2.4 rough idle

01-29-2012, 03:49 PM
2001 Grand am 2.4. Runs fine when the engine is cold but when it warms up the idle is rough. When throttle is added from idle, RPM's drop, it hesitates, then either stalls or goes. Car runs just fine when there is a load on the engine (highway speeds). Sometimes it will even stall coming onto off ramps off the freeway. It intermittently shifts hard. SES light is on p0108. Have replaced the MAP sensor and checked for vacuum leaks, still have the light and the same problems. Any ideas what could be bad. Thanks for any help.

01-29-2012, 07:00 PM
Look up for vaccum leak... Good luck.

01-30-2012, 09:39 AM
p0108 means map voltage was ober 3.5 volts with engine load low.engine run time at least 40 seconds and throttle angle was under 30% with engine speed over 3000 rpm indicating a no load condition or throttle angle was under 2% with engine speed under 3000 rpm indicating a low load condition.map sensor voltage was above 3.5 volts during one of these load conditions.no code p0122 or p0123 present.check map sensor signal light green wire with the engine at idle.the voltage should be between 1-1.5 volts.if the voltage is wrong check the vacuum at the map sensor.it can be caused by a cracked or broken vacuum line to the sensor.also check map sensor ground black white wire ground voltage should be under 0.05 volts.if all these tests pass then do a sweep test by slowly applying vacuum to the sensor and watching the voltage output for a spike.note this is a one trip code that turns on the mil after first failure.

01-31-2012, 12:01 PM
Should I do these voltage checks on a warm or cold engine? I don't have a vacuum pump so I can't check vacuum however the hose is brand new and all seals look good.

01-31-2012, 01:44 PM
Ok I went out and shot it. Both brown and green wires were 4.5v. Ground was .04v. I do not have a vacuum pump however my OBD2 reader shows 28.6" at idle and it steadily drops when throttle is added. I did replace the cracked hose and the problem persists. Also bought a new MAP sensor and the problem is the same. New code also popped up "p0300 multiple random misfire". Any more ideas? Thanks for any help.

01-31-2012, 07:24 PM
connect negative terminal of meter to negative battery terminal and positive of meter to the light green signal wire by either backprobing the signal wire or by using a bed of nails or another probe that piercing the wire to get a reading with the connector connected......at idle you should get 1-1.5 volts dc and then connect to the ground wire for voltage drop on ground side which should be under 0.05 volts.

01-31-2012, 10:52 PM
Yes I did that. 4.5v on green wire at idle.

02-01-2012, 08:05 AM
you should have close to 5 volts koeo.disconnect connector and take a paper clip and put one in the brown wire the 5 volt reference wire and one in the black white wire and turn key to the on position engine off and see what the voltage is....it should be close to 5 volts.you need to tee in a vaccum gauge to the map sensor and see what vacuum is getting to the map sensor and need a vacuum pump to test the map sensor.....for every 5 inches of vacuum applied it should drop .7-1 volt.I use a mighty vac.was the new map sensor a new one or one from a bone yard or old car?a bad map will cause a misfire.

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