One of the first things to do is perform an ECU code readout with a portable scan tool to see if any fault codes are set. The tool can be purchased at most auto parts stores. Post the actual codes here on the forum so that we may be able to help you further. If there is one or more fault codes set, they can help point to the malfunction. If you have a copy of the FSM for your vehicle, the code readout procedure is described there along with a listing of codes. You can download a copy of the FSM sections from the Nico Club web site; here's the URL: http://www.********.com/FSM/
(replace the asterisks with "nico club" spelled as one word). The section EC.PDF is the one you need to read.
The common causes for rough idle are:
- Intake system vacuum leak. To check the intake system for a vacuum leak, attach a vacuum gauge to a full vacuum source. With the engine fully warmed up, the reading at idle should be 18 - 20 InHg. At 3,000 RPM, it should be 21 InHg. If readings are under 18 InHg, check the intake manifold nuts to make sure they are tight. The gasket may have failed; spray a water mist at the gasket to see if the gauge reading changes. Also check the intake plenum bellows at the throttle valve and at the MAF for cracks or loose clamps.
- Dirty fuel injectors.
- EGR valve. Check all the vacuum hose connections.
- Marginal performing Ignition coil.
- Bad spark plug wires or incorrect wiring.
- The cam timing may be incorrect.
If you've just replaced a head gasket or if you have a high mileage vehicles with over 100,000 miles are the most likely to start having engine miss problems (it can be less in some vehicles), then it's advisable to have a compression test performed to rule this out. If you have low compression on one or more cylinders, this could be indicative of a more serious condition that would need to be fixed immediately.
The normal procedure for a compression test is to remove all the spark plugs, insert a compression gauge into the spark plug hole, and then turning the engine over about three times. Record the reading and do this for all the cylinders. If there is more than a 20% difference in any of the readings, a further test would be required to determine if the variance problem is due to an issue with the piston rings, the valves or the head gasket.