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Thread: Need help narrowing down cylinder misfire.

  1. #1
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    Need help narrowing down cylinder misfire.

    Just got a 99 bmw 323i. I’ve got multiple cylinder misfires on 1,2 and 3. Didn’t mess around with changing coils around to see if the issue is coil related because I don’t believe that is the issue. There is also a popping noise coming from the intake which i can only assume are small backfires. It only really does it at start up.

    Anyways, while the car is running, if I unplug the MAF the cars idle will straighten out. No popping. No choppy idle. I got to looking and the intake boot is torn in 2 spots. Big tears too. I just ordered a new intake boot for the thing and those will be in shortly.

    I guess my question is, could the major vacuum leak cause the computer to trick the engine into running fat/lean and create backfires? Is unplugging the MAF and seeing an improvement a sign of a vacuum leak? Also, would a major vacuum leak cause a multiple cylinder misfire? Just to add a little bit of info. When the MAF is plugged in and I take the oil filler cap off it will bog down bad and stall. If I do the same with the MAF unplugged it will act as it should and change the idle slightly.

    I’m new with bmw’s so any help would be appreciated.

  2. #2
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    Disconnecting the MAF causes the engine computer (DME) to revert to an overly rich fuel/air mixture. This masks vacuum leaks. The first step in diagnosing a problem is to scan the DME for codes. Report back. The next step is to smoke test the intake. Find a reputable indie shop that specializes in bimmers at www.bimrs.org.

  3. #3
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    You would always mark and move coil packs and spark plugs if you like as a diagnosis starting point because its simple imagine if you didn't do that and that was the problem all along. Then move to the next step, then smoke check the intake system with the engine cold.

  4. #4
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    Cylinders 1,2,3 are one the same bank. While major vacuum leaks would definitely cause misfiring, the fact it's on the same bank makes me think of the O2 sensor downstream of that bank. You could try swapping them around. If the error code follows the sensor (cylinders 4,5,6) , it'll confirm the o2 sensor's issue.
    "If you have integrity, nothing else matters. If you don't have integrity, nothing else matters." Alan K. Simpson.
    5spd E46 "3XX": 328i engine, 330i Intake and exhaust, CAI, 323i diff.

  5. #5
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    Quote Originally Posted by MarcoZandrini View Post
    Disconnecting the MAF causes the engine computer (DME) to revert to an overly rich fuel/air mixture. This masks vacuum leaks. The first step in diagnosing a problem is to scan the DME for codes. Report back. The next step is to smoke test the intake. Find a reputable indie shop that specializes in bimmers at www.bimrs.org.
    Ok. I’ve diagnosed it. Here are he codes I got

    P1250 - No DTC definition found
    P0300 - random multiple miss
    P0130 - oxygen sensor circuit bank 1
    P0302 - cylinder 2 miss
    P0303 - cylinder 3 miss
    P0301 - cylinder 1 miss
    P0170 - fuel trim
    P1188 - no DTC definition found
    P1182 - no DTC definition found
    P0142 - oxygen sensor heater circuit bank 1 sensor 2

  6. #6
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    hmm - just to clear up some terms - diagnosing means you have identified the problem and the fix.
    what you've done is had codes read, by a OBD2 generic scanner, which produces P codes. P codes are not very reliable on a BMW.

    is this the same vehicle that you test drove and it overheated?? i'd say that's your bigger issue, but, hopefully it's not your issue at all because you didn't buy the car....

    fuel trim codes, misfires - real common on an e46 of this age/mileage. you'll need to check for vacuum leaks by doing a smoke test, and you'll need to figure out what's going on with the o2 sensor circuit - could be a broken wire, etc. the car could very well need ignition coils and spark plugs (basically, a tuneup).

    the bigger issue is the overheat, though, if that's the same car.
    '95 325iS - auto to manual swap done!

