Vehicle: 2006 Acura MDX, V6-3.5L, Automatic Transmission/Transaxle

Mileage: 194,551

Problem: The MDX was in the shop because the owner noticed the engine was idling rough. The rough idle was worse when the engine was cold. Additionally, the malfunction indicator light (MIL) would occasionally light up and then go out. The timing belt had been replaced not too long ago but the problem didn’t start after the belt was replaced.

Case Details: The technician noticed the MIL had come on during the test drive. Above 1500 rpm, the engine smoothed out and showed no lack of power. Checking for diagnostic trouble codes (DTCs) he found misfire DTCs for cylinders 1, 3, 4, 5 and 6, but the data stream showed no misfire counts. The technician checked the valve timing three times, and all the timing marks were spot on.

Since clogged EGR ports can cause random misfires but no EGR DTCs, the technician tried running an EGR test with a scan tool, but it was not a supported test on his scan tool, so he checked the EGR voltage with a voltmeter in the closed position. It was within specifications (between 1.1-1.2 volts). A quick smoke test determined there were no manifold vacuum leak issues.

The technician called an ALLDATA Tech-Assist consultant who advised him that a common cause of rough idle on these motors is valves that are out of adjustment. He said to check the manifold absolute pressure (MAP) voltage readings when the engine was warmed up and idling with no loads. Any reading more or less than 0.9v – 1.1 volts indicates that the valves may be out of adjustment. The consultant also advised not skipping the basics (spark plugs, fuel pressure, compression, head gasket etc.).

The MAP voltage turned out to be 1.3 volts at idle which indicated the valves could need adjustment.

Confirmed Repair: Many of the valves were tight so the technician made the appropriate adjustments. After the valve adjustment, the idle was glass smooth and the MAP voltage was 0.9 volts. Fixed!