I have a 1989 F250 with 351W engine. 160,000 miles but has been running quite well and passed California smog check a year ago. Suddenly it has started idling roughly and blowing black exhaust, also prone to stumble from idle. I checked for vacuum leaks but found none, replaced PCV valve and it's hose because it seemed brittle, new air filter. I've had suggestions about MAP sensor and IAC valve problems but don't know if I can test them or if I need to replace them. Any ideas? It runs very strong once above idle.

 

Check fuel pressure and the fuel pressure regulator. You can also test the MAP sensor using the frequency scale on a multimeter, or a 6 cylinder tachometer (see a repair manual for details on the tachometer test method). The IAC won't cause any changes in the fuel mixture, but you can clean it if you want to with some throttle body cleaner. It also wouldn't hurt to check the computer for any trouble codes: http://www.thorssell.net/hbook/eectest.html . If your check engine light works, you can leave off the test light/analog voltmeter and just count the blinks on the check engine light.

 

the easiest way to test the map sensor is to just unplug it, but you can use a meter to see if its good or not too.

 

That doesn't tell you anything about the sensor itself, though.

 

On Ford applications, a multimeter that can read
frequency is normally required to check the sensor’s
output. But you can also use an ordinary tachometer
because a tach can display a frequency signal.
Here’s the procedure:
1 Set the tachometer to the four-cylinder scale
(regardless of how many cylinders the engine has).
2 Connect one tachometer lead to the middle terminal
on the MAP sensor and the other tachometer test
lead to ground.

Connect the two jumper wires as in the diagram above,
attaching each end terminal on the sensor to its
respective wire in the wiring connector.
4 If you want to measure engine vacuum so you can
correlate it to a specific frequency reading, connect
a vacuum gauge to a source of manifold vacuum
on the engine, or tee the gauge into the MAP
sensor hose.
5 Turn the ignition ON and note the initial reading.
The reading on the tachometer should be about
454 to 464 at sea level, which corresponds to a
frequency output of 152 to 155 Hz.
6 Start the engine and check the reading again.
If the MAP sensor is functioning properly, the
reading should drop to about 290 to 330 on the
tachometer, which corresponds to a frequency
output of about 93 to 98 Hz. No change would
indicate a defective sensor or leaky or plugged
vacuum hose.

 

what do you want to see, if you unplug it and it work then its a given, its broken. these trucks will run fine without it if there is no other problems, the computer kinda goes into a safe mode when there is no signal from it, but if its getting a screwed up signal the computer still uses that info.

now it may also be a vacum hose and the best way to find that out would be metering resistance between the wires and see if they check out with the manual.

 

If there's a vacuum leak in the MAP line, unplugging the sensor might make a difference in how the truck runs, but you know what? A new sensor will cause the exact same problems as the first one that was "bad" by your unplug-the-sensor test method. The best way to test the sensor is either with the frequency scale on a multimeter (resistance won't tell you a darn thing about a MAP sensor, and can even ruin a perfectly good sensor) or using an analog tachometer following the directions as subford has posted. That way you know exactly what signal the sensor is sending to the computer, and there's a better chance that a good sensor won't be falsely diagnosed as a problem.

 

My problem turned out to be the fuel pressure regulator. With a pressure gauge attached to the fuel rail and the fuel pump hot wired to "on" everything checked out fine. When I started the engine the fuel pressure started at about 40 PSI but within about a half minute it jumped to over 100 PSI and the engine began to stumble. I managed to replace the regulator without taking off the intake plenum. It was tedious, but much easier for me than disassembling the intake. As an added bonus, the truck now starts much easier since the new regulator holds fuel pressure in the fuel rail instead of immediately bleeding off like the old one did. Thanks for all of the suggestions.

 

Glad you got it fixed