1997 Ford Expedition Lean problems under load and at idle

  • V8
  • 4WD
  • 180,000 MILES
I have had an on-going problem with my Expedition that is kind of irritating. I live at 4600' in elevation and work at 100' in elevation, so I drive 4500' down and back up every day. At least once, if not 3 or 4 times, on my way back up the hill my car will act as if it doesn't get any fuel for about 5 or 6 seconds. If I floor it or take my foot off the throttle and put it back on it goes away and runs normal again. If I leave my foot in that same position it will eventually pick back up to normal but it takes a total of about 15 seconds before I am at max HP again. It will also drop to about 200RPM at stop signs for 5 or 6 seconds when I exit the freeway sometimes. I can cruise though for hours at 65 on flat ground and it never happens.

I have already changed the fuel pump, filter, TPS and coil packs and ran my OBD again and it comes up with 3 lean codes, one O2 slow response code and P0401, which I have narrowed down to the sensor and will change soon.

I did some reading on some of your other forums here and it appears that a bad mass air flow sensor could be causing both the fuel delivery and the O2 sensor problem.

My question is- Is the mass air flow sensor circuit connected into the fuel delivery part of the computer to a point that it could shut off the fuel almost completely? I had thought that it kind of just adjusted the fuel flow slightly to optimize performance. I have spent quite a bit on parts trying to figure this out and those things are about $80. I will clean it very good tonight first, I have carb cleaner already at home but I don't want to buy yet another part if I don't have to.

If it is this, I am hoping it will solve both the RPM decrease adn the O2 sensor problem.

Also, the O2 code is sensor 1, bank 1. Which sensor is that in case I do need to replace it?

Do you
have the same problem?
Thursday, June 24th, 2010 AT 10:13 AM

1 Reply

Hi tallan1976. Welcome to the forum. You've already identified this problem is related to elevation, not parts on the truck. The mass air flow sensor has the single biggest say in how much fuel is commanded from the injectors. Only Chrysler has been able to make an engine run right without using one. The sensor calculates air volume by weight, and you know it's working because the engine runs normally most of the time.

There is a range the MAF and DPFE sensors work within. As you change elevation, barometric pressure changes and these sensors can go out-of-range. The same thing happens on engines that use a MAP sensor. GM uses theirs to constantly measure barometric pressure while driving. Chrysler uses only that sensor for fuel delivery but at engine startup, it takes a single barometric pressure reading. Their Engine Computers get a combination reading of manifold vacuum and barometric pressure changes while driving. In rare instances, they will record a new barometric pressure reading at wide-open-throttle when no manifold vacuum exists. Otherwise the value stays in memory until the engine is restarted.

If the problem is nothing more than a sensor reaching its limit, a simple trick you can try to prove it is to stop the engine and restart it just before you are about to reach the point where the problem is likely to occur. The new barometric pressure will be learned and the sensors will operate around that value. It sounds like a relearn procedure is taking place in your Engine Computer but the conditions to make that happen are the symptoms you're noticing. Years ago GM cars and trucks were having a similar problem when driving in the mountains, but the only symptom was the Check Engine light turned on. People would stop at a dealer for help only to find the light was off when they restarted the engine. It had taken a new reading from the MAP sensor at engine startup and would run fine. All that was needed when heading back down the mountain was to restart the engine once or twice on the way down.

I believe I heard a story once about a different part number MAP sensor that could be installed for vehicles that either regularly where driven only at high altitudes or regularly made the transition between the two extremes. It would seem to me if that is true, it would only apply to vehicles that have Engine Computers that can be reflashed to use the new sensor values. Your dealer would be the place to ask about that possibility.

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Thursday, June 24th, 2010 AT 2:53 PM

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