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Thread: Overheating diagnosis help

  1. #1
    apriliaforum newb
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    Overheating diagnosis help

    Hello everyone!

    My Aprilia Pegaso 650 Cube (mod 1999) troubles me with an overheating problem.

    The story:
    One day during normal riding to work, the bike overeheated (almost to 120C). I started searching for the cause and first thing I noticed was that the radiator was cold from the middle up, expansion tank was overfilled and no apparent leaks.
    I removed the radiator, flushed the system, cleaned everything, checked cooling fan operation, checked temp sensor and gauge and refilled the cooling system according to service manual.
    After that the bike run fine. During daily 40 km mixed daily ride to work it rarely reached 80C (on highway it was about 60-70C). EXCEPT the fact that it needed periodical coolant refill on radiator with no leaks detected.

    After a while I noticed that when riding with passenger in highway (120-130 km/h) when I pin the throttle for an overtake, temperature rises rapidly and it doesn't drop even if I slow down considerably. Upon stopping the radiator is cold, the expansion tank overfilled (when the bike cools, the coolant returns as normal to radiator), cooling hoses feel normal at touch and no leaks.

    From what I have read here, my first thought is blown head gasket. BUT... no oil contamination, no coolant contamination and when I ride alone the same route at the same pace, the bike hardly gets to 60C!!!

    Is this typical of head gasket problem or I should check something else?

  2. #2
    apriliaforum Junkie
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    It seems clear your coolant is going somewhere, and if you have no leaks, either it's being squirted out of the expansion tank when you're not looking, or you've a gasket issue.

    Is your thermostat OK?

    I suggest you do a pressure test on your whole cooling system. Most automotive parts shops can do this for you, either by measuring the composition of the cooling water, or adding a dye and inspecting under UV light. The problem correlating with carrying a passenger is almost certainly just because you're at full load for longer than normal.

    Are you 100% certain you get all of the air out of the system after flushing?

    Cheers... Paul

  3. #3
    apriliaforum newb
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    Paul,

    I haven't noticed the coolant overflowing through the expansion tank (it could happen while riding?) and I am not sure about the thermostat, but it's the next thing that I must check. But if it was faulty, it could happen when driving solo or even when I am at traffic

    As for the air in the system, I followed exactly the procedure outlined in the service manual. Could it also be a faulty radiator cap not sealing correctly?

  4. #4
    apriliaforum Junkie
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    Yes, for sure. It's reasonably rare, but it does happen. If you can get another cap, try it.

    Cheers... Paul

  5. #5
    apriliaforum newb
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    Has anyone been able to find a source for a new thermostat or a similar replacement?

  6. #6
    apriliaforum newb marlouw78's Avatar
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    Quote Originally Posted by gstt500 View Post
    Has anyone been able to find a source for a new thermostat or a similar replacement?

    Would also like to know.

  7. #7
    apriliaforum newb
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    I wasn't able to find the thermostat from an Aprilia dealer, but I have found that it is available in the following sites:

    https://www.ultimateparts.net/aprilia/a.php?i=348526

    https://www.motosp.gr/partcategory.p...r=1999#content

    And from what I have read in various threads, some owners removed the thermostat completely (at least temporarily until they find a replacement) without any problems. The only difference is that the bike will be slower to reach operating temperature.

  8. #8
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    After all the checks the problem has been identified as a leaking head gasket..... The combustion gases escape to the coolant and that makes the cooling system not work correctly.

    Before I go for the head gasket replacement, I would like your opinion on the following:

    1. Aprilia head gasket is 75€ and Athena 35€. Should I go for the factory one?

    2. The head gasket from Aprilia is Part No AP0230604 and is the one used by Pegaso IE. In this thread: https://www.apriliaforum.com/forums/...=1#post2884322, the guys opted for the older one. Should I do the same?

    3. Do you suggest taking the head and cylinder to a machine shop to clean and level the mating surfaces?
    Last edited by rogtek; 04-18-2019 at 04:45 AM.

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