Bfritz97

So my 97 Cherokee has had a CEL for random misfires going on and off for a while. It has now been on for a few weeks without going off.

I decided to test the MAP sensor with a hand vacuum pump and by backprobing the MAP signal wire and found out that the voltage was too low with or without vacuum. I don't remember the exact numbers, except with the engine off it was only reading 3.9 volts, so I replaced it. However, the replacement still read only 3.9 v with the engine off and I didn't test it with a vacuum gauge.

I was also messing around with a multimeter with the MAP sensor unplugged.
-There is a little over 5V power, which is normal (probe from power terminal to battery ground or ground in connector)
-With the sensor unplugged, there is also ~5V from the sensor output terminal
-With the key off, there is no resistance between the ground in the connector and the battery negative terminal, but with the key on, the resistance is 0.21 (with multimeter set on 20k--sorry, not an expert with multimeters, but in short, the resistance goes up) As soon as a relay clicks (I assume the ASD), the resistance drops to 0.

Any or all of the things I listed above may be perfectly normal, but I'm not sure

I'm also at over 5,000 feet altitude, for whatever it's worth.

Basically, I am a little lost and suspect some sort of wiring issue since I replaced the sensor and am still seeing low voltage and rough idle. It is also possible that the issue isn't even related to the MAP sensor.

Any help is appreciated

CCKen

Is this what you did to test your MAP sensor?


Testing MAP:

Using a digital voltmeter, back probe the MAP sensor connector while it’s attached to the MAP sensor and take the following readings.

With ignition switch ON and engine OFF, input voltage should be 5.0 +/- .5 volts at terminal 3 (Orange wire).

Test the MAP sensor output voltage at MAP sensor connector between terminals 1 (Brown/Yellow tracer [ground] wire) and 2 (Dark Green/Red tracer [signal] wire).

With engine running, the voltage should drop to 1.5 to 2.1 volts with a hot, neutral idle speed condition.

Snap the throttle. Voltage should go to 3.5 volts at Wide Open Throttle.

Notes:

MAP Sensor exerts high authority over fuel delivery since it provides the PCM with vital information on engine load. Higher intake vacuum (lower MAP Sensor voltage) equals lower load/less fuel delivered; lower vacuum (higher voltage) equals higher load/more fuel delivered.

The MAP Sensor is a linear sensor; meaning as manifold pressure changes, signal voltage to the PCM changes proportionally. Zero manifold pressure is 0.5 V and full scale is 4.5V.

Bfritz97

I tested it acording to this procedure: http://troubleshootmyvehicle.com/jeep/4.0L/map-sensor-diagnostic-tests-1
Basically the same thing but with a vacuum Guage instead. I think I also tested it with the jeep idling and the voltage was too low, but I'm not 100% sure as this was a few weeks ago.

Anyway, the other day I put some electric grease in the connector and also unplugged and plugged in the pcm connectors. The CEL went off and the suddenly the jeep cranks for way longer than usual, but then starts fine, almost as if the pcm is relearning, so maybe the issue is fixed. I will see what happens.

I also tried testing continuity between the map connector and pcm plug, and I did verify the ground was ok, but I didn't know the connector #s at the pcm to check the rest. If problems persist, I will likely start here to check the wiring. If someone can provide a link to that info, that would be great.

Like I said, I will wait and see if the CEL comes back on and go from there