750i bad idle when warm

[GoldenNut]

Hi everyone,

I am facing a problem with my 1987 750i for a while now and I am not sure what to do next. When the car is started, the idle behaves as it should, the throttle pedal is responsive and the exhaust doesn't smell that bad (for a 35 year old V12). However, if I drive the car for a bit and it starts to warm up, it seems to runs rich, after the throttle pedal is pushed it takes a while before the car takes off, the fuel consumption is higher than usual and the exhaust smells like unburnt fuel (the car also didn't pass an emissions test, therefore it didn't pass the mandatory state inspection, the emissions guy said that it is running rich). Also, when the car is warm, the idle in P or N goes up and down repeatedly (between approx. 300-800 rpm). When I removed some of the easy to reach (new, less than 1000 km) spark plugs, they were dark and smelled like fuel.

I have already replaced a lot of parts that could have an effect on this, some before this issue started: new spark plugs , new ignition cables, new distributor caps and rotors, new fuel pressure regulators, new fuel pumps and filters, new O2 sensors, cleaned the throttle bodies and resynchronized them (this helped with the throttle response, but it still isn't great), resealed the intake manifolds, along with valve covers and banjo bolts, replaced a cracked intake boot between the throttle body and MAF, replaced a cracked vacuum line. I alway used either original BOSCH parts or other well-known brands/OEM parts.

I wanted to check the coolant sensors at the back of the engine and I found out that the one furthest from the center of the engine was unplugged and the connector of the one closest to the center was loose. I plugged in both sensors correctly but the issue still persists. I wanted to measure the sensors, but reached only the one that was unplugged and it read over 7kOhms (the engine was cold, ambient temperature around 15C). Could this be the problem? I was also thinking about buying new injectors, because the old ones were prety filthy and definitely original, but they probably aren't causing this issue as it seems to be temperature dependent.

Is there something else I could easily check with a multimeter? I sadly don't have a diagnostic tool or a smoke machine and the mechanics in my area aren't very helpful when it comes to this car.

Sorry for the lengthy post, hopefully someone will be able to help me.

[shogun]

7kOhms at 15 degree C seems to be wrong. The one closest to the center is important as it is for the EML, and the EML also controls idle.
if the coolant and temperature sensors are not connected or bad, then the system will go to pre-set emergency values, which means it always uses data for a cold engine. Not warm. That is why you could have problems when engine is warm.
Here are the test data which you can test on the DME pins, remove the cap on top of the DME connector and use a multimeter, then pins are marked with numbers http://www.europeantransmission.com/...out1988_90.pdf
sensors from fender to middle of the engine: http://bmwfans.info/parts-catalog/E3...g_system_pipe/
# 8 12621710535 2-POL M14x1,5 , black 5000 Ohm, for temperature gauge sender, spec by Hella: M14x1,5, 2 pole, NTC-Sensor; resistance [Ohm]: 5000, 544 ; color: schwarz, [V]: 12. This one is not so important as it is only for the cluster temperature gauge.
# 7 temperature sensor water 12621288158 M14X1,5 engine coolant temperature sensor for DME, (20 deg. C, 2500 Ohm.) (White Top) 12621288158
# 5 temperature sensor, water, 13621707366 3-POL-2500-OHM , EML coolant temperature sensor

also check the ground cable from engine to chassis on right side of engine near the engine mount, bad ground can also cause hunting idle.

[shogun]

here a pic with numbered temperature sensors in the back of the M70 engine, easier to understand
Wasserbrcke.png
Nr.... Sensor type.......................................connect s to
1 intake air sensor bank 1......................DME1
2 intake air sensor bank 2......................DME2
3 double temperature sensor coolant.......each one sensor for each DME
4 temperature coolant............................EML
5 temperature coolant............................temperature gage in instrument cluster

in front of the engine we have these
Stecker.png

Nr................Sensor type.........................................goes to
1......crankshaft position sensor CPS bank 1...............DME1
2......crankshaft position sensor CPS bank 2...............DME2
3...cylinder identification sensor CID / donut bank 2....DME2
4...cylinder identification sensor CID / donut bank 1....DME1

Bosch 2500Ohm Sensors, NTC 2500Ohm at 20°C
Temperatur [°C] ......R [Ohm]
-40........................45.313
-30........................26.113
-20........................15.462
-10.........................9.397
0............................5.896
10..........................3.792
20..........................2.500
30..........................1.707
40..........................1.175
50...........................834
60...........................596
70...........................436
80...........................323
90...........................243
100.........................187
110.........................144
120.........................113
130..........................89


