Crank Position Sensor?

Discussion in 'Engine' started by brucemillar, Jul 26, 2015.

  1. brucemillar

    brucemillar MB Club Veteran

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    Folks

    Not a Merc but the same principle. Please bear with me, I need help.

    I have a 3.5 V6 Petrol - SWB Mitsubishi Pajero. It had a head gasket failure.

    Off it went to have the Gaskets replaced along with:

    New Water Pump
    New Timing/Cam Belt
    New Crank Shaft Pulley (these de-laminate so need replacement).
    Heads refurbished - pressure tested & skimmed.

    All good so far.

    When they put the car back together, she fired up and ran for 10 minutes with no issues. She then cut out and refused all efforts to re-start. Cranking but not firing.

    Now she will start but is back firing and running like a dog, any attempt to give throttle results in back firing.

    Garage have checked and re-checked the timing - spot on. They even tried retarding and advancing it with no change.

    They checked all injectors - all good.

    Now they say that they suspect the Crank Position Sensor?

    Why? Well when they removed the Crank Pulley they say the blade that activates the sensor was bent and heavily marked, due to the pulley wobbling about. The thinking is that this must have damaged the CPS which has now failed. They also say that there is no other component that could cause this behaviour as it is backfiring on all cylinders, so must be getting a wrong reading or no reading from the CPS.

    The CPS is £120 no refundable from Mitsi.

    Is there anything else that could be checked?
     
  2. carat 3.6

    carat 3.6 Hardcore MB Enthusiast

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    Might be an obvious question but they have checked that the plug leads are on in the correct order? The cps must be giving a signal for it to fire at all.
     
  3. OP
    OP
    brucemillar

    brucemillar MB Club Veteran

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    Hi. Yes the car was running fine then it shut down all by itself. When it was restarted the backfiring started up.
     
  4. carat 3.6

    carat 3.6 Hardcore MB Enthusiast

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    I dont think it's the cps, weak spark maybe?
     
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  5. Pontoneer

    Pontoneer MB Club Veteran

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    Secondhand cps from a breaker worth a try ?
     
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  6. Dichtung

    Dichtung Hardcore MB Enthusiast

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    Has it VVt Variable Valve timing?
     
  7. martyz

    martyz Hardcore MB Enthusiast

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    They changed the cam belt and it ran ok,I would suspect it has jumped,were the tensioners renewed ?
     
  8. carat 3.6

    carat 3.6 Hardcore MB Enthusiast

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    Thinking about this a little, were you present when it ran ok? Or have they just told you it ran for 10 mins?

    They say they have checked the ignition timing and the injectors, and that these are working as they should. But these are told when to fire by the cps, so the if the cps is faulty the injectors and timing would be way off spec.

    A jumped cambelt is a possibility as Martyz has said, but I would hope the garage has re-checked this.

    I suspect ignition timing is the problem here, I'd be tempted to check the dizzy is in the correct position.
     
  9. OP
    OP
    brucemillar

    brucemillar MB Club Veteran

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    Hi all. Many thanks again for all your replies.

    It does not have Variable Valve Timing on this engine.

    The tensioners were renewed/replaced along with the belt. The timing has been checked multiple times by different mechanics as they all agree that it does sound like a timing issue. The timing is spot on.

    A sensor from a breaker is no good as you have to strip the front of the engine to get it out then re-build to test it. So is a financial no no ;^(

    Yesterday we again checked.

    Engine cranking with no issues or tightness with fuel off (pump unplugged).
    Injectors all fueling with no leak of.
    Fuel pressure as per the manual.
    Good spark (new plugs) on all six cylinders.
    14.9 bar compression on each cylinder (that is very good)
    MAF ruled out as we have the same issue with it unplugged.

    The chopper/blade on the old crank pulley is heavily scored and bent. That suggests that it was hitting the sensor for some time. So the thinking remains that this can only be the sensor that must have been on it's way out and failed due to earlier damage. That is the only bit in the repair cycle that has been disturbed, when the old pulley was removed.

    I have no choice but to go with a new sensor and hope that this fixes it. I cannot think of anything else tat could cause this to backfire and run so poorly apart from the timing.

    Any other thoughts?
     
