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Thread: How bad can a vacuum leak be? The car hardly even runs.

  1. #1
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    How bad can a vacuum leak be? The car hardly even runs.

    The local Auto Zone was nice enough to scan my codes for free and these came up: P0174 and P0143. I tried driving with my MAF sensor unplugged and it made no difference in performance. The car misfires so badly that I had a hard time even maintaining the speed limits. I have never heard of an air leak causing such a drastic misfire and drop in power. What else could it be? I know, almost anything- the car is 16 years old and has 265,000 miles on it. Tomorrow is my day to fix it. Where else should I be looking/testing?

    '01 330 ci

  2. #2
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    OBDII Code P0174 can be a vacuum leak and it can be other things, including a failing Fuel Pump. The second code, P0143, is for an O2 Sensor, which can be triggered by the first code. First thing I would check is your Fuel Pump. You can easily determine if your current Fuel Pump is the one that was factory installed: Remove the rear seat bottom. Remove the cover for the Fuel Pump on the passenger side, it's just four bolts, 10mm if memory serves. Inspect the top of the Fuel Pump. If it's filthy dirty and the clamp for the fuel OUT line is one of those 'crimped on single use clamps', then you can be almost certain that your Fuel Pump has served the car well for 265,000 miles and is in need of changing.

    Don't be alarmed if the top of the Fuel Pump is rather dirty - that dirt and grime has no effect on whether it functions or not. And here's the best part - a new Fuel Pump is less than $150 and you can have it installed in under an hour.

  3. #3
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    Thanks for the tip. I'll start there. I could remove it and hook it up to a 12 volt power supply to see if it runs steadily.

    I don't think the problem is with the fuel filter because the engine feels like it's misfiring. Not running out of gas.

  4. #4
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    Does it run just as bad when the car is cold? I would start by checking the big rubber boot after MAF. Remove and inspect including areas hidden by folds.

  5. #5
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    It runs the same at all operating temperatures. OK. I'll check the easiest stuff first.

  6. #6
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    A vacuum leak allows air to enter the intake system after the maf. The maf tells the dme how much air is entering so it can add the correct amount of fuel for proper combustion. If you have air getting in after the maf it's called unmetered air. This creates a lean condition (too much air not enough fuel) and lean conditions can easily lead to misfires. The o2 sensors will detect the lean condition and tell the dme to add more fuel. This added fuel will be displayed on a scanner as an increase in "short term fuel trims". Normal STFTs should be around 0-2% range. Maybe a bit higher. In the case of a vacuum leak it's not unusual to see trims upto or beyond 25%. That's what my 330 was recently doing. That was caused by a hole in a rubber boot about the size of my little finger. Larger leaks can cause very severe misfires and misfires are potentially catastrophic to your engine. At some point the o2 sensors reach the maximum amount they can read. This triggers a code for o2 sensors. Vacuum leaks can easily trigger 4 5 or 6 codes at once. It's not unheard of.

    Most common culprits for vacuum leaks are the lower intake boot and upper intake boot. The lower one has a ribbed elbow that connects to the idle control valve. Those ribs get old and they tear. That's where my leak was. Other common culprits are the CCV system and the DISA valve. The plastic parts of the CCV get old and brittle. They break and allow unmetered air into the engine. The DISA doesn't let unmetered air in but it has parts that can fail and lead to a whole host of other issues.

    Get someone to put a scanner on your car and read the live data. Look at them short term fuel trims and see what they are reading.
    Last edited by flyfishvt; 06-08-2017 at 08:19 AM.

  7. #7
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    Quote Originally Posted by M6BrokeMe View Post
    ...the engine feels like it's misfiring. Not running out of gas.
    These engines will misfire when they do not get enough fuel.

  8. #8
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    Well today I have found out some things that aren't the problem. I removed and inspected: both rubber boots and they are OK. "F" connector is good, Disa, throttle body, idle control valve, dip stick O-ring had a big cut in it so I replaced it. All the vacuum lines I could reach looked and felt OK. Re-installed everything except the airbox and started it up. Same stumbling idle. I unplugged each coil pack one-by-one with the engine running and found that unplugging the front 2 coils made no difference in the idle. Unplugging the others each made a difference in the idle.

    I sprayed MAF sensor cleaner all around the intake area to see if it would change the idle if I sprayed a leaking vacuum line. No luck.

    The only line I could not see was the one which plugs into the exhaust flapper valve. I think I have to jack the car up to see that one. Is that right?

    That was a lot of work for me today and I'm not done. Now I'm going to take out the fuel pump and attach it up to a 12 volt power source to make sure it runs consistently. It occurred to me that I could have water in my gasoline. Not likely. I'm really searching for answers now.

    I definitely have at least some vacuum because the idle changes when I take off the oil filler cap. I also get a change in idle when I unplug the connector from the MAF sensor. That doesn't prove it's good but at least I know it is generating a signal.
    Thanks to you guys who offered suggestions. I'll keep you posted.

  9. #9
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    That's some really good work. Looks like your issue is with those front 2 coils. Remove them and inspect for hairline cracks. If you haven't done it lately it's probably a good idea to pull all the plugs too.

