525i 24V-M50 still not running since new year

[EdNL]

Hi, I hope someone has a good tip for me:
My 1990 European 525i 24V with M50 engine hasn't driven any kilometers since newyears day.

This is what happens:
The car starts just fine. It revs up to about 1000RPM and then almost immediately it dies. If I try to keep it running by depressing the pedal quite much, it stutters and runs very violently, even backfiring a lot in the intake.

This is what I've done in an attempt to diagnose the problem:

• Carsoft 3.4 diagnostics: No faults in the log of the DME
• Replaced fuel pump, meassured fuel output (0.8 liter/30 seconds) at injection rail --> OK (Bentley)
• Replaced starter motor (check thread about starter motor with white smoke)
• Checked the ignition coils resistance (Bentley Manual)
• Replaced the spark plugs
• Checked the air filter --> was clean

Any ideas?
I really wonder why it backfires so violently in the intake? That would mean a severely bad timing. The strange thing is that it starts just fine, just before it dies.

The problem started a few days before newyears day. I wanted to start the engine. It would rev up to 1000-1500 rpm and then it would die. Second try, it would die again. Third try I depressed the gas pedal and then it would run. After that I comfortably drove 100 kilometers at 60-90mph. Old years eve I drove the car from the road to the place next to our house and since then it's been there, sadly...

[Jon K]

Sounds like an intake issue - check idle control valve, check to make sure MAF is connected properly, check to make sure elbow rubber intake hose has no cracks, check to make sure crank case ventilation hose is connected, check for HUGE vacuum leaks.

[EdNL]

Sounds like an intake issue - check idle control valve, check to make sure MAF is connected properly, check to make sure elbow rubber intake hose has no cracks, check to make sure crank case ventilation hose is connected, check for HUGE vacuum leaks.

Thanks!
I'll check it out today.

But I wonder: Can problems in the intake electronics/valves cause such violent backfires into the intake manifold?

Ed, the Netherlands

[mystical_wings]

sounds like the MAf to me I have that problem before and sounds exactly the same.

[molesz]

look arround the ICV (idle control valve) and attached hoses, I had the same symptoms when the vacuum hose from the ICV detached from the intake manifold.

[Beemr750]

Sounds like a ign issue if it idles. Check distributor etc if you have it.and other ign components,also timing settings.Check frt crankshaft pulley being tight and not shifting.

[Gayle]

Hi, I hope someone has a good tip for me:
My 1990 European 525i 24V with M50 engine hasn't driven any kilometers since newyears day.

This is what happens:
The car starts just fine. It revs up to about 1000RPM and then almost immediately it dies. If I try to keep it running by depressing the pedal quite much, it stutters and runs very violently, even backfiring a lot in the intake.

This is what I've done in an attempt to diagnose the problem:

• Carsoft 3.4 diagnostics: No faults in the log of the DME
• Replaced fuel pump, meassured fuel output (0.8 liter/30 seconds) at injection rail --> OK (Bentley)
• Replaced starter motor (check thread about starter motor with white smoke)
• Checked the ignition coils resistance (Bentley Manual)
• Replaced the spark plugs
• Checked the air filter --> was clean

Any ideas?
I really wonder why it backfires so violently in the intake? That would mean a severely bad timing. The strange thing is that it starts just fine, just before it dies.

The problem started a few days before newyears day. I wanted to start the engine. It would rev up to 1000-1500 rpm and then it would die. Second try, it would die again. Third try I depressed the gas pedal and then it would run. After that I comfortably drove 100 kilometers at 60-90mph. Old years eve I drove the car from the road to the place next to our house and since then it's been there, sadly...

No ideas for you, but what a great post! The information is all there and organized and easy to read. This should be saved as an example of how to do it.

[Brandon J]

I agree with Gayle. Very nice organized post.

