Here are some items that may have issues:

• Wastegate valve, solenoid and spring
• Motronic temperature sender
• Blower control box
• Throttle position sensor
• Starter relay
• Output shafts
• Additional air slide valve
• Thermostat/radiator coolant hose
• Hydraulic brake booster
• Crankshaft position sensors
• Engine control unit (ECU)
• Electrical/visco fans
• Ignition wires
• OBC relay
• Ignition switch
• Thermo time switch
• Oil pressure sender

Wastegate stuck closed makes car stutter when accelerating on boost. Boost will raise too far and ECU protection cuts fuel injection. Opposite can happen too, valve is stuck open and little or no boost is seen. Soaking wastegate in thin oil (WD40) may eventually free wastegate valve and make it again operational. When only 0.2bar is seen on full throttle, solenoid is not working/line always open. Double-check solenoid and all related connections before looking for a replacement parts. L-jetronic wastegate spring can wear too and show less than rated boost.

On thermostat housing blue temp. sender gives information to ECU. Faulty sensor makes engine cutout at >3500 rpm (‘boiling protection’). New Bosch 0280130025/Vdo 1709967 sensor solves this, good one measures 200 ohms hot. It may be good idea to replace all sensors on housing. BMW 11651277228 water temp. sensor is located on engine block. Late cars have blower inside front bumper for after cooling turbo (water >70C). Faulty timer unit is common cause for non-running blower. Replace or repair unit, it is at driver’s side of firewall. Check also blower in wire fuse next to battery.

Throttle position sensor should always be adjusted first. Faulty sensor can make erratic behaviour on eh-transmission, like downshift on cruise for no reason. Replace sensor if adjusting it does not make a difference. This sensor is also used for turbo management and according to BMW is vital component for engine performance. While adjusting sensor, check also idle switch and throttle body mechanical adjustment. All this info like other good guides are in Bentley repair manual for E28-series. There is no manual made for E23-series.

If car is occasionally unable to turn starter only fuel pump running, either starter relay is bad, solenoid is sticking or voltage drops in black/yellow wire to solenoid. This usually happens when engine is hot. Orange relay is located below dash on left side of steering pillar. While replacing relay verify that new relay is fit for placement. Also replacing fuel pump relay next to main fuse box is good idea. One more  to replace is white Motronic relay on passenger side of firewall. Motronic relay does not interrupt function of starter if it becomes faulty.

Shudder on acceleration when up shifting may come from worn output shafts. This happens especially when transmission is in sport mode. Shakeup can be heard and felt easily but is difficult to point out. Check shafts for play between ends and replace as needed. Dynamic vibration in relation to engine rpm probably comes from propeller shaft instead. On 745i turbo output shafts are working pretty hard, M106 engine outputs same torque on 1500 rpm as M30 engine on 4000 rpm. Like brakes, all E23-series share very same shafts, there is no exception here.

Additional air slide valve can cause problems on cold startup. Car is unable to hold idle unless revs are held up by pedal. Slide valve can be stuck and out of function. Cleaning valve with carburetor cleaner should help, but this may be non-permanent solution. Replacing valve with new one may be good idea. Replace also related rubber hoses, these are likely hard and cracked already. Also inspect small vacuum/pressure hoses on intake manifold (heater control, crankcase ventilation, fuel pressure regulator and turbo management).

Big coolant hose between thermostat and radiator can get damaged from touching water pump wheel. Wheel cuts hose open, coolant pressure is lost, ending likely to headgasket failure. This is more probable on aftermarket hoses that don’t have reinforcement on radiator side. In any case check gap between hose and wheel, adjust hose if needed to avoid any trouble. Bending hose away from wheel while tightening hose clamp should do it. Older M30 engines had hose support on lower timing chain cover, but there is no place for such on later models.

E23-series was first production car with ATE hydraulic brake booster. Similar booster can be found on other BMW’s later on. Booster is common failure item and pond of Atf under car is result of this. When front o-ring of booster doesn’t hold anymore, atf is able to escape on low pressure side of booster. As long there is oil in reservoir, brakes should operate as normal. Booster can be rebuild or replaced, new part is expensive. Brake master cylinder is good to renew at same time. It is needed to remove brake lines and master cylinder while working with booster.

Crankshaft position sensors are located in auto transmission bell housing and connect to engine wiring harness on top of intake manifold. One sensor is for reference point while cranking and second is for engine speed information. Faulty sensors can cause no start conditions, either not starting at all or problems starting hot engine until cooled down. Original sensors can be replaced with Bosch part 0261210001, this is with 820mm cable and should fit for both applications. OE sensor with 610mm cable is discontinued and longer version is offered as replacement.

