Opinions needed re: engine hesitation/vibrations

[shiver]

hey guys,

i would just like your help in diagnosing two problems that my car has, i am probably going to take it to the shop, but i would just like input from the M42 guys.  i know it is hard to diagnose problems over the internet, i will try to make it as descriptive as possible, so bare with me.

problem # 1:

sometimes when i start the car it will idle funny- going up and down between 800 and 500 rpm and then die.  i can prevent it from stalling if i give it some gas.  also, when i do get the car rolling, it seems to hesitate.  when i push down on the gas pedal, the car will slowly accelerate, it almost feels like there is a slight resistance.  then suddenly, the problem wears away and the car accelerates normally.  this problem occurs at different times, sometimes when the car is parked for a long time, and other times when the car has just been restarted after a drive.

problem # 2:

i was out for a drive last night and i noticed when accelerating in first, at about 4500-6000 rpm, a vibration occurs.  it also happens in second gear at around the same rpms, but is less noticeable.  the vibration almost feels like it comes from the rear end of the car, but i'm not 100% sure.  i checked the wheels and they were all torqued properly, and all the weights were still there.  also, the there is not vibration at high speeds when i'm coasting, so i am almost certain it is not wheel related.  i have no clue what this might be, if anyone has experienced anything similar and has diagnosed it, i would appreciate your input.

thanks for reading all of that, your help is much appreciated.

[bmwman91]

Well, #1 could be your idle control valve, although unlikely if it causes problems on-throttle.  Your oxygen sensor could also be going out, or there might be a really bad vacuum leak somewhere in there.  It is fuel-control system related most likely.  The air flow meter might even be the culprit.  There are a lot of things it could be.

#2 I would say check your driveshaft's center support bearing, tranny mounts, and guibo (flex disc...sits between tranny's output shaft & the driveshaft).  Look for cracks...the guibo is a little harder to check on-car, it might been to be removed.  I'd just replace it anyway if it has never been done.  The support bearing is an item that dies very frequently as well, but it easier to see if the rubber webbing has torn.

[Deutschbag]

Are you running aftermarket wheels? If you are, and you don't have hubcentric rings on them, you can still have vibration even if they're balanced properly and they're torqued correctly.

[shiver]

i'm rolling on stock weaves.  the vibration occured recently, and i've been running my wheel/tire setup for a while now.  i think i'll check out the things bmwman91 pointed out.

thanks guys.

[Febi Guibo]

your problem #1 could be a lot of things, but I would very carefully check your AFM unit; take it out and  check the seal around the top black plastic cover to see if its been opened and/or fiddled with; while it's out, clean the inside with carb cleaner very carefully.

Take it from me, don't open the electronic part up and mess with the sensor itself, just make sure the barn door is clean and works smoothly.

[shiver]

just a quick update.

i haven't had time to bring the car to a shop, so i'm still in the dark with regards to what the problems are.

anyway, last night, after going out for dinner, i started the car (1-2 hrs of sitting), and problem #1 occured again.  it was really bad, and the car was sputtering/stalling.  but this time, while accelerating (i was still getting the hesitation/resistance), the car was really sputtering and the check engine light went on.  i shut car off for about a minute, and when i restarted the check engine light disapeared, but the car continued to run rough.

this morning, the car ran without the sputtering/fluctuating idle, but i'm not convinced that the car is running too well.  it feels like (maybe its my imagination), that the car is down a few ponies.

i'm really anxious to get it into the shop and all sorted out.  i'll update my findings re: the problems/solutions.

thanks

[silverblades181]

I have problem #1 to a lesser extent I guess. I'm going to replace the O2 sensor and try to clean the AFM some way...keep me posted on what the shop says/does.

[bmwman91]

Well, from my experience messing with the air flow metering system, issues like that come from the sensor not working at all.  Check the connector and wiring for breaks...something may be coming loose.  O2 is unlikely.  When my wide-band died I was running with NO sensor at all for a week until the replacement arrived.  The car ran fine...just VERY VERY rich.  When the o2 dies the car goes into emergency-enrichments to ensure that the motor does not run lean.

