My 535i bucks like a bronco

[misfortune]

I'm at freakin wits end. My car has been spitting and sputtering for close to a month now.

I've been experiencing power loss quite frequently while driving. Its really really really embarassing. The symptoms, as best as I can describe them, are basically that I will lose power for a short time. I'll mash the gas down and get no power at all. The power will then come back for a short period, maybe less than a second then will go away. THe car will rock in response to such uneven power delivery. It gets really violent and inconsistent. Eventually the car will stall out completely.

Sometimes at a stop light, the car will begin surging on idle. I'll put it in gear and it'll begin the bucking once again. My indy has no clue himself. He's done a smoke test, checked the MAF/AFM, checekd the throttle position sensor, changed my fuel filter, changed my ignition coil and whatever else. While its bucking the CEL comes on, then it will go away. When it was really really bad before, the CEL stayed on and it threw the 07 (bad afm) code. Its not the afm. He says its not the 02 sensor. (he's pretty hardheaded but very very thorough and I trust him)

Anyhow when the car does run, the power delivery isn't quite right. I mean I know how the car should run and I'm not getting the proper 535i jolt off the line. It also revs ok, but the needle on the tach sort of stutters across and I don't feel like the power is all there.

The car doesn't show the symptoms as bad after a day on the road, but the early going is ****ing hellish. I'm talking 20 minutes of nothing but bucking around. I'm so sick of this ****, and I have a chunk of money to throw at the tires/wheels/body work/full on repaint, (thanks to side jobs and insurance) but no point if the car isn't running. Please help .info guys. I have no ****ing clue where to go from here

[bill g]

Sounds just like a bad AFM. The AFM itself may check out OK, but what about the wiring between the AFM and ECU - did your guy check it ?
For example if the AFM signal wire is touching earth or being shorted or interrupted you will get this sort of behaviour.

Bill G

[BillionPa]

sometimes you dont want a mechanic to check into problems like this but rather an electrical engineer.

first a couple of questions
1: DO YOU HAVE EML?? i assume not?
2: is the alternator output voltage steady when the car bucks?
3: has operational fuel pressure been tested?
4: how many miles on the oxygen sensor?

i would replace the following unless it has been replaced recently:

air flow meter (preferably replaced with a MAF sensor)
coolant temp sensors (if you have 3, one is for EML, one for DME, one for dash, first 2 are important)
crankshaft timing pulse generator
ignition pulse generator
ignition coil
all rubber vacuum lines and fuel lines in engine bay (probably not a big deal but its good to do)
fuel pressure regulator
fuel filter

check the electrical wiring to the fuel sending unit for corrosion.
the answers to questions 1-3 will be important in figuring it out, as well as a detailed list of things he has replaced (not things he has checked)

[ryan roopnarine]

ummmm.....
what does the exhaust smell like when it's doing all of this?

[Dave M]

I had similar symptoms a while back. Bucked hard periodically. Check out thisold thread, maybe it will help.

Good luck with it.

Dave

[KenB]

fuel pump dying?

[misfortune]

Still have the same problem. Someone help. I've bothered swhersted and he's busy.

Its less frequent, I had some swap the fuel pump, but it still happens. Yesterday after stop and go traffic for close to an hour it was doing this....badly. It also threw an error code that I didn't get to check. I kill it, go inside for an hour come out, and its back to normal: less power, a bit of backfiring, but still running ok.


Re-reading dave's post, and doing some thingking on my own....this must be electrical. It happens when it gets hot, but when it sits, its fine. Any off the hip suggestions?

[leicesterboy15]

Does it smell or smoke like its over fuelling? One of my intake hoses was replaced with a cheap one and it started to leak. This meant the car was over fuelling as the air was escaping (thats my theory). It used to buck and run rough like you suggest (no CEL though). The culprit was a small vacuum hose that ran from the top of the front of the engine through the exhaust manifold and into the engine (I think that was where it went, it was a while ago). I replaced this hose and all was well. Maybe the MAF is a red herring due to a vacuum leak like this? Is it surging like it regains power or is it dying like it looses power (there is a difference)? My old Merc used to hesitate like it was dying and this turned out to be a bad HT lead.

Can't you test the MAF by disconnecting it (anyone confirm) before shelling out on another?

[Ferret]

What you're describing is usually caused by a massive vaccuum leak that may not be immediately obvious.

However I've seen a bad crank position sensor do this too - when it warms up it paritally packs in and causes the engine to run like ****.

[Derek A.]

that thred was geared towards an M50 not your M30.

Have you verified fuel pressure ?

Have in inspected the distributor cap and rotor for cracks ?

have you pulled a spark plug to see what the motor is doing - running lean or rich?

You said the indy checked the AFM - yet your car is throwing a code. What did he do- did he swap it out with a known good unit? Did he put it ona diagnostic tool and verify the values ?

Are the valves set right? Cam wiped out?