Now do try what others have suggested. One thing I noticed you said is that there is backfire, whence you changed the spark plugs and checked the individual coils. Checking the individual coils resistance does not always give the true results. Some coils are bad only under load or higher rpms. Direct ignition coils are even harder to diagnose. The method used? Buy 1 known good one, either used or new. Then switch with each cylinder one at a time. If you have a bad coil, then you will know as the car will run well when you switch the bad one. It is also a possibility that if a coil goes bad, it takes the ECU box too. Dealerships normally practice swapping in a new ECU as well. Sorry to say that if it is the ECU box as well, you will just have to buy a good one. No way to tell if it is bad unless you try it out on a good running car.

You have done well to try to cover the fuel, air and ignition parts. Did you check the codes after you ran the car for 100km? Maybe there are new fault codes. Hmm, it is a little baffling as I don't have your car in front of me. I don't know how the car sounds or backfires, and even when the backfire occurs. It could be from the MAF, crank sensor, to vacuum leaks, to a failing fuel pump. Failing fuel pumps can still run intermittently, I use to kick the bottom of the tank of my old Grand Cherokee to get the fuel pump running temporarily. That pump was replaced 3 times.

Good luck with your diagnosis.

[EdNL]

I agree with Gayle. Very nice organized post.

Thanks for the compliments
I figured that I would have the best chance of getting good feedback that way. Thanks also for your tips/ideas.

I've done some more diagnosing, but haven't found the culprit yet:

• I disconnected the cable from the MAF, so the engine would run in limb mode (Bentley says so), didn't help, but would run (a very poor) idle at +/- 500rpm with lots of backfires, etc. Seems like the MAF is not the main cause.
• Vacuum hoses all seem to be connected just fine.
• I reran the Carsoft diagnostics and noticed not a single fault appeared and also no information on the ECU was given at all.. The ECU is probably gone.

I will receive a new DME/ECU with warranty tomorrow and will try that. I just hope it won't blow up. Luckily I found one for a very reasonable price, 75 euro.

I learned a lot about the M50 anyway. My previous M20 525i was much less advanced. It had a single ignition coil instead of six and hence it also had a distributor. Furthermore the throttle position sensor only had 3 modes: idle, somewhere between idle and kickdown and the third mode: kickdown. The M50 has a variable sensor which records anything between idle and kickdown and returns a variable voltage. That's why the MAF is not neccessary to run, whereas it is for the M20 and M30 engines.

On the other hand: Working on the M20 was easier I think. Replacing a startmotor without having to remove the intake manifold, better access to the idle control valve. Big advantages of the M50 ofcourse: more power, more torque, "theoretically" less prone to electrical (high voltage) faults because of distributorless design, and the chain instead of the belt is a big pro.

I've driven BMW for many years now, E30 316 (carburettor, 960kg, really nice, started my BMW addiction ), then an E30 320i (2nd type with cat), an E30 320i (1st type without cat, lot faster and more fun and better MPG), E34 525i M20 and now the E34 525i M50. This is the first time that a BMW has failed on me, not being able to drive in it. It's been off the road for exactly a month now.

I will also test the throttle position sensor tomorrow. I don't think the idle control valve can have much to do with it, because the problem occurs everywhere, at idle but also at kickdown and anywhere in between. But I will give it a look too.

A question for the Carsoft 3.4 experts:
I tested it on my father's 530i M30 and it returned a full spec-sheet for the ECU/DME with all Bosch partnumbers. It then showed there were no faults recorded.

When I check the ECU of my 525i M50 it returns nothing at all. It just says "press enter to continue". No specsheet at all, no error code, no "OK, it's fine" or "No response" or something like that. Just a blank screen, that's it. I wonder if that is a big sign that the ECU/DME is in fact dead already.

All other systems test just fine. The combi instrument panel, the door-lock and power windows computer (cool feat: controlling those from outside the car).

Well, will keep you updated.

Greetings from the Netherlands
Ed

[Espen]

Did you locate the fault?
Could be handy for others reading this forum.