weird fix for erratic idle, backfiring engine

[dreamaker]

I've been a registered member for a year or more and have found so much useful information that I can't thank everyone enough for the help.
One of the problem's I've been fighting on our 1989 525i is the infamous hunting idle, rough running backfiring @#$#@## engine.
I have searched this sight including the archives for a fix and have tried most every suggestion I could find. Nothing worked!
It got so bad, I had to go exchange vehicles with my daughter because it wouldn't go faster than 25-30 mph.
I almost let her leave before me since I'm the dad and can handle the situation, right?
The mall she was at is 15 miles from our house.
So after limping home, I went on a two week throw money at the BMW binge.
That day I replaced the plugs with platinum Bosch. Helped a bit.
Next day it was the O2 sensor. Helped more.
Few days later same symptoms appeared so I looked for intake leaks, adjusted the throttle cables, looked for loose connections and everything else I'd heard of or read. Nothing was working, so I figured a new distributer rotor was called for, again, no help!
So I got some more money but this time I actually found a salvage yard that had some things I thought I could use. Damn the torpedo's and all those diagnostic tool's people talk about, I was going to do it the stupid way!
Besides, the parts at the yard were cheap and warrantied for a year.
Nearly bought a whole parts car. I did that once with a 1980 280 zx and my wife said that wouldn't be happening twice.
Can't blame her though, it's not like we have a garage or anything to hide a heap. I mean "parts car".
So, I got a fuel pump to go with the fuel filter I'd purchased the night before, went home, installed them and it changed nothing!
Pump only cost 50 bucks and we've owned the car since "95, so it probably needed it anyway.
Next morning I'm at the yard again, this time it's a mass air flow meter I'm thinking will do the trick. I can't take all the credit for this decision though, the stomp test clued me to the need for this part.
Plop down another 50 bucks [like I said, these parts are cheap] and off I go, but not before I rip the rubber rear door seals off my newly adopted parts car and give the guy at the counter another 20 dollars.
Won't help the idle, I thought, but when I get that puppy up and running it will be a lot quieter.
MAF did not help and I was ready find a smart guy with those diagnostic tools I mentioned earlier.
However, I remembered reading in the quick fix section of Bruno's site about some vacuum conections underneath the car running from the engine to the transmission.
Out come the jack and the stands and the creeper and I'm looking for something that I haven't a clue what it might look like or where it might be and that's when I find it, not a loose vacuum line, but a cut electrical wire running from the front of the engine on the drivers side under the fuel regulator across the front of the motor and down the other side to a sensor under the exaust manifold just behind the air conditioning compressor. I soldered in a splice, used some heat shrink on it and cranked her up!
The car ran like she was 5 years old again, maybe younger.
I got in and punched it to see if it was going to stall or miss or backfire or something but nothing, nothing but a smooth ride. I'm not sure what this wire connects, but the oil pressure sensor light that had been coming on for the last couple of weeks isn't coming on anymore. I know what you're thinking, but it's more complicated than that and I've written enough already. I would like to know what that wire connects if anyone can figure it out from my description
My daughter is thrilled to have her car back, my wife is thrilled to not have another parts car in the driveway, [although I do have visitation rights down at the salvage yard], and I'm thrilled not to have to crawl under the car... 'till the next time.

[DaveVoorhis]

adjusted the carb

That might have been part of the problem right there...

Your car doesn't have one.

[dreamaker]

That might have been part of the problem right there...

Your car doesn't have one.

Your right! Thanks for the correction. I meant the throttle cables.

[Dave M]

Well, great job. Can't provide anything but encouragement, as I don't know a lick about the M20.

Not spark, not air, must be fuel!!

At least you found a parts car,

Dave M

[Espen]

I have the same problem on my M50 engine, my temp fix is to discconect the MAF.

[E34-520iSE]

You need that looking at PDQ Espen!

Shaun

[fujioko]

Espen…The battery on my 92 with an M50 was practically dead from a week stereo instillation (still not finished).

Anyway I went to start the car and there was just enough juice to crank the engine once… No big deal, I put the battery charger on and let the battery charge for a few hours.

Latter when I tried starting the car … BOOM! A huge back fire!... the car started and ran like crap. The idle was hunting and the check engine light was on.

Immediately I knew I blew a vacuum hose or something.. As some of ‘ya know vacuum leaks cause hunting and rough idles.

The culprit was the IAC (idle air control) tube that plugs into the intake manifold. This hose is attached underneath the manifold and is hard too see. Sure enough the hose was half way dislodged from the manifold orifice.

In the past this hose thingy has been a problem because the locking tab is busted. I have it rigged with ty-raps now and it should never pop off again.

My point is… a back fires will damage weak parts and eventually lead to vacuum leaks.

Give the old M50 a good look over.. it might be something simple to fix.