MY 1990 740 B230F engine runs erratic at idle after about 15 minutes of opertaion. I removed the cylinder head because the plug threads needed to be chased. Installed new plugs, new head gasket, new intake gasket, measured head for warpage and found none. Installed and problem of running rough at idle began to occur(this was not a problem before I pulled the head). Subsequently I have replaced distributor cap rotor, plug wires, coil, timing belt, fuel filter, fuel pump check valeve, fuel pressure regulator, all to no avail. Problem still exists. Continued trouble shooting ran one can of sea foam through gas tank. Compression checked good, air filter good. Listened to injectors with stethescope while running and could hear clicking indicating proper operation. Ohm checked injectors 16.0 each. All hoses and connectors good. Found electronic control unit temp sensor to be reading high ohms throughout temperature range. Replaced temp sensor. Air mass sensor reads 2.8 ohms between terminals 2 and 3. Fault codes, test mode 1, table 1, 2-3-2, fuel trim(lambda control). Could not find sensor only line leading from exhaust across back of engine to bellows under intake manifold which led to egr system controller. 1-2-3 signal missing to/from coolant temp sensor. Table 2, 2-2-4, engine coolant temp sensor signal absent or faulty. Test mode 2, 3-3-4, throttle position switch operates properly. 1-4-1, rpm sensor good. 3-3-2, tps okay in idle position(when throttle is moved). 1-1-4, A/C control on/off good. Any suggestions from someone to possibly pinpoint problem.

have you checked the intake air bellows hose between air box & throttle body. Mine had large hole in underside visible only when removed and carefully inspected. the idle was rough but intermittent, so bad at times the steering wheel would shake severely at warm idle. '91 745 N/A 155K and now idles perfect.

thanks for your suggestion,the bellows was good.

do you know where the oxygen sensor is located. unable to find it on the exhaust sys. yet the error codes indicate there is one.

thanks

peter boe

I'm not sure excatly where o2sensor is but if you follow exhaust manifold back it should be easy to find. Do you have clean / properly working iac valve.

I found O2 sensor when I looked under the drivers side of the car to verify fuel pump operation, the O2 sensor was located just forward of the catalitic converter.Also found that temp sensor was the wrong part number & am replacing now, will also tonight measure O2 sensor millivolts while operating engine to verify if it is working properly. All valves pessed fault code monitoring, what does iac stand for?

Found it, (iac) idle air control is good. belive o2 sensor readings my be bad, researching this some more, no sense in replacing a good o2 sensor.