  10. #10
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    Pretty sure those first two coils being unplugged should jave made a difference in idle

    Sent from my LG-D415 using Tapatalk

  11. #11
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    Have*

    Sent from my LG-D415 using Tapatalk

  12. #12
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    Quote Originally Posted by TopNotchPanch View Post
    Pretty sure those first two coils being unplugged should jave made a difference in idle

    Sent from my LG-D415 using Tapatalk
    Absolutely

  13. #13
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    I have confirmed that unplugging the #2 coil pack does in fact have an effect. It's hard to tell because it's idling so rough even when it's plugged in.

    It still is true is that unplugging coil pack #1 makes no difference.

    I also came to the realization that this may not be a true test since part of my error code is "fuel cutout". I think that means that the DME as deactivated the injector to that cylinder to save the catalytic converter. So in that case, unplugging a perfectly good coil pack would have no effect. I'm going to find out the real way to test these things. Still there is a lean condition for bank 1 (cylinders 1,2 and 3) and tomorrow I'm going to check the fuel pressure. On second thought... low fuel pressure would not cause a lean condition only in one bank and in only 1 cylinder. I'm getting a bit tired and confused. Not ready to give up and take it to the shop though. I'll get this.

    As a side note, it is interesting that a lean condition causes misfiring. Actually it's a lean condition being over compensated for by the DME which is causing a rich condition.
    Last edited by M6BrokeMe; 06-08-2017 at 08:06 PM.

  14. #14
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    Today I continued to fight the good fight. I borrowed a fuel pressure tester from Auto zone an confirmed that my fuel pressure is good.

    I pulled 2 coils from a junkyard vehicle that had been wrecked and tried them in my car. No change is symptoms.

    Question: Could I have water in the gasoline? Next I'm going to jack the car up and look for vacuum leaks in the exhaust flap vacuum line and the line that leads to the fuel pressure regulator which is part of the fuel filter.

    Wish me luck. The saga continues.

  15. #15
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    A vacuum leak can un-balance the entire engine performance system....leading to and including wrong data subject to other critical engine operating systems. It's like a stacked house of cards....if one card falls, many other cards will follow!

  16. #16
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    Quote Originally Posted by M6BrokeMe View Post
    Today I continued to fight the good fight. I borrowed a fuel pressure tester from Auto zone an confirmed that my fuel pressure is good.

    I pulled 2 coils from a junkyard vehicle that had been wrecked and tried them in my car. No change is symptoms.

    Question: Could I have water in the gasoline? Next I'm going to jack the car up and look for vacuum leaks in the exhaust flap vacuum line and the line that leads to the fuel pressure regulator which is part of the fuel filter.

    Wish me luck. The saga continues.
    You should get that car on a scanner and look at the live data.

  17. #17
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    Yesterday I successfully used the fuel pump to completely pump all the gas out of the tank and filter. Then I poured a couple gallons of fresh gas back in and drove the car around the block. No luck. Still misfiring away.
    Today I have spent about 10 hours trying to download Carsoft BMW ver 1.4.0 and run it on my windows 10 laptop. That has been a total nightmare. The program is made for Windows XP and is configured to be used on a machine that has an RS 232 port. So that was a no go. That's OK because I ordered a bluetooth enabled OBD2 reader last week and it should arrive tomorrow or Tuesday. Then I can download a free app. to my phone and do my diagnostics wirelessly.
    I have been reading up on fuel trims and am convinced that they are critical in diagnosing any engine because the fault codes are not much help. Thanks for reading my trials and tribulations. I hope they will help somebody some day.

  18. #18
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    Yes. You are right. That's my next step.

  19. #19
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    Today I was able to get my BMW mechanic/friend to put the scanner on my car for free(!). With the car running he showed me 6 readouts- one for each cylinder. I ha never heard of this before but it gave a percentage reading for the misfiring activity for each cylinder. He said 4 of my cylinders were misfiring and that I should change the spark plugs. I thanked him and sputtered off to the auto parts store for spark plugs. I absolutely did not believe that I had 4 spark plugs go bad at once but I dutifully followed his advice.

    And not the car runs perfectly. Mission accomplished. I guess the old fashioned way of doing things is the best way sometimes. Any old timer who knows nothing about fuel injection, ODB, oxygen sensors or catalytic convertors would have told me to change the spark plugs.

    I had to try looking for vacuum leaks, bad fuel and everything else before just blindly following the advice of the mechanic. I don't understand it but I am glad my car is running well now. The sage ends. Thanks for watching. I hope this helps somebody in the future.

  20. #20
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    always start with basics first, doesnt matter BMW, Audi, ford, GM the engine is an engine and requires the same things. air fuel spark. you had failed or failing plugs, which lead to missfire, which lead to computer shutting down injectors for those cyl to save cat. for a missfire always start with plug. move affected cyl plug to another cyl and see if missfire follows. then do same for coil, if those dont change check vacuum components (althought most will cause missfire on all cyls.) pull compression and leak down. you didnt say if plugs were soaked in oil. is your valve cover leaking? that will cause the plugs and coils to crap out.
    Level 1 Certified BMW tech.

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