Thermostat Woes and Rectification. Gen 4.

Discussion in '3rd & 4th Generation 1990-1997' started by Tinkicker, Jun 9, 2020.

  1. Tinkicker

    Tinkicker New Member

    United Kingdom
    Apr 6, 2020
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    Selby North Yorkshire
    Thought I'd post it here rather in the mechanics corner because most people who will be interested will be here...

    So I have a restored 1997 VFR750 fv that had the common problem of the temp gauge barely making off the bottom of the scale. I had a brand new FV in 1997 and know the normal needle position with engine at normal operating temp is between 1/8 and 1/4 of the way up the scale, so obviously something amiss.
    Common problem is the thermostat stuck in the open position and since the gauge needle changed with speed, it seemed this was the culprit.
    So I ordered a new stat.
    Removed the old one and sure enough it was stuck open. Fitted new one and took the bike on a test ride feeling nice and smug...
    That warm, cozy feeling soon passed because the gauge stayed in the same zone as before, only a bit more stable than before, it did not fluctuate as much.
    So it appears I am suffering the VFR double whammy of a bad thermostat and a bad temp sensor. That's about the way my luck usually goes...... So when the new sensor arrives I'm going to have to drain down the coolant again.

    For others, hearing tales about having to remove the carbs ect according to the book, I will regale you with a tale on what is required to change the T stat on a gen 4 while it is fresh in my mind.

    1. Drain the coolant. To get at the coolant drain plug on the water pump, you will need to drop the lower cowl, unless you can get to the hose clip and just pull the bottom hose.

    2. Remove the R/H side cowl to gain access to the pressure cap and thermostat housing. Remove the soundproofing pad. It is best to have the cap off before you remove the drain plug, the coolant comes out with some force initially and will end up all over the floor unless you are ready to catch it with a bowl.

    3. If you are intending to replace the coolant, remove the expansion tank retaining bolt, lift the tank off its rubber mounts, a tip it over into a bowl to get the old coolant out. Refit the tank.

    4. Remove the radiator hose from the thermostat housing. (It is right in front of you and accessible with the carbs in place).
    5. You need to put a screwdriver into the housing water outlet and just bend the supporting bracket down a little to get a ring spanner under the carbs and onto the front bolt. Loosen the bolt.
    6. Loosen the rear bolt.
    7. Using a 5mm hex key, remove the bolt securing the housing to the supporting bracket and you can then move the housing on its rear hoses and gain enough access to get to the bolts with your fingertips and take them out.
    8. Remove and discard the old O ring and thermostat.
    9. Fit new T stat with air bleed hole to front of bike at highest point and fit a new 47mm x 2.5mm Viton O ring - far cheaper and easier to source than the OEM Honda one. I got five for the price of the Honda one from Ebay with free shipping. The Honda one seemed to be unavailable at the time I bought the T stat.

    10. Rebuild is the reverse of removal.

    Time of job taking it easy with my meagre home tools was around an hour, if I had my work tools such as ratchet spanners it would have been considerably less. Most time was taken up by the removal and refit of both side fairings and the lower fairing.

    It really is an easy job on the gen 4. Carbs do not need to be disturbed at all, unlike what the official manual says. Probably because I bent the support bracket down 5mm to get an 8mm spanner in, then bent it back when everything was tightened back up.

    Hope this helps someone thinking of tackling the job.
    Last edited: Jun 9, 2020
    Spud750 and RedMenace like this.

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