Engine Acceleration like Horse Canter Ride

[BMWFL]

I have very frequently found answers in the forums but not this time, so I am hoping someone can point me in the right direction.

I am the 3rd owner of an E34 manufactured Jul 1991 with a M50B25 Non-Vanos engine, automatic transmission, with around 93k original miles exclusively using 93 octane fuel. Car is running with new Amsol synthetic oil and new Mahler air filter.

The car feels heavy and sluggish, it accelerates with difficulty and what it is a mystery for me is that it does it in "horse canter-like" manner. At constant throttle one can feel acceleration for about a second, then acceleration diminishes, and then it accelerates again acting like small spikes. It happens around every second as said.

This is also visible in the RPM, in the Speedometer and in the MPG gauge, in all these places the needles act accordingly, fluctuating in these spikes. While this issue is constant, weather seems to affect the "heaviness" of the car, being noticeable worse in hot and humid days.

I took the car to my BMW specialist and he does not have a clear idea on what the cause may be. The car had a 405 ECU on it and I swapped for another of the same. The car ran better virtually eliminating the spikes, but had an Engine light which the specialist's OBD-I scanner identified with a bad ECU. His suggestion was to replace the ECU and then check again.

So after reading a little about ECU's, I learnt that the 402 is the most "recent" one for my car (in-spite of having a lower number than the 405), and decided to try with this number. I currently have (2) 405's and (2) 402's, and after testing I get the same results with 3 of them (two of the 402's and one 405), the only ECU that feels it works better is the 405 producing the "Engine" light with a bad ECU code.

I have a feeling that the ECU is "checking" the engine every so often (perhaps once a second) and the issue may be a bad sensor reading, or perhaps also having to do with the MAF (from weather behavior), but so far everything seems to be working fine.

I assume that the other 3 ECU's work fine. One of the 402's has a Turner Chip installed, and although it produces the same acceleration "spikes" as the other 2 (perhaps a little less noticeably), it seems to idle at lower RPM's and when coming to a stop it feels like the engine will die, but it compensates allowing more gas and repeats this cycle. I do not feel is a gas pressure issue because it does it at all throttle ranges, from idle to full acceleration on the freeway, but I may be wrong.

I replaced the catalytic converter and the oxygen sensor about 10k miles ago, I have a new converter and oxygen sensor at hand for replacement if needed. Mechanically, everything else seems to be fine with the car, my mechanic does not seem to have a simple answer to this problem. I hope you can share with me your thoughts, Thanks!

[ross1]

I think you are describing surging. Typical cause of this is a vacuum leak.
I suspect all this DME talk is overthinking a simple problem.
Inspect the intake "boot" between the throttle body and the air flow sensor for cracks. If none then a smoke test will find any other places where unmetered air is getting in.

[BMWFL]

Thanks for the suggestion, I will check for leaks. I replaced the intake boot recently for a new one, but now that you mention it when accelerating I hear like a small exhaust hiss or leak on the driver's side, I thought it was from exhaust and was going to have it checked, I'll start there and make sure is not a vacuum leak. Thanks again!

[cirrusblau]

I think you are describing surging. Typical cause of this is a vacuum leak.
I suspect all this DME talk is overthinking a simple problem.
Inspect the intake "boot" between the throttle body and the air flow sensor for cracks. If none then a smoke test will find any other places where unmetered air is getting in.

If it's a vacuum leak, he should also experience erratic/surging idle right?

[Hofmeister]

If it's a vacuum leak, he should also experience erratic/surging idle right?

thats correct. I wish op ws closer id check it out for him. im in tallanasty

this might be a long shot, but check to see you dont have water or oil in the spark plug wells, my windshield washer gasket leaks onto cylinders 5 and 6 in heavy rain even if the car is parked. ill end with weird issues for a bit and eventually itll start runing only on 5

[fo3]

I've got a m50b25NV in a 09/90 e34. DME is 405
Specifically white label 0261 200 405
1730156
1735844
173 5614.003 078

Yours should be the same as mine because 09/90 - 09/91 were the same, and they made changes for 09/91-09/92. Yes, even all non vanos m50s differ. Ours are not exactly the same as an 09/91 m50b25NV

Pretty sure they're not all compatible with later ones (I got my numbers written down and saved because it matters ie lots of rechip places need these exact numbers, not just a simplified '402' or '405' )
90/91 NV e34s are a bit of a unicorn for parts in some respects. Different Air flow meter, couple of different sensors, and unrelated things like belts and tensioners.