Bosch 5kOhm Sensor, NTC 5kOhm at 25°C +/-2%
Temperature [°C]......R [Ohm]
-30..........................88.700
-25..........................65.300
-20..........................48.500
-15..........................36.400
-10...........................27.600
-5.............................21.100
0...............................16.300
5...............................12,700
10...............................9.300
15...............................7.800
20...............................6.200
25...............................5.000
30...............................4.000
35...............................3.200
40...............................2.600
45...............................2100
50...............................1.800

[GoldenNut]

Thank you for the tips. Measured the resistance on both DMEs, they were within spec (2.38 kOhms at 22 degrees C). Also tried to measure the coolant sensor for the EML (number 4 on the first picture in your post) via the connector at the EML but couldn't get any reading. I tried pins 31 and 19, and 45 and 19 mentioned here https://www.bimmerforums.com/forum/s...frence-pin-out. Did I use the right pins?

Also, the engine ground is cleaned but it didn't help.

Changed the injectors for the Bosch updated design with four holes, the engine now runs a lot better but the idle is still not fixed and I think it still runs rich - the exhaust still smells quite bad and on idle the consumption has to be above 20l/100km because the OBC showed about 19l after a short drive and it started to rise when stationary.

[shogun]

check once more the EML pinout based on your built year in the ETM 1987 https://shark.armchair.mb.ca/~dave/BMW/e32/
also check the throttle valves https://www.bimmerforums.com/forum/s...tle-flap-shaft
my one was stuck a bit and caused bad idle, also had leaks

[GoldenNut]

Finally got around to work on the car again. I once again tried to measure the temperature sensor on the EML pins 45 and 31 but it didn't read anything. Then I noticed that the wiring from the sensors for the EML and the temperature gauge was not factory (the wires were red and white on both sensors, these were not the factory colors), so I went through the electrical diagrams and noticed that the connectors for the temperature gauge sensor and EML sensor were swapped. When I plugged them in correctly, I could measure on pins 31 and 45 on the EML harness around 3 kOhm, which is within spec given the temperature was around 15C. The engine now does not run as rich (at least I think, have not had it tested for emissions yet) and the throttle response is much better. I still have the problem with the idle though. I found a good deal on a pair of MAF sensors which are supposed to be tested and working correctly so I will try to swap them (or atleast keep them as a spare). Also, I will try to somehow smoke test the engine, maybe there are still some leaks (like the one you linked Shogun, thank you).

[shogun]

good find, thanks for the info
MAF; check the wiring to the MAF, we have has several cases where the wires to the MAF were broken just before the plug from the wire loom goes into the MAF counterpart plug, there is a sharp bend in the plug wire before the MAF, the outer sheath of the wire loom is tone hard now and if that breaks, it cuts into the thin wires inside the loom.
Testing can be done at the plug, see here
http://bmwe32.masscom.net/sean750/MA...leshooting.htm
http://www.ow.no/index.php?option=co...d=19&Itemid=13

[GoldenNut]

I would like to add more information to this thread, maybe it can help someone in the future. I ended up replacing the crankcase ventilation valve and the corresponding seal (number 1 and 2 here https://www.realoem.com/bmw/enUS/sho...diagId=11_4618). This solved the problems with idle (i also replaced the coils at the same time but i dont think they were the issue). Also found out that one of the donut sensors was dead (not sure if completely, the engine still ran on all 12 cylinders), fuel consumption and throttle response improved after replacing with a good used one.

After that, I found out that my car was worked on by monkeys. After plugging the connectors to the temperature sensor the right way, the engine would never warm up, it almost didn't move out of the blue area even after driving for several kilometers. I bought a new thermostat and went to replace it. When I took off the housing, there was NO THERMOSTAT, only the outer part of it so it wouldn't leak. Then a few days later, when removing the exhaust, I noticed that the O2 sensors were swapped. After rectifying these issues and putting in Shoguns chips the car runs awesome. The only thing bothering me right now is an exhaust smell in the cabin. There are no exhaust leaks and i think the car doesn't run rich (fuel consumption around 12l/100km), almost everything regarding the ignition system is new (only MAFs are untested, will do after finishing up suspension work), there are new catalytic converters (aftermarket). It is really annoying because I have to change my clothes after every drive. The weird thing is, when all windows are closed i can't smell anything, only when the windows/sunroof are open.

[shogun]

exhaust/engine smell in cabin https://www.bimmerforums.com/forum/s...disintegrating