On the other hand, if the AFM dies, it is a different story.  The idle will likely be very unsteady, and the engine will cease all power production under 2 conditions: 1)you try to rev it over ~3500RPM and 2)you open the throttle more than say ~20%.  If this sounds familiar, then I would put money on the AFM.

The carbon-track inside the AFM could be worn out.  It is repairable (I cannot remember where the writeups are...someone help him ou with that) without spending any $.

OH, and problem #2, I almost forgot.
There is a common problem where the muffler hanger will come in contact with the spare tire well.  At some RPM's the harmonics of the engine vibration will displace it enough to cause a lot of commotion back there.  Check to see if the PVC coating has been work away on the spare tire well around the front hanger.  If so, you can try bending the straps with a hammer, LARGE screwdriver, and some big pliers.

[shiver]

engine will cease all power production under 2 conditions: 1)you try to rev it over ~3500RPM and 2)you open the throttle more than say ~20%

by ceasing all power, do you mean it will die, or not accelerate too well?  i've taken the car to about 6k rpms with no problem, and have gone WOT a couple of times.  its just that the car doesn't seem to have the same pick-up as before.  

i think the o2 sensor is on its way out, i am smelling unburnt gas when the car is idling.  i don't know if that's whats causing the problem, but i think i'll replace it anyway- hopefully my mpg will improve.

with regards to the AFM dying, i don't think its completely dead.  the funny idling only occurs intermittently, and as i mentioned before, i was able to rev to 6000+k rpm no problem at WOT.

can't wait to get this all sorted out :confused:

[bmwman91]

You were able to rev to 6k when this problem was in its full effect?  If so, then yeah, I doubt it is the AFM.  The O2 dying will cause a little sluggishness, but nothing horrendous like it sounded like.  I guess it is probably the O2 that is the culprit.

[shiver]

actually, i was able to rev to 6k, but not as it was happening.  i never really got it to 6k when the problem was actually occuring.  but right after, when it would go away, the car could go to 6k.

well, there is a strange new development.  just some background info before i get into the whole story.  the PO had a mark D chip in the car.  when i purchased it, the owner removed it.

anyways.. today i left my lights on.  this obivously caused the battery to drain, and the car not being able to start.  but, i noticed when i jumped the car and got it running again, the time/date was reset.  does this mean that other things in the car reset as well?

when replacing chips, does the car have to be reset in anyway?  this might have caused the problem of intermittent rough idle and bad acceleration... i'm not really too sure if this even has anything to do with the battery dying, but ever since this morning, the car seems to be running better and no idling/accelerating problems have occured.  i'll wait a couple of days before i draw any conclusions.

[bmwman91]

Hmmmmm.

If a sensor goes south and you replace it without resetting teh ECU then yeah, all the self-adaptations will make it run poorly.  Any time you unlpug the battery or remove the ECU it is reset.

I was asking about the 6000RPM DURING the problem.  It still could be comethign AFM related (bad connector or something).  I know on mine, every time I reset the ECU it idles like garbage for a good 10 minutes.  I think it has issues running the ICV and it has to re-learn.  After that it is fine, although it never affected my on-throttle performance.  It could be that the EC is resetting for some reason intermittently, forgetting all its adaptive programming.

This is a tough one.  I have a spare AFM if you want to borrow it and see what happens, although I would do a thorough check for damaged connectors & wires.

[silverblades181]

It happens to me too. The car will idle very poorly, threatening to die at times, and I find that's when I'm in a parking lot, and running very slowly...I WOT it and nothing happens...until bam! it's back and it revs again. Then the problem won't occur until I shut the car dow and leave it there for a while. I also think it's more prone to happen when it's hot and humid outside. My wires, spark plugs etc. are all new. It doesn't do it everytime.

[bmwman91]

Wow, that is really weird.  This is definitely a problem with one of the EFI's input systems.  I wonder what it is.  For once, I am totally stumped on a control-system problem.

[shiver]

silverblades, that is exactly what happens to me!  oh, and the problem did come back, bringing it into the shop this week, so we'll see.

thanks for all the pointers.