Here are some items that may have issues:
- Wastegate valve and spring
- Motronic coolant sensor
- 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
Wastegate stuck close can make car stutter when accelerating on boost. Boost will raise too far and stock control unit cuts out fuel injection. Also opposite can happen, valve is stuck open and little or no boost is seen. Soaking wastegate in thin oil like WD40 may eventually free wastegate valve and make it again operational. Spring on wastegate can wear-out and show inability to build boost, only 0.2bar may be seen on full acceleration and car simply feels slow. Double-check all wastegate related connections before looking for a replacement part.
On thermostat housing blue temperature sender gives information to control unit. Faulty sensor makes engine cutout at fixed rpm. New sensor from Bosch (0280130025) or Vdo (1709967) solves this, good sensor measures ~200 ohms when hot. It may be good idea to replace all three sensor on housing. Later cars have blower inside front bumper for cooling turbo after shutoff (above 70C). Faulty timer unit is probable cause for non-running blower. Replace or repair original unit, it is near 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 and fuel pump can be heard running, most likely starter relay is going bad. This usually happens when engine is in operating temperature. Orange relay is located below dash on left side of steering pillar. Replacing relay solves this issue, verify that new relay is fit for placement. Also replacing fuel pump relay next to main fuse box is good idea. One more relay to replace is Motronic relay on passenger side of firewall. Motronic relay should 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 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 OEM sensors can be replaced with Bosch part 0261210001, this is with 820mm cable and should fit for both applications. OEM 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 for overpriced items on Ebay, in euros 200 to 300 is reasonable price for used part. 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 can give several issues on cold or hot engine. Special wire set was used in 745i because of knock-control unit, there is additional impulse sender on cylinder 1 wire. Wire set is not available as new, yet sender can be transferred over to another set. Other than cylinder 1 wiring is all same to M30 engines. New sensor is available in Europe at least from BMW. Kingsbourne in USA used to make complete wiring for 745i but no more because sender being NLA there. Measuring resistance between connectors should show 5 k-ohm +-20%.