My car was sluggish when I bought it, main cause was vacuum leaks. Seriously - replace every rubber thing in your car from vacuum hoses, inlet air pipe, IAC hoses, crankcase vent hoses, fuel lines, suspension, O rings etc just as a starting point!
Then look at ignition coils, fuel pump etc.

E:

thats correct. I wish op ws closer id check it out for him. im in tallanasty

this might be a long shot, but check to see you dont have water or oil in the spark plug wells, my windshield washer gasket leaks onto cylinders 5 and 6 in heavy rain even if the car is parked. ill end with weird issues for a bit and eventually itll start runing only on 5

This is how I found out my spark plug boots were bad. I would have been nice to replace coils but it's hard to budget getting parts from UK/USA to australia as there's no cheap in country parts supplier. Coils would be a poor freight to product cost unless I was making a huge order. If you're in the US get complete coils for $200. I settled on $70 for new boots and plugs.

DMEs rarely go bad unless coils are damaging it so I'd buy coils on OP's case if vacuum leak/rubber repairs/replacement doesn't fix it.

[ross1]

If it's a vacuum leak, he should also experience erratic/surging idle right?

Not always

[BMWFL]

Thank you Hofmeister, a vacuum leak seems to be the general consensus so far. I am getting so annoyed about these issues that I may end up driving to Talla just for your advice if you are open for that!

I love mechanics, the learning, and have a few tools to play with, but between the wife and 4 young kids I rarely have the time to do it myself anymore. I love this car, I was thinking on a paint job next and fixing a few other cosmetic details once I have the mechanics all worked out.

At present it is my daily driver, and although I don't put a lot of miles on it per week, I still hate these performance issues. It still has under 100k miles and the engine feels strong, I'm planning on keeping it for long time if possible. Thanks again!

[BMWFL]

Thank you too Fo3, I didn't know that these 09/90 - 09/91 were such unicorns. So far I have swapped two 405's and two 402's and haven't noticed any change in the "surging" behavior (now that I've learnt the right term for it from Ross1), what I mean is that it does it regardless of the ECU that is running under but I have not detected any other changes in the car. The only difference I could detect when swapping was a stronger erratic idle with a 402 ECU running with a Turner chip, but I am told that perhaps it would be expected.

If you don't mind, I will like to know more about your findings on our cars, I am sure it will be very helpful to me. Thanks again!

[fo3]

I know you said you can't work on it yourself, but the bare minimum you could do to check coils, boots and plugs.
1, check no water or oil in spark plug wells.
2, visually inspect boots for cracking, and spark plugs for wear/fouling/cracking.
3, check primary side coils (terminal 1 and 3 ie the two outside pins) should be 04-0.8 ohm.

Get your mech under the car to check/replace or clean IAC hoses to manifold, checking manifold plugs and orings (there's 2: one from crankcase vent and the other from fuel pressure reg E: due to the thin plastic inlet manifold these plugs are a loose fit at best so these o ring fittings commonly leak)

Or just get a mech to smoketest for induction leaks anywhere like main manifold o rings to the engine and throttle body. Also do a fuel pressure test.
See if you can get them to use something to test secondary windings of coils (if too hard just replace the coils, they're probably 28 years old anyway and you can buy bavauto ones or ngk ones pretty cheap).

But sparkplugs, boots, coil primary windings are an easy 30min job for yourself, easier than changing DMEs.

[BMWFL]

Thanks again fo3, I am getting to work on your suggestions. As mentioned I have limited time but at least I have now a very clear idea where to start looking and get doing.

I am browsing for hoses on Bavauto and while there I will also look at coils and plugs. I will also look for water and oil in spark plug wells. After receiving this valuable feedback I will replace everything rubber, or as much as I can, and I will leave the rest to my mechanic. I will also perform a fuel pressure test.

I do not know how long all of this will take me, as soon as I make real progress I will report back my findings. Thanks again!

