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Mechanical Noise Fooling Knock Sensor? Faulty AMM? 700 1986

posted by  jadnhm on  Tue Aug 21 02:33 CST 2007
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last visit: Tue May 22 18:03 CST 2012
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Hello fellows,

I have an '86 760 turbo that has a lot of mechanical noise in the engine. It's been my experience that this is not uncommon in early B230FTs, esp ones that may have been less than perfectly cared for. (this is the case with mine)

The biggest noise appears to me to be rod bearing noise, and/or perhaps piston slap. The valve train also makes a lot of noise (which I'm told is indicative of oil getting to the valve train?).

Anyway, it makes a lot of noise. It runs great, it just sounds like hell. Pretty much everyone that's 'met' the car has asked me if it's a diesel or not.

Anyway, I've recently completed a bunch of little things I've wanted to get done for some time which I thought were negatively affecting my fuel mileage.

For example:
- All vacuum hoses have been replaced, with the exception of the turbo's wastegate actuator, which appears to be in very good health.
- new O2 sensor (old one died)
- there was a small exhaust leak, pre-O2 sensor, which I suspected was leaking fresh air into exhaust stream, or letting exhaust out of the stream
- fixed a broken wire in the injectors (I chased this for a few months before finding it)
- new wires & rotor button ~20k kms ago (timed then as well)
- new AMM-turbo and turbo output hoses
- brakes (needed to do pads on all four wheels for safety inspection, replaced one front and one rear caliper) so rolling resistance should be down
- finally tracked down a small fuel leak in tank return line
- good fuel injector cleaner (Sea Foam)
- maybe some other little things I can't remember

As I checked off each item on my list I expected my fuel mileage to improve but it just doesn't seem to be improving. So now I'm looking at other things.

I was wondering if it's possible that my knock sensor is being fooled by all the mechanical noise in the engine (which sounds to my non-piezo non-computerized ears a little like engine knock) as pre-ignition knock and retarding the timing completely?

I also considered the possibility that my AMM isn't reading right?

Any ideas at all would help a lot. I'm planning on doing a fairly large road trip in the next few weeks (~3000kms) and I'd love to get back to getting 500km on a tank (right now I'm getting about 400km/tank)(60L tank so that's about 55L fuel)


-Joel
--
1986 760 (B230FT) 420k+








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    Mechanical Noise Fooling Knock Sensor? Faulty AMM? 700 1986

    posted by  Rolyak on  Tue Aug 21 06:24 CST 2007
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    last visit: Fri Oct 24 08:03 CST 2014
    [RELATED]

    There is lots of good info about the knock sensor in the FAQ's. Not sure which system you have but it sounds like only the Chrysler system could be fooled by mechanical noise.

    I too have been experiencing low fuel mileage, found my O2 sensor stuck on .9V, replaced it and AMM and the O2 sensor began to do cross counts.(.1V-.9V) Still mileage was a bit low but it's been very hot in Memphis (100*+) so AC and thermo-fan running are combining to keep MPG's down.

    For grins pulled the plugs (25,000 mi) cleaned and regapped and the last two tanks indicate about 2 mpg increase. (around 24 mpg)

    Also trying Shell gasoline, swapped in a set of newer, cleaned and tested injectors, cleaned throttle body, replaced the intake gaskets. Will be keeping a close watch on MPG's.

    You can also build an LED tester (similar to Volvo 5280) that will test your knock sensor and coolant temp sensor plus a couple other items. See the FAQ's.

    Piston slap: I have 468,000 miles (333,000 of my own after my rebuild)and it's still slapping away although I would not be surprised if a skirt might fracture at some point?







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      Mechanical Noise Fooling Knock Sensor? Faulty AMM? 700 1986

      posted by  jadnhm on  Tue Aug 21 08:03 CST 2007
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      last visit: Tue May 22 18:03 CST 2012
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      Come to think of it my plugs are pretty old (~85k, though they are platinums...) - Maybe I'll get some new ones.

      It sounds like you had some benefit from a new AMM, and I think I have a spare lying around so I'll swap that in and see how I do as well.

      Also, I've read about the LED tester trick, so I may look into that as well.

      Thanks for the reply.
      --
      1986 760 (B230FT) 420k+








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    Mechanical Noise Fooling Knock Sensor? Faulty AMM? 700 1986

    posted by  someone claiming to be fixit2003 on  Tue Aug 21 06:10 CST 2007 [RELATED]

    Should be able to see if the timing is actually affected by putting a timing light on the crank pulley. Providing, of course, the pulley is OK and hasn't started to slip like alot of them do after about 150 k miles or so. Timing is 12 deg BTDC at idle for my '88 turbo. Don't know if your's is the same or not. Check the turbo bypass valve it's a good bet your's is bad. Also very good idea to remove and clean the throttle body and idle air control valve.








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      Mechanical Noise Fooling Knock Sensor? Faulty AMM? 700 1986

      posted by  jadnhm on  Tue Aug 21 08:01 CST 2007
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      last visit: Tue May 22 18:03 CST 2012
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      I didn't know the blow-off valve could affect the mileage like this.

      Does the FAQ explain how to check this item?

      I just realized that I haven't cleaned my IAC in a long time, so I will pull that out and clean it up good. I'll do the throttle body at the same time.

      I'll let everyone know how things go.
      --
      1986 760 (B230FT) 420k+







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