Engine control unit (ECU) is electrical failure point in these turbos. Engine cranks and nothing else happens. Unit is located behind right-front speaker. Before going after another, check crankshaft position sensors and ignition system. Skilled electrian should be able to repair dead ECU, taken spares are on hand. When searching for another unit look from German Ebay, usually there are few for sale. Watch out for overpriced items on sale! With this 30-year-old equipment all are going to fail sooner or later.

Non-operational electrical fan on front of radiator raises coolant temperatures on hot climate. Test two temp sensors on low right hand side of radiator and replace faulty sensors. If fan still not operational next items to check are fuses, low speed resistor fixed to frame and fan itself. Fan can be rebuilt with new bearing and some pop rivets. Access fan by removing front bumper and turbo-blower piping underneath. For engine cooling also test visco fan on water pump wheel. If on cold start fan can’t be heard moving air around replace visco coupling straight away.

Bad ignition wires give issues on cold and hot engine. Special wire set was used in 745i because of knock-control unit, there is impulse sender on cylinder 1 wire. Wire set is not available as new, yet sender can be transferred over. Other than cyl. 1 wiring is all same to M30 engines. New sensor is available in Europe from BMW. Kingsbourne in USA used to make complete wiring for 745i but no more because sender being NLA there. Only NGK does make set for 3.4 745i, part is RC-BW1104 / 44239. Measuring resistance between connectors should show 5 k-ohm +-20%.

Bad solders in OBC relay box is common cause for intermitted stalling or no-start issues in E23/28 series. Box is responsible for antitheft system by cutting power to main relay. Main relay is powering ignition and fuel, without it in operation engine stalls or won’t start. Watch out for bypasses made to OBC relay wiring with bad connections. Connecting 1.5mm2 green and green/red wires in plug going to relay makes permanent feed to main relay bypassing antitheft circuitry. Best way is to follow these wires up to black plastic connectors and re-connect there.

Ignition switch is responsible for powering many elec. functions from starter to main relay. It can cause issues with engine main electrical system and accessories. These are sometimes obvious but also often intermitted. To access switch remove steering wheel, lower plastic column housing and turn signal relay. Switch is held by single stop-screw, use loctite threadlocker when replacing screw. Replace plastic wire straps too not let wires pinch between column housing. Steering lock cylinder is independent from switch and stays in place during this procedure.

Engine starts cold everytime but stalls at hot start. This comes worse if car has been sitting some after long drive. Bad thermo time switch (TTS) can cause cold start valve (CSV) to open when it really should not. CSV needs to stay closed above 35C water temp, remove brown plug and re-start car. If car starts fine without any stall in rpm, replace TTS. New part for 745i is Bosch F026T03100 (8sec / 35C), it is remanufactured part with electronics inside. Be aware new part triggers cold start valve for at least one second, try to start hot engine by holding throttle 1/2 way open.

Oil pressure sender is located on intake side, next to cylinder six. This somewhat common cause for M30 oil leak, that can escalate over time. Sender is used as input for dashboard oil pressure red light. It may be difficult to spot top of engine, but underside car it is easy to see, as it drips from where engine and transmission connect. Even original BMW sender might not last long, had one fail after only 24000 km’s. When sensor fails oil comes through the body, there is oil inside rubber cover. Do not over tighten sensor to aluminium cyl. head.

In general you can utilize BMW provided TROUBLESHOOTING TEST PLAN (please see Files/BMW M106 repair files/img115.jpg). Example for erratic idling during warm-up phase, the table lists coolant temperature sensor as one of possible culprits. Bad connection on two pins may give intermittent High impedance, that engine sees as very cold conditions. This makes idle running rich, rough and low rpm. Idle may dive while on move because of too rich mixture. As stop gap solution you can use two small flat Abiko connectors instead of old AMP connector, keep wires in correct positions!

Information on important E23 relays for locating original and replacing with new part:

Relay	BMW Part#	Location	Color
Fuel pump	61311373588	Main fusebox side	Blue
Starter	61311373588	Under steering wheel	Orange
EH transmission	61311243086	Behind right front speaker	Metal
Sunroof	61361388911	Under steering wheel	Metal
DME main	61361729004	Firewall passanger side	White
Power windows	61311373588	Under steering wheel	Blue
Unloader relays	61311373588	Main fusebox K5&K7	Blue