Start at the top and work to the bottom, without skipping any steps. The steps are listed in order of likelihood.
Idle or hard starting problems are most commonly caused by intake leaks and/or a sticky or defective Idle Control Valve (ICV). The only reliable method of locating intake leaks is to have a smoke test run on the intake and crank case and to test the brake booster with a gage and vacuum pump. The complete list of possible causes of an intake leak is:
Intake boot
Throttle body gasket
ICV hoses & connections
Brake booster, hoses, and connections
Crank case breather hose
Evaporative control hoses, valve, and expansion tank
Fuel pressure regulator & hose
Injector seals
Valve cover gaskets & bungs
Oil filler cap
Dip stick o-rings
Oil return tube o-rings
While leaks in some of those can be found by inspection or by spraying carb cleaner on suspect areas, not finding leaks that way doesn't eliminate the possibility. Only a smoke test will really work.
Once the possibility of intake leaks is eliminated, the ICV needs to be removed and cleaned with carb cleaner until the vane inside moves freely. When the ignition is switched on you should be able to feel vibration from the ICV. If no vibration the ICV is bad, there's a problem with its wiring or connector, there's a problem with the TPS, or the DME (or Idle Control Module (ICM) on an ETA car) is faulty.
For the DME (or ICM) to control idle, the idle switch in the TPS must work correctly. The switch should close when the throttle stop is 0.030-0.060" off the idle stop screw. Hearing a click doesn't mean the TPS is working. Check with a meter or test light.
The fuel system should be tested via the suite of tests in the Bentley manual as invalid rail pressure can be a contributor to idle and starting problems. A simple injector check is to pull the injectors, jumper the fuel relay to run the pump, and see if the injectors are leaking. You can also point the injectors into a towel, remove the coil wire, and crank the engine to see if all of the injectors appear to be spraying in a similar fashion. The best approach to possible injector problems is to have the injectors cleaned and flow tested. Since raw fuel can or will be released in these tests, have a fire extinguisher handy.
While a bad check valve in the high pressure pump can result in longer than normal cranking, if the fuel system is working as it is supposed to the rail will reach normal pressure in a few turns of the engine. A weak pump, clogged filter or leaking FPR in conjunction with a failed check valve can result in longer cranking and/or idle problems. The O2 sensor can be a contributor to idle problems. The O2 sensor is a scheduled maintenance item with a useful life of no more than 100k. If the sensor has that mileage or more (or is of unknown age), replace it.
The AFM can be a contributor. If the vane doesn't move freely or the resistance track is worn the DME may be receiving invalid data from the AFM. And if someone has fiddled with the bypass air adjustment the DME may be unable to stabilize idle. The bypass air adjustment should only be adjusted per the procedure in the Bentley and with an exhaust gas analyzer. And even then everything else associated with engine management has to first be operating properly. If the AFM becomes a suspect, replacement with a good used unit is the best approach.
Improperly adjusted or malfunctioning valves will affect idle and starting. As can compression issues from ring or cylinder wear. A valve adjustment is called for every 15k. A useful diagnostic is to run compression (dry and wet) and leak down tests on the engine. Aged ignition wires, plugs, distributor cap, or rotor can cause problems. Insulation does break down with time and heat. And since the youngest E30 is going on 20 years old, if the ignition system is original or the plugs are old replacement is indicated. Although not usually a problem, a bad DME temp sensor is a possibility. That generally won't cause an unstable idle, but can cause hard cold or hard hot starts and/or a rough idle. As can problems with the timing reference sensors. Although not commonly encountered, a bad harmonic balancer on an M20B25 or M30 engine will cause problems.
When all other possibilities have been eliminated and idle or starting problems persist, replacement of the DME, or if applicable the ICM, is indicated.
In many cases the cause of problems will be a combination of factors. So it is important to test and repair all of the possible causes.
Last edited by thejlevie; 02-09-2013 at 07:59 PM.