- - - Updated - - -

My thanks to cirrusblau, ross1 and Hofmeister for their help. I appreciate the generous advise given and their sharing and instruction to lesser experienced guys like me.

[BMWFL]

OK guys, time to knock on your door for advise again. I have checked for a vacuum leak and found none, also nothing from a smoke test.

The car does not surge at idle, except one short spike perhaps every 2 to 3 minutes. The surging comes while driving, accelerating and also has happened less noticeably when down-shifting.

Does anyone has a suggestion on what other cause than a vacuum leak can cause this problem?

Thanks!

[fo3]

Like I said before, if not air/vacuum leaks, start looking at fuel pressure and ignition. Also things like battery health, neg earth (main one to engine and block, small one from the injectors to valve cover and I think one under the manifold that goes to a bosch plug I think)

Ever heard of diagnose dan on youtube?
Shows in one video a good mech can test a coil if they have a scope (don't really need an induction 'fly swatter' like he uses, probes on the secondary terminals to the scope should work).

https://www.youtube.com/watch?v=YVCaNtbuBoc

If you can't find a good mech with a scope, then try get some coils and boots and replace yourself. Unfortunately bavauto has closed, so the cheapest option is off the table.
Next cheapest option is take a gamble on s/h coils.
newer bremi coils are $30-40 at places like rock auto, turner/ecs and fcp (fcp costing more but they include boots unlike rock/ecs, and honestly you will need boots, and fcp having the lifetime warranty).
I'd pay more for NGK if not buying from FCP euro as bremi has had some failures in recent stock so get NGK if you're not getting a long warranty.

Of course fuel pressure check (and leak down, and flow checking is worthwhile)
Maybe take measurements of sensors (crank pos sensor, cam pos sensor, throttle position sensor, air flow meter)

After that, it could be DME like another diagnose dan vid (it shows coil testing without the fly swatter induction meter as well)
https://www.youtube.com/watch?v=RO12NiE5g1E

Late edit (took me ages to work out how to edit this post without it all being attached to the ending url link!): try this link for manuals to get specs for the sensors I mentioned. http://www.mediafire.com/?ijviizbkk26be

[ross1]

This isn't going to be a DME problem* so don't overthink this.
If you are CERTAIN there are no vacuum leaks then time to examine other things.
What is this "spike" while idling? An increase in speed** or a dip?
Weak coil(s) is a possibility and could cause a dip(dropping a cylinder momentarily)in idle speed. If you wish to experiment with this I have a box with probably 50# worth of coils for these cars. PM if you want some on the cheap

* assuming a correct DME I see you've been swapping these around. One of them having a "tuner" chip which in my eyes unless a well known and regarded one is always suspect. Also, I'm told, using a DME intended for an auto trans car in a manual application can cause problems with throttle being slow to come down while shifting, a "hanging" throttle some have called this.

** there is only ONE thing that can cause idle speed to increase and that is AIR, so either your idle valve is operating or you have a leak.
Is the idle valve clean?

[BMWFL]

Thanks again fo3 and to you ross1.

After replacing all hoses the mechanic performed a new smoke test and there are no more vacuum leaks.

The "spike" accelerates the car, it is seen in the MPG gauge, the tachometer, and depending on weather (the car seems to behave more sluggish in hot humid days) one can also feel the pull in each spike, more noticeably on cooler days. At a constant speed the surging is not felt or seen in the instruments, only when accelerating.

The car has an automatic transmission and idles perfect, there is no change in idling when parked, in neutral or when the transmission is engaged but the car is stopped like in a traffic light.

I will work on what you have recommended and post back.

Thanks again!

- - - Updated - - -

Can this be a MAF related problem?

[BMWFL]

I made a YouTube video, hope it helps.

https://youtu.be/S5FGWaIWzw0

Thanks!

[Hofmeister]

says video is unavailable

[Mixzzz]

ugh...It could be something like MAF or wiring problems to any of the sensors. You can try unplugging the MAF and going for a ride...It become more powerful but it will at least be smooth.

Giving second thought it seems like some sensor has died causing sluggishness and at those spikes the sensor "comes back to life" and dies again. Or the wiring problem could do the same