Hello,
I searched the archives but didn't fine exactly what I needed. I have an intermittent problem which always occurs with the check engine light coming on. The problem started one day when the engine stalled at idle, which it had never done before. Then I noticed that it would buck a little under accelleration sometimes and the check engine light was on for the first time in the ten years I've owned the van. After a little checking on the Web, I cleaned the mass air sensor, and it seemed to fix the problem for a few miles. Then it came back. It can run perfectly for 5 miles with no light, then really act up so that I have to keep my foot on the gas at stop lights. I tried disconnecting the MAS as suggested by one person to put the system in "Failure Mode and Effects Management (FMEM) to see if the problem went away- it didn't, so I concluded that the MAS was OK. I went out and bought a new oxygen sensor since the old one is probably the original with 180,000 miles on it, and an OBD tester. The codes I got were: 157, 158, 172. I have checked for obvious vacuum leaks and the pcv valve seems fine.
Any ideas?

 

OK, here's the update: I replaced the oxygen sensor (luckily it came loose with a big kick to the wrench), reset the trouble codes by disconnecting the battery, and went for a test run- the CEL came on immediately. When I rechecked the codes, I just got the #157- low voltage output. I replaced the MAS with a rebuilt unit, reset again, and everything looks good after a three mile run- it does run noticeably better than it has for awhile. So I guess the test where you disconnect the MAS isn't valid, or at least wasn't in my case. Hopes this helps someone.

 

good troubleshooting and DIYer repair.

the 4L Aero is suseptible to dirt contamination of the MAF sensor. many of us have success at time cleaning the MAF and saving an expensive replacement.
search Aero forum for a number of good write ups on MAF cleaning.
it's worth a try. try cleaning the old sensor and saving that MAF if good as a spare.

yes, the O2 sensor is a mandatory replacement item. Ford used to pay for the first one up to 80k miles under the Fed. Emissions Warr.
saves fuel if they are replaced every 50k to 100k even if a code is not present. sensor becomes contaminated and damaged after years of high heat. slows switching response time of the unit and it's accuracy.