Blown MAF hotwire, "slow return to idle"
Now that my truck is fixed I can sip a beer and share what I've learned.
2007 XL Ranger 2007 2.3L standard 2WD
PCM part number: 7L5Z*12A650*ASG
Calibration ID Number PYEL0A4.HEX
Calibration Verification Number 3E AD FZ AB
Latest TSB (June 21st 2010) “high idle ~1500 to 2200
I drove away one morning and my MAF hot wire burnt out, likely the moment I turned the key. Whenever the clutch was depressed or the transmission was in neutral the RPM would race to 2250RPM and sit there indefinitely, unless I came to a stop, where it would take between 7 to 30 seconds to return to idle.
12V ignition thru fuse---A---hot wire -------B---ground
PCM MAF Vref------C----- [sensor] ----D----PCM maf return
Vref-----------------E---[thermistor]----F----PCM intake air temp
As shown in the attached diagram (see above ugly text diagram if image deleted in future) the hot wire is fed by fused ignition 12V on one end and grounded on the other...it is not visible to the PowertrainControlModule (PCM) so it cannot directly throw a code. In my truck no indirect or pending codes were thrown, and my highway mileage remained normal.
The 2007 Ford Ranger schematic shows the 6 pins of the MAF connector as numbered but on the part itself letters are embossed. 123456 = FEDCBA so it is reversed from what you would expect.
AB are the hotwire pins. Expect 3K6ohms resistence. I found infinity on my blown part.
CD is the MAF companion wire measured by the PCM. Expect about 9Kohms
EF are the Negative Temperature Coefficient (NTC) thermistor which measure ambient air temperature
The NTC thermistor Ford uses is really cheap and sloppy so they can save micropennies.
-15C 68K1 (note the huge difference here.. the other one was more than double this resistance. Too Cheap!)
The thermistor is combined with the MAF to make a 6 terminal expensive part that you have to buy if the thermister blows. The Canadian price is $250. The US price is $100. Ford thinks they can use such a cheap sensor because the PCM algorithm can “learn” the characteristics of the new part and build a lookup table for it volatile “keep alive memory” (KAM)
You need to reset you KAM and since there is probably a supercap in the PCM to retain the memory long enough for you to do basic things like swap a battery, I left my battery disconnected overnight. I've read that after ~2010 KAM has been outlawed by government mandate in favour of EEPROM.
My truck never properly re-learnt whatever the parameters set at the factory were and the failure can be summarized as
“IDLE RPM = 1000RPM below 5km/h, 2000RPM forever above 5km/h”
This makes upshifting hard on the clutch, and the programmer who wrote this algorithm is an imbecile. I could easily write better microcontroller algorithm code than Ford and have fun doing it...what's their problem?
Kitchener Ford in Waterloo Ontario listened to me, told me of an update that addresses this and they were correct. Cost me $66. Ford Canada told me there was no update. Parkway Ford insisted on $222.
A note on TPS and IAC
You can follow the debug procedure for the Throttle Position Sensor (TPS) and compromise your wiring. I just bought a new one since they do wear out and eventually I'll need one. Now I have a spare. I doubted TPS was the problem andI got no codes, so really I had no good reason to think it was bad.
The Idle Air Controller (IAC) is driven by the PCM but not readable by the PCM, so it can throw no direct codes. If the PCM commands it to open, the PCM has no direct way to tell if its bidding was carried out. The simple test, and the basis of the Ford Diagnostic procedure is to disconnect the connector to it while running at idle. If the engine then stumbles and even stalls then you know the IAC was doing its job before you disconnected it. The IAC is downstream of the air filter so it is somehow dirty or clogged ask yourself how that could possibly happen. It takes only a moment to pull it off and check with ballpoint pen that the valve is not binding.
Hope this helps others.
Last edited by Andre; 02-21-2013 at 10:12 AM.
Reason: spelling correction, formatting, additional info.