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  1. #1
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    Could a vacuum leak prevent the car from starting?

    With the help of you all I've ascertained that I have a vacuum leak, which I have yet to fix. However, today there was a symptom that I'd never encountered before. The car won't start. It turns over and catches a little bit but won't idle. I keep revving it and it doesn't really respond at all (RPM at 350), then the RPMs shoot up to 3000, then fall off again, despite the same amount of gas. The weather has been cooling down here and I understand that is a factor, so maybe it's just getting progressively worse. Guess it's time to fix the leak, but should I be worried about another problem? What with the shaking as it's idling so rough the word "nikasil" won't leave my poor brain alone.

    Thanks,

    Bark '93 740i 109k


  2. #2
    RLC
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    Previous posts and other info included

    It all boils down to 3 basic things any engine needs to run continuously: Ignition and a stoichiometric combustible mixture contains the exact quantities of air (oxygen) and fuel required for complete combustion. For gasoline, this air-fuel ratio is about 14.7 : 1 by weight. If the fuel concentration is too rich or too lean relative to the oxygen in the mixture, combustion cannot take place. (Air - Fuel - Ignition)

    Make sure that the battery is fully charged. Then

    1) Checking Battery connections under the rear seat.
    2) Try disconnecting the battery cable for an hour to see if that resets everything and see what codes come up on restart attempt.
    3) Check the Ignition System you mentioned that it was raining...you wana see if moisture may have gotten into the distributor cap somehow.
    4) Check spark plug wires as well as the connectors at the coils.
    5) Remove the ICV (Idle Control Valve) and clean it with carb or injector spray.
    6) Check for signs of a leaking Fuel Injector or loose connector to any one injector etc., by spraying carb cleaner and listen for RPM increase. Same can be done for Vacuum Lines. Check and replace as many as possible. If one is perished, then it is likely the rest are.
    7) Check the Starter and Alternator conections for tightness.
    8) Check Oxygen Sensor connector just below the intake manifold to the rear of the engine.
    9) Check the black box near the firewall on the passenger side to be sure that it is screwed down tightly. This is where the ECU is housed as well as the main and fuel pump relays. Check for possible sign of moisture having entered the box. (also while you're there Check the Fuel Pump Relay.
    10) Check all Relays and Fuses in the black fuse box. Check all Gorunds.
    11) check the Throttle position sensor connector on the throttle body, as well as signs of binding linkage near the accelarator and cruise control cables.
    12) Check Oil Filler Cap, Dipstick, Fuel Filler Cap Security and seal.
    13) In the Black box on the Passenger side next to the spark coil, thier is a Orange Relay. You will have to remove the cover with a phillips screw driver. Switch it with one of the Orange relays in the Fuse Box. While your there, Pull the main relay ( White ) in and out a few times. To clean off the contacts.

    It is possible to check and adjust fuel mixture using your fuel mixture Via your O2 sensor and Digital volt meter. Here's how:

    Thought you needed an expensive scanning device such as AutoXray or Diacom to view your O2 sensors voltage readings? Think again! You NEED to do this if you have an adjustable fuel pressure regulator or if you have a Q-jet equipped thirdgen and are changing the metering rods to achieve optimal performance.

    You will need: A digital voltage meter, approximately ten feet of wire preferebly 12 gauge or smaller, wire connectors (Screw type "european" connectors preferebly), Jack, jackstand, wire strippers.


    1) Disconnect negative battery cable
    2) Raise vehicle, after wheels are chocked and Parking Brake Set.
    3) Locate plastic connector for the O2 sensor near exhaust manifold, Disconnect.
    4) Strip end of materials wire approx. 2 inches to expose core
    5) Put exposed end into one end of the weather pack connector for the O2 sensor and reconnect.. Its a tight fit and you might have to trim or expose some more wire. It also might take a little force for it to snap backtogether.
    6) Route wire away from exhaust manifolds or headers back into passenger compartment. If you plan on using this tuning aid temporarily simply route through door jamb. If not, there is a hole under the power booster in which you can route the wire.
    7) Once inside of engine compartment trim wire to an accessable length. Connect wire to the positive probe of your Digital volt meter MAKING SURE that no part of the wire is exposed in any way so as to not ground the o2 sensor accidentally. European wire connectors worked well for me (the kind with a screw in each end to hold the wire in a plastic connector. Avail. at radio shack)
    8) Use the extra length of wire to make a ground for the Digital volt meter. Under the dash there are plenty of places to ground the wire. Connect it to the negative probe of the digital volt meter.
    9) Lower car, reconnect battery. Start car and turn your Digital volt meter on.

    The range is as follows 3mV being the leanest, 1V being the richest. Part throttle does not matter so much as the computer always keeps it somewhere in this range depending several on variables and inputs from other systems sensors. At Wide open throttle the MOST desireable voltage should USUALLY be somewhere between .850mV an .900mV Make adjustments accordingly to reach the desired settings. Remember, the lower the voltage the leaner, the higher the voltage the richer. This also has an effect on spark timing. For instance if your car was running lean prior to adjustment and you had the base timing set to just before knock you may be able to adjust your timing for more advance with more fuel. The same is true vice versa. In addition, if you have an old O2 sensor take this opportunity to replace it as an O2 sensors sensitivity greatly diminishes with increased mileage.

    It is imperative that you DO NOT ground the O2 sensor wire. This will KILL your O2 sensor.

    Hope this helps. Good luck.



  3. #3
    Paul
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    Re: Previous posts and other info included

    Just wanted to note that, a lot of that stuff posted above applies to 735 and 750's but not so much the 740. there are no spark plug wires or distributor cap, so you dont have to look at those, but you may want to check you coils, there is 8 of them, kind of hard to diagnose a bad one, but they are suspect to failure. There is also the PCV valve thing, which is a large cover on the back of the intake manifold, the seal inside it can break and allow large air leaks causing poor running, also check the tube that runs form teh PCV cover tot he fuel pressure regulator and any other vacuum hosed attachments for leaks back there. Also the gaskets below the intake manifold are suspect to leaks and so is the gasket from the throttle body to the intake manifold. You can test thsoe by spraying carb cleaner or what not on them while car is running. Your 740 may have a bad MAF, my car idled like [Oops!] when the MAF went bad, once it was diagnosed and fixed, it was much better.

    try the stomp test and see what codes it gives you, they are semi misleading but it might give you a hint.

    I boguht the peake research scan tool for my car, its got a ton more info available throught he diagnostic port than the stomp ttest will give you. I havnt actually used it yet for diagnosing anything, and at $139 its kind of pricey, but it looks to be a good solid tool that will be a good aid when th situation presents itself.

    good luck,
    Paul

  4. #4
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    Re: Previous posts and other info included

    What's the MAF and how do I check on it's condition? Thanks for all the help.

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