1993 525i M50 Petrol engine strange behaviour

[Daddario]

Hi guys! Hope you can help me with this strange problem of past few months turning car recently unusable.

After startup there's about two minute sequence when engine is running higher RPM (1OOO). After these two minutes RPM dropped to about 700. I haven't thought about it since engine was running well. This changed, when winter came. When this RPM drop occured when engine was still cold there was evident loss of power when accelerating from halt eventually with occurence of "explosive metal tube sound" somwhere in engine compartment. I just solved this by letting the engine to get warm by idling. I had to push and hold the throttle to get higher RPM because dropped 700RPM would cause near engine stall. From month to month i noticed poorer acceleration. The incredible M50 sound when valves opened was the same but acceleration response was poorer and poorer. Then yesterday on 4th gear at 120 kmh engine lost power completely. Constantly loosing RPM even with throttle pedal pushed. RPM stopped slightly below 5OO and no way of getting them up. When tried to do so by throttle the nearly forgotten explosive sound came again. I shut down the engine and start it again. RPM 75O. Running well. A again went to a road. After mentioned two minutes it happened again. Can someone help, please? I absolutely have no idea what causes engine runnig well for the first two minutes and what then causes the engine power drop and stalls. Thanks.

[BillionPa]

could be the crank sensor. if it misses a beat it could cause backfire (metal tube sound?) and engine stalling.

did you get any sort of check engine light?

1000rpms at startup and 700rpms when the coolant reaches a certain temp is indeed withing proper idle speed parameters for an M50

other information would help, as much as possible.

what kind of motor oil, how many miles since it was changed.
how long since last coolant change.
auto or manual transmission.
recent maintenence.
etc, etc.

if you have not changed your spark plugs in a while, pull them and check for fouling. large amounts of carbon deposits could be responsible for causing preignition, which would cause the knock sensor to dial back the timing and significantly reduce engine power output.

[Daddario]

I personally suspected MAF went wrong. As I found on this forum, disconnecting it would force ECU to act in some kind of deafult mode. And so I did it. First two starts same song. Two minutes ran well willing to accelarate. After that idle RPM dropped now to about 550. Unable to get them up followed by the like shooting sound. Second try the same. On third try (like ECU was trying to get used to new conditions) after two minutes RPM dropped to 55O. But no stall. Willing to accelerate. Acting calmly. But sometimes when accelerating from this low idle shooting sound occurs. But without impact on engine run. I am still not sure if MAF is the cause since the shooting persits. I rather decided to not to drive my BMW under these circumstances. I read in Haynes manual that O2 sensor starts to act when it gets warm and it's about two minutes. Could it be the cause of such problems. When I bought the car, the guy told me I might need to change it in a while. He had the car on diagnosis and it was showing bad O2. Although when I made emmissions test nothing pointed to that so I decided to let it go since engine was running well. But still the two minutes interval is a mystery to me. It doesn't matter if engine is cold or warm. It's still there.

Car has 207000 Km. Oil changed at 202000 km (5W-40 fully synthetic). Coolant changed three months ago. Manual transmission. I didn't check spark plugs and don't know when they were changed. Because engine started always on first try I didn't care about them. Maybe I should. Recent servicing includes: Changing brake master cylinder, clutch cylinder and completely new clutch set.

Thanks for your help.

[Daddario]

According to check engine light I noticed it during the stall to flash briefly for just a second and also when trying to get from the stall. But not regularly...

[BillionPa]

get new plugs and new O2 sensor, new air filter, new fuel filter.

the problem seems to lie with the O2 sensor given that interval, however you must not neglect the rest of the car. O2 sensors are very important for performance and fuel economy, not just emissions.

plug the maf back in.
while you are working on the car, run a low amp trickle charger on the battery.

after you get all those parts replaced, pull the harness off the ECU for an hour to clear its lambda map.

then start the car and let it warm up for exactly 120s and start driving. go directly to the highway, drive on there for 10 mins, and then go back home using back roads.

this will allow the new lambda map to be generated with a hot sensor.

double check inside the engine bay for vacuum leaks or any electronic component that looks.... old. if your crank position sensor has rust all over the tip, get a new one! if you have the time and a multimeter, check the resistence of the coil packs on the spark plugs, and make sure one of them isnt way out of line compared to the others.

[Daddario]

Just waiting for parts to be delivered I told myself to try the self-diagnosis for any fault codes stored. I turned the key to the last position before start-up, pushed throttle for five times fully down and up within five seconds. But nothing happened. Tried many times. Turning ignition on and off again. But nothing. Does this point to a faulty throttle position sensor? Or the screw on throttle body is tightened tooo much and won't let throttle plate to return to zero position? Anyone has any experience with that? Thank you.

[pheno]

after you get all those parts replaced, pull the harness off the ECU for an hour to clear its lambda map.

Sorry OT but what is lambda map ?

[BillionPa]

the MAF sensor determines how much air is entering the engine.
the O2 sensor determines how much air is leaving the engine.

the Lambda map takes O2 sensor data to change the way the MAF sensor measures air.

that doesnt mean the MAF is wrong, it could mean the fuel pressure regulator is a bit off, or the injectors are not flowing to spec.

the way it works is the MAF is saying you are sucking in a certain amount of air, the ECU injects the specified amount of fuel, but the O2 sensor is telling the ECU it doesnt match for optimal combustion and adds a point to the lambda map at that RPM and MAF reading for a new fuel injection amount.

the more points on the map, the more accurate it will be, since it interpolates missing points.

[pheno]

Thanks for very clear explanation. So basically if I imagine following scenario: I've been running for a long time with this ECU and it has stored many values to lambda map. If I do ECU reset which clears lambda map and I find my car runs like crap can I assume there's something wrong in the car (some injector , spark plug,etc. faulty?

[Daddario]

Hi guys!

So I've finally found time to change the plugs. My weapon of choice were NGK BKR6EK. Few unpleasant surprises were expecting me. I started from 1st cylinder. Spark plug that was there was so loose I can unscrew it by hand holding the spark plug extension socket. It was dumping in black thick oil and was heavily carbonized. I wiped the spark plug hole out with clean cloth to suck the oil. Spark plug itself was Czech make Brisk, Premium line. I will post some pictures tomorrow. I am not sure if it was even supposed to be in this engine. Second cylinder, same song. Third was so tight and dry I had problems getting her out. Thread was rusty and plug heavily carbonized. Fourth loose again, dumping in oil. Fifth more tighter than third, more rusty, carbonized and dry. Sixth, the most loose, most oily and carbonized. I fitted new ones. Used a bit of oil to thread on third and fifth to get them smoothly in. Tightened the with torque wrench to 5ONm. And started up the engine. I swear I never heard it running so smooth and calm at idle. Beautiful transition from idle to higher RPM. I noticed slower engine warmup than usual. I plugged the MAF back. Idle OK. But when pushing throttle I felt like slowered reaction between throttle move and engine acceleration. A hesitation. Like mixture was for a moment too lean. But no more intake shooting. I'll try to borrow a MAF from junkyard and try it instead of mine. Any other clues what might be causing this? Thanks again, guys.


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