Engine dies when given throttle after coolant repairs

Discussion in 'Mechanics Garage' started by vfrtodd, Dec 25, 2015.

  1. vfrtodd

    vfrtodd New Member

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    New member here. I have a '99 VFR800fi that I've had for nearly 10 years now. It is just shy of 100k miles. I've run into a problem that I can't diagnose after I made repairs . I'm not a mechanic, but have fairly average wrenching skills.

    My original problem was a coolant leak somewhere internal - under the tank and outside of view. After some disassembly, the problem was a broken metal connector joint between a hose (after the thermostat coupling) and the engine casing (front bank). I replaced all of the coolant hoses in that area, both connectors, gaskets, and the thermostat while I as in there. But to get there, I fully removed the fuel tank (disconnecting all of the hoses), the airbox (fully out), and the throttle bodies (enough to tip it up out of the way to the left side, but not fully removed from the bike). Because it had been awhile, I replaced all four spark plugs while I was at it.

    Once reassembled, the engine started back up just fine to my delight. It idles okay and can come up to operating temperature (which I did to check the coolant leak). But as soon as I twist the throttle, it dies. If I twist it ever-so-slowly, it will gently climb in revs. But anything other than that (e.g. a normal departure throttle), and the engine will just die. I was pretty meticulous reconnecting the airbox vacuum hoses (thankfully numbered) and the fuel tank following the shop manual. I have since disassembled it again looking for an inadvertent hose that may have decoupled during reassembly, but everything seemed intact. I'm on the verge of taking it to the shop but wanted to see if I could get any pointers on where to look next, whether it might be a vacuum or a fuel problem, or possibly anything even more specific.
     
  2. Knight

    Knight New Member

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    Check the vacuum lines such as the breather and fuel return that they not pinched. That could create a vacuum when running until the vacuum overcomes the positive fuel flow and it dies. You can try opening the tank when the bike is dead. Do you get a "whoosh" of air and the bike starts and allows more rpm for a time? That would be a dead giveaway.
     
  3. Grumpy old man

    Grumpy old man New Member

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    my money is on that You have not re-connected the map sensor

    It is the sensor that the 4 hoses from the throttle join with through the little 5 port plastic "T"
    Check both the rubber hose connections and the electrical plug

    with this unplugged the bike will idle fine but as soon as you give it throttle it will die
     
  4. Mark919

    Mark919 New Member

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    +1 on this suggestion!
     
  5. vfrtodd

    vfrtodd New Member

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    Thank you, gentlemen. Had to leave town, so can't try those immediately but will check both when I return and let you know. As of last night, the 4 hoses from the connector were all connected (at both ends) - that was one of the first spot checks I had done. I could have missed the electrical (though I didn't leave any unplugged that I could see, but it's always a possibility and I'll definitely check that). It's also quite possible it wasn't fully seated. And I'll check the fuel lines too. That would be easy to check pinching by simply raising the tank up. Much appreciated!!
     
  6. vfrtodd

    vfrtodd New Member

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    Finally back yesterday and was able to reassemble everything. I verified the electrical connections were in place. Same results unfortunately. It accelerates up to about 3500-4000rpm before dying when twisted at a normal rate. When twisted slowly, it can go as high as I want it to go, though it would stall when it returned to idle. I left the fuel tank up to make sure hoses weren't pinched:
    [video=youtube;PQSwb77QrLE]http://www.youtube.com/watch?v=PQSwb77QrLE[/video]
     
  7. Jeff_Barrett

    Jeff_Barrett Member

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    Test your fuel pump ... it's possible it's not keeping up when you crack the throttle?

    It's like the bike is running out of fuel / fuel pressure to keep it running when you crack it hard.
     
  8. Terry Smith

    Terry Smith Member

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    It seems unlikely that a fuel pump that was previously fine is now giving grief; all you did was disconnect the fuel hose and reconnect it. I take it there's no gushing leak of fuel???

    I'd be looking at the throttle position sensor and the MAP sensor and checking that they are connected and there's no obvious wiring damage. Are you getting an FI warning light at all? I forgot my MAP sensor hose connector and it threw up a fault pretty quickly.

