts on teh code/codes Ive been getting?

Well, lean is too much air and/or not enough fuel, and/or something off in the sensor readings about it. A vacuum leak will produce the codes (too much air). So that was my first thought on the codes (other than things you’ve done already).

As Caddyman noted, on occasion this can come from fuel delivery problems - check the fuel pressure, give it a new fuel filter and run some good injector cleaner through it.

Is the air filter new? It doesn’t have an aftermarket oiled filter such as a K&N does it? These can foul MAF sensors, normally from having a bit too much oil on them.

It is also possible that you do need a new O2 sensor - those can be tested by somebody with the right scanning equipment. Then again, some people figure its just as economical and less hassle to toss on a new one. I would double check the vacuum leaks & fuel system and then maybe throw a new O2 sensor at it.

ok this is all good information. I will work on this stuff over the week and see if I can get this worked out. Ill let you know. Thnaks for helping