I'm working on my nephew's 1997 F250 5.8L A/T. I'm having an idle problem. The engine starts and idles fine until... I give the accelerator the slightest touch, then it dies immediately. I've checked engine vacuum, and with engine hot and at idle, it holds about 19" of vacuum, steady. It will restart fine and idle fine again unless I just breathe on the accelerator - if I apply a little more pedal, it will stutter a bit, but then rev normally, only to stall again when returning to idle. I've checked the operation of the TPS through it's range and it's smooth, the IAC was recently replaced (not by me). The EGR port into the intake was blocked off by a plate installed between EGR valve and gasket (also not by me), so I wouldn't expect it to factor into the symptoms I'm seeing. I decided to investigate the MAP sensor next. After back probing MAP reference wire (4.8v) and signal wire (also 4.8v) and applying vacuum manually with a MityVac, there was no change in voltage on the signal wire. I figured my MAP sensor was toast, so I replaced it with a new Motorcraft unit, but I still have the stalling problem. If I leave the MAP connector unplugged, the engine will idle (higher than normal), and I do not have the stalling condition on acceleration, although the engine seems to be running very rich by the odor of the exhaust. With the connector unplugged, KOEO, I probed the signal wire terminal and still get 4.8v (the same as the reference voltage). Is this normal? I'd expect the signal voltage to be 0v when it's unplugged from the sensor. Anybody have a clue why I have this reading on a sensor signal wire.

 

You might check the color wire that is supposed to be the signal wire to the PCM and start at the PCM to see if the voltage is coming from there. If so, problem is with the PCM. If not , possibly someone mixed up the wires or there is a path from the 5 volt reference getting to that signal wire..

 

Hi hanky - thanks so much for your response. I think I need to find a source for a PCM wiring diagram.
I just spoke to the (well regarded) mechanic who previously looked at this truck. He's the one who replaced the IAC and TPS sensors, and spent a good amount of time trying to chase down the problem. He's pretty well convinced that there is a sticking valve or something else in the valve train causing immediate vacuum loss and the instant stall. The truck has about 100K on it, and I'll admit the valve train is a little rackety when running. But why would it only present the problem on very light throttle or return to idle? By light throttle, I mean only enough to raise RPMs by 25 or 30 revs. Let's be real here, any V8 will seem to run reasonably well with a dead cylinder - although we all recognize it's only a matter of time before catastrophic things begin to happen. Anyway, he said he checked the MAP sensor circuit with a factory Ford diagnostic tool he has and it passed the diagnostic tests (he even swapped out the MAP sensor and it made no change). He seems to think that the MAP circuit may be intentionally designed to start with reference voltage on the signal wire which is then modified by the sensor's reaction to vacuum changes. Doesn't really make sense to me that it would work that way, but maybe you have a different opinion. I can tell you that I back-probed the signal wire with the engine idling (vacuum line plugged) and applied vacuum with the MityVac and saw no change in the signal voltage from 0" to 25" of vacuum nor any change in the way the engine ran. Do you know what a good MAP should be reading at various vacuum levels and what it's typical signal range should be? I'm still heavily leaning toward a problem in the MAP circuit. Your thoughts?

 

Ford MAP sensors operate with "Frequency" not voltage. Voltage readings are of no value here.
Readings taken from signal wire of connected sensor.


Your meter needs to have the frequency function to do this test.
1. Engine first started = approx. 152 Hz.
2. Approx 5 in vacuum applied to Map sensor =approx 141 Hz.
3. Approx 10 in vac applied =127 Hz.
4. 15 in vac applied = 114 Hz.
If you do not get these readings, the sensor is defective and should be replaced.

I believe your mechanic could use a refresher course.

Do you know if the TPS was checked for an open in it's circuit when off throttle is operated ?