'93 Ranger 4.0 DTC 157

[MillerAPF97XLT]

Working on a friend's truck, scanned it the other night and pulled a KOEO code 157 (also got a 565, I think, for canister purge failure). Truck has a VERY new 4-wire MAF sensor (a Motorcraft, even), installed by the last mechanic under the hood before me. Owner complains of rough running and engine not wanting to wind past 3,000-3,200 RPM. Would help to know what the PCM wants for reference and signal voltages to and from the MAF. Got a multimeter and backprobes, just need the numbers and wire functions. I'm kinda hoping either for a bad-out-of-the-box MAF, a crummy connection or bad wiring between MAF and PCM, in no particular order. As for the 565, the purge valve and purge hose elbow look original, with plenty of weather-checking on the elbow. Owner said he thought he might have stuff to replace the elbow with at home. Could the purge valve be stuck open, so the engine is running with some of the air un-metered? This one is going to be a bit of a challenge, since the owner still drives it and it's not sitting in my driveway... Any/all input/advice is greatly appreciated. Thanks in advance - Chris

Edit - Also, could the IAC be stuck open, again allowing the engine to run on un-metered air and decreasing the signal voltage from the MAF?

 

[MillerAPF97XLT]

Update - Thanks to the Interwebs, I've been able to discern what terminal at the MAF does what and what the voltages SHOULD be. Hopefully, I'll get a chance to look at the truck again Wednesday and see what it's actually doing. I'll post more then. Meanwhile, what if I were to disconnect the purge valve and plug the fitting on the throttle body to see if that clears up the problem?

 

[arco777]

Are you sure the Motorcraft MAF is made for the truck? I have verified that MAF sensors from other Fords will fit and even let the engine run, but can result in poor performance. Externally they look identical so you have to verify part numbers.

The evap hoses are easily replaced with parts store fuel vapor rated tubing. The solenoid is usually pretty reliable, but the fittings can break off it if it gets yanked on. Then you'd have a vacuum leak and an inop evap system.

The IAC can get gunked up and is easily cleaned out with carb cleaner and q-tips.

Code 157 shouldn't show up in KOEO, only KOER. That said, it does seem to be MAF related.

Try unplugging the MAF connector before starting the engine. The engine should start and run (utilizing speed density fuel injection strategy) but will run rougher and illuminate the CEL shortly after start. Does it make any difference running/starting with the MAF plugged in? If there is no difference, try another MAF that is verified correct, and check for wiring issues.

 

[MillerAPF97XLT]

At lunch today, I checked grounds, input voltage and signal voltage on the MAF sensor - two good grounds, battery voltage input and about 1 volt at idle, 2.5 volts at about 3000 rpm. Smooth all the way, until it got to 3000. That's when the miss showed up. I'm beginning to believe the 157 may be an old code, needs to be cleared, and that the miss is a whole 'nother problem. So, if the 565 code is current, could a canister purge failure contribute to a medium-engine-speed miss? The coil pack, plug wires and plugs look in good shape, and are reported to be reasonably fresh. What could cause such a miss, that shows up at and over a certain engine speed?

 

[Centaurious]

The ignition system is a 'wasted spark' type which means that each plug and wire see twice as much use. Our engines require double platinum plugs at a minimum and the wires last half as long. this means that wires that still 'look' good may not be that great.

Interesting Fact: The 4.0 OHV engines came from the factory with 2 different Plugs. One bank had Plugs with platinum center electrodes and the other bank had Plugs with a platinum button on the ground electrode. Must have been a 'bean counter' thing.

You should run the KOEO, KOER, and MEM test procedures as the KOER test may give more clues.

 

[MillerAPF97XLT]

The latest - did another KOEO read, got 157 and 565 again. Did a KOER read, got 157 and 998 (hard fault) and a message, "EEC IS IN FMEM AND CANNOT RUN DIAGNOSTICS".Also tried to view datastream and everything showed "NO DATA". Suddenly, I'm wondering if the problem is a toast ECU. Ideas?

 

[MillerAPF97XLT]

Another thought - Is it possible this truck has a California-compliant ECU, and I'm trying to read it as a federal truck? Could that cause the 998 code and the no-data condition? How would I be able to tell whether it was a California ECU or not?