    Last dumb question...did you clean out all rags that were used to stop parts falling into the intakes?
     
  9. Knight

    Knight New Member

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    As a double check of vacuum, open the gas tank as soon as it dies, listening for a rush of air. Then make sure the problem still occurs while the tank remains open.

    What Cadbury said, maybe the sensor connections are corroded or one of the wires is shorted. Inspect the wires carefully for tears. Spray the connections with an electrical cleaner (???)
     
  10. Jeff_Barrett

    Jeff_Barrett Member

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    I had originally suggested that MAP and others the TPS so I'm assuming he checked that since.

    EDIT: Whoops ... different post. Saw another of these the other day and suggested MAP. My bad. Yes ... check the MAP/TPS first and make sure no obstructions in the airbox / intakes. I just went full SOW.
     
  11. vfrtodd

    vfrtodd New Member

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    I think it's the MAP or TPS. While the video didn't show it (it wasn't on for me), when I actually tried to drive it a block, the FI light did indeed first blink then stay lit. I'm going to try to diagnose that based on this:
    http://www.vfrdiscussion.com/forum/index.php/topic/16-fi-light-how-to-read-error-codes/
    The MAP plug was a pretty tight fit to get around and plugged in, so it's possible I have a broken connection in the line. When driving, it exhibits more of a TPS problem, though, as the revs no longer matter, but the position of throttle (e.g. higher throttle but lower revs to go up a hill - twisting the throttle even at 3k-rpm would start to die).
    Oh, I first did crack the tank open as soon as it died. No vacuum whoosh.
    And yes, all the safety rags were removed from the air intakes.
     
  12. Grumpy old man

    Grumpy old man New Member

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    When you tightened the clamps on the card boots (insulators) did you leave the recommended gap in mm between the ends of the clamps as over tightening these can cause air leaks

    I think they are around 7 mm gap between the ends of the clamps when tightened to spec

    Also how were the rubber boots (insulators) no cracks chips ,tears etc I have seen these crack when the throttle bodies/carbs are removed as they become brittle with age and heat cycling

    The cracks can be very hard to see

    I bought new ones last time I did the thermostat and just replaced them along with all the thermostat hoses

    Back to the MAP sensor the hose from the plastic 5 way to the actual sensor can split at the sensor end

    I have also inadvertently pulled a vacuum hose off a cylinder takeoff point when wrestling with the 5 way splitter before
     
  13. nearfreezing

    nearfreezing New Member

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    If I recall correctly, it's possible to accidentally swap the connectors for two of the sensors in the airbox/fueling system. Double check and make sure you connected the right sensor to the right connector in the wiring harness. The two that are easy to swap might be the MAP 3P and ECT 3P sensors.

    EDIT: This is a write-up for the 6th gen, but it might apply to the 5th gen....
    http://vfrworld.com/forums/showthread.php/29432-Thermostat-Replacement-6th-Gen-VFR800-VTEC

    "Note that there are two connectors that are interchangeable -- the IAT sensor and the Bypass control solenoid valve. I originally had these switched, and that threw an IAT sensor FI blink code. Drove me NUTS trying to figure it out. Meatloaf was kind enough to send me a spare IAT sensor to troubleshoot. Thanks for that!"
     
    Last edited: Jan 6, 2016
  14. vfrtodd

    vfrtodd New Member

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    nearfreeezing, you nailed it. The two connectors are the same and indeed they were swapped. That simple. I feel like a bonehead. But in a relieving kind of way. Thanks all!
     
  15. Allyance

    Allyance Insider

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    That's strange, Honda normally uses different connectors (color or pins) for each device/system.
     
  16. vfrtodd

    vfrtodd New Member

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    Yeah, they normally do. In this case, there's a small orange(?) tab inside to help distinguish, but the pin connections are identical. My own earlier quote should have tipped me off: The MAP plug was a pretty tight fit to get around and plugged in
     
  17. Terry Smith

    Terry Smith Member

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    Thanks for posting your resolution to this one Todd. Another one to file away in the "just in case..." memory.
     
  18. nearfreezing

    nearfreezing New Member

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    Glad you fixed it!
     
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