  10. OP
    OP
    brucemillar

    brucemillar MB Club Veteran

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    No I wasn't there when it ran. It has no Dizzy. I have looked at the timing and does all line up with the marks. I have used them for years and have to trust what they tell me. They agree that the sensor must be sensing for it to fuel but cannot think of anything else to check.
     
  11. grober

    grober MB Club Veteran

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    Bruce could you post the exact date of manufacture of your car please and engine no if possible.
    Further thoughts- my first inclination was to ask the same question as carat 3.6 -- Thinking about this a little, were you present when it ran ok? Or have they just told you it ran for 10 mins? the implication being it was a bad rebuild- but leaving that aside for the moment one thing occurs- just how long did the vehicle sit after it blew the gasket and was subsequently repaired - could you be dealing with a bad/contaminated fuel issue---- is the fuel supply pressure OK from the pump- blocked fuel filter- water in fuel etc etc .
     
    Last edited: Jul 27, 2015
  12. OP
    OP
    brucemillar

    brucemillar MB Club Veteran

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    Graeme

    I believe that it is a 6g74 engine the truck is an import. 3.5 V6 Petrol built in 1997/8
     
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  13. OP
    OP
    brucemillar

    brucemillar MB Club Veteran

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    Not sure if this helps?
     
    Last edited: Nov 10, 2015
  14. martyz

    martyz Hardcore MB Enthusiast

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    Just a thought,seeing as the crankshaft pulley is damaged,the only thing I can think of
    is movement,is it possible the woodruff keys in the sprocket and pulley have worn?

    I had a similar experience with an imported Nissan diesel a few years back,the keys were
    chewed!.
     
  15. OP
    OP
    brucemillar

    brucemillar MB Club Veteran

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    A very good thought, thank you. Both were damaged and replaced with new ones. There is no movement there at all now.

    This is a common issue on Pajero's along with the crankshaft pulley delaminating. It starts to wobble which in turn damages the woodruff key and gear and the actual bolt. The fix is to replace the lot.

    The garage have asked me to go up there in person and they will run through the timing with me stood beside them, they are so confident it is correct. I will do that anyway as four eyes is always better. They say that it is sparking too early (timing) which is causing the backfire through the inlet. If the timing is correct then the only other input is the CPS? which tells it when to spark.
     
  16. martyz

    martyz Hardcore MB Enthusiast

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    I know this might sound strange,but it has caught me out,count the teeth on the sprocket,I had an extra two teeth on a replacement,same diameter,and fitted perfectly.
     
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  17. OP
    OP
    brucemillar

    brucemillar MB Club Veteran

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    Just come back from the Garage.

    We re-checked the timing against ALL the timing marks on cams and crank. All perfect. Wound the engine over with a socket & bar. Timing returns to perfect after every revolution on all marks.

    Then we checked the new sprocket on the crank shaft. Counted the teeth, this is identical to the old sprocket on teeth count and diameter.

    Checked the Crank Shaft Sensor chopper blade, new against old. This has three blades and is located on a woodruff key and two dowels so cannot be incorrectly fitted. The new chopper is perfectly flat on all three blades. The old chopper has all three blades bet forward (towards the radiator) by about 10 degrees - this looks to be mishaped/bent rather than preformed. All three blades were heavily scored and have obviously been hitting the sensor. The woodruff key was also bent so that the chopper and sprocket would be almost half a tooth out of allingment when fitted. That is why a new sprocket and chopper plate were fitted.

    We have a good spark on all cylinders and good even compression on all cylinders. All injectors are fueling nicely with good fuel pressure. The engine cranks with no noises or siezing when fuel is disconnected.

    So we have to believe that the Crank Position Sensor is damaged? There is nothing else that we can think of that would cause it to backfire and not run.

    /
     
  18. martyz

    martyz Hardcore MB Enthusiast

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    I can't get my head around the cps being faulty as you are getting a signal from it and
    a spark,I now feel the distributor is not timed correctly 180 degrees out? I feel your pain
    Bruce.
     
  19. Bellow

    Bellow Hardcore MB Enthusiast

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    Cam position sensor?
     
  20. OP
    OP
    brucemillar

    brucemillar MB Club Veteran

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    Bellow.

    Agreed. I think we will go with the CPS first. If that does not help then it's the CAM sensor.

    Sent from my iPhone using MBClub UK
     

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