  7. #7
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    Quote Originally Posted by shadowpuck View Post
    hmm - just to clear up some terms - diagnosing means you have identified the problem and the fix.
    what you've done is had codes read, by a OBD2 generic scanner, which produces P codes. P codes are not very reliable on a BMW.

    is this the same vehicle that you test drove and it overheated?? i'd say that's your bigger issue, but, hopefully it's not your issue at all because you didn't buy the car....

    fuel trim codes, misfires - real common on an e46 of this age/mileage. you'll need to check for vacuum leaks by doing a smoke test, and you'll need to figure out what's going on with the o2 sensor circuit - could be a broken wire, etc. the car could very well need ignition coils and spark plugs (basically, a tuneup).

    the bigger issue is the overheat, though, if that's the same car.
    It is the same car. I bought it for $400 and took on the challenge of fixing it. I don’t think he over heating is caused from anything catastrophic. Cooling fans were not engaging. Compression was fine. Runs smooth. Cooling system just isn’t working right. I can work on the car no problem. I just like to narrow down things before I start throwing money at it.
    Last edited by Tsadkins105; 03-15-2019 at 10:35 PM. Reason: More info

  8. #8
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    You're misunderstanding the system.

    The M52TU and the M54 engines do not put up with overheating. A single trip to the red will kill this engine, as often as not. When it overheats, the head warps. When the head warps, it pulls the threads out of the aluminum block. Expensive and time consuming fix, usually far cheaper to just replace the engine and the entire cooling system.

    The sensor responsible for turning on the electric fan lives in the lower hose; so if the hose is cold, the fan will never come on, and the engine will kill itself

    Is this a manual or automatic? (The fans are entirely different)

    Just so you know, the average 2000 E46 I see needs ~$3000 to $4500 worth of work, just to make it a decent car. If the engine is toast, add another several thousand.

    There is nothing more expensive than a cheap BMW.

    If it runs smooth, why do you have misfires, mixture codes, and an overheating engine? I wouldn't call that smooth.

    Oh yeah, you need a good BMW specific scan tool. OBD codes don't cut it. Make sure you get a cable to connect to the underhood 20-pin diagnostic port, or you can't talk to all the computers.

    P codes are worse than useless.
    Last edited by bmwdirtracer; 03-16-2019 at 12:52 AM.

    Chris Powell
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    Master Auto Tech, owner of German Motors of Aberdeen
    BMWCCA 274412
    German Motors is hiring ! https://www.bimmerforums.com/forum/s...1#post30831471

  9. #9
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    It is a manual car.

    I understand the complexity of the engine. That’s what brings me here. I’m used to small blocks. The only reason I got the car was for $400. I own a body shop so I can get salvaged vehicle parts without any hassle. That includes engines. I can get a used engine for 1k. I do not out source work. I can do most engine repairs. I’m just clueless when it comes to bmw and all their fancy wiring and what not. If this was a small block Chevy I wouldn’t even be asking these questions lol. If it needs head gaskets, water pump etc etc I can do it. I will never get an engine like this reworked. I’d just buy used. But I agree with you. The overheating is the most concerning. I just noticed it after a few minutes of test driving after I put the driveshaft up in it. I was just mainly concerned if anyone has dealt with this issue before or if there may be a common issue with that engine. It doesn’t display any of the common signs of a toasted engine.

    Now that I remember, the guy I got it off of told me he put some coolant in it because it was low. Maybe he didn’t bleed it properly. I know these cars are super picky about the coolant system bleeding. Idk I’m racking my brain here.

    I have yet to manually wire the fans to check if they will come on in the first place. It got late so I came in and started researching the issue.

  10. #10
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    About the overheating, I'd say it depends how much and how long. As in did the needle go all the way to the red or somewhere between operating temp and the red. And how long it stayed there. I guess properly bleeding the circuit would be the first step, to then be able to monitor its behavior: will the overheating go away, will the coolant level go down after a few spirited drives or not, will there be traces of oil in the coolant...
    "If you have integrity, nothing else matters. If you don't have integrity, nothing else matters." Alan K. Simpson.
    5spd E46 "3XX": 328i engine, 330i Intake and exhaust, CAI, 323i diff.

  11. #11
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    i'm with chris - any trip to red is a cause for major concern for me on any m50/s5x/m52/m54 engine family engine. especially on the m52 and m54. that's clearly the bigger issue....
    '95 325iS - auto to manual swap done!

  12. #12
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    Quote Originally Posted by Breeze1 View Post
    About the overheating, I'd say it depends how much and how long. As in did the needle go all the way to the red or somewhere between operating temp and the red. And how long it stayed there. I guess properly bleeding the circuit would be the first step, to then be able to monitor its behavior: will the overheating go away, will the coolant level go down after a few spirited drives or not, will there be traces of oil in the coolant...
    I would never try to run the engine in the red. In any vehicle to that matter. I shut it down when I noticed it. Now that’s not to say the guy before me didn’t run the heck out of it. But I got it realizing that it might be gone considering what I can get a replacement for. The lower sender, thermostat, cooling fan or improper bleed is where I’m hanging my hat right now. Cheap fixes. If it still over heats I’ll do a more surgical check to find the issue. Then I’m only in it for time. I’m not dumping hundreds of dollars in it when I can just get a replacement for that.

  13. #13
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    The fact that the lower hose is stone cold when the car's overheating means the coolant's not circulating. That's either a bad water pump, thermostat, clogged radiator, or air pocket. Certainly, you can try bleeding it first; instructions are in the FAQ section at the top of this forum....or, use a vacuum-fill system, best of all.

    Best bet is the water pump. BMW used both plastic finned ones and metal ones, the plastic impeller falls to bits. It's wise to do the thermostat and water pump at the same time. ALWAYS use OEM parts, never crap from Autozoo. Try FCP Euro.
    Use blue coolant.

    Chris Powell
    Racer and Instructor since, well. decades, ok?
    Master Auto Tech, owner of German Motors of Aberdeen
    BMWCCA 274412
    German Motors is hiring ! https://www.bimmerforums.com/forum/s...1#post30831471

  14. #14
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    Quote Originally Posted by bmwdirtracer View Post
    The fact that the lower hose is stone cold when the car's overheating means the coolant's not circulating. That's either a bad water pump, thermostat, clogged radiator, or air pocket. Certainly, you can try bleeding it first; instructions are in the FAQ section at the top of this forum....or, use a vacuum-fill system, best of all.

    Best bet is the water pump. BMW used both plastic finned ones and metal ones, the plastic impeller falls to bits. It's wise to do the thermostat and water pump at the same time. ALWAYS use OEM parts, never crap from Autozoo. Try FCP Euro.
    Use blue coolant.

    I went ahead and got a sensor, thermostat and water pump. I did the 12v test on the t-stay it took a bit longer than bthought it would to open. I think the plunger lost all its lubrication. Got a water pump with a metal shaft.

    I tested the fan connection with a multimeter when the car was on but not running. Said it was getting 10.66 volts. Not sure if that was the correct way or not. But there’s that. Fan worked though when I hooked it straight to a 12v source. I need this overheating issue corrected before I even dive into the other nonesense.

  15. #15
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    The electric fan (efan) has power as long as the battery is connected. Why? This allows the engine computer (DME) to keep cooling the engine even when the ignition is off. When you look closely at the efan connector you’ll see 2 large wires and a small wire. The first wires are power and ground. The third wire is connected to the DME. The DME uses pulse width modulation to vary the efan’s speed.

    Chris, please forgive me, but there are several other factors that go into the efan’s speed. One is the engine temperature (the sensor is in the head between cylinders 5 & 6). I don’t remember the others but jfoj over at E46Fanatics has a list.

    That voltage is way low; your battery is suspect.

  16. #16
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    Quote Originally Posted by MarcoZandrini View Post
    The electric fan (efan) has power as long as the battery is connected. Why? This allows the engine computer (DME) to keep cooling the engine even when the ignition is off. When you look closely at the efan connector you’ll see 2 large wires and a small wire. The first wires are power and ground. The third wire is connected to the DME. The DME uses pulse width modulation to vary the efan’s speed.

    Chris, please forgive me, but there are several other factors that go into the efan’s speed. One is the engine temperature (the sensor is in the head between cylinders 5 & 6). I don’t remember the others but jfoj over at E46Fanatics has a list.

    That voltage is way low; your battery is suspect.
    I would agree with you that the battery is suspect but I used that battery to ensure the fans worked. And they did. The battery is fairly new. The fans never came on which leads me to believe that it’s something inline Between the plug and battery ie fuses or wire.

    I pulled the glovebox down and inspected the fuseable links above the fuse back. three fuses from left to right have a white corrosion type film on them. I suspect this to be the issue although wouldn’t it prevent current all together? From my understanding, #37 50amp blade fuse goes to #101 50 amp fuseable link. Then forward to the plug.

  17. #17
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    I’m pretty sure the fan is pwm controlled by the dme.
    '95 325iS - auto to manual swap done!

  18. #18
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    It is controlled by the DME. Here’s a document about the efan operationwritten by a Canadian tech: https://drive.google.com/file/d/1Cac...BTPQDJx6u/view

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