Modern Vespa

The issue I am having with my coolant temperature arose from detecting what I think is the scent of gasoline in my oil. It was recommended by several folks on the board to test or replace my thermostat, since having the engine run below normal operating temp could result in unburned fuel in the oil system, and the scoot would run better at temp since that is how it was designed anyway.

I replaced the thermostat and upon testing the system, let it idle and run to normal temp (just below half on the gauge) and cycle several times through the thermostat opening and closing, checking the coolant reservoir for warmth, proving the system was operating as intended. All was fine.

Upon riding the scoot in cool temps (55-60 degrees) the scoot's gauge sits well below normal, mostly at or below 1/4. That is running at all speeds, highway, freeway, city, etc. I brought the issue up to a couple people at a group meetup recently, and the suggested that I need to bleed the coolant system, which I never did when I installed the thermostat.

What I did do was to run the scoot to temp and let idle for 15 minutes with the reservoir cap off, but did not keep my eye on checking for air bubbles.
in the system.

Then today, I came across this thread about coolant replacement:
https://modernvespa.com/forum/topic29213?highlight=coolant

Since there is no bleeder nipple as there is on the GTS, how can I be sure I have bled out the air in the coolant system of my GT and how can I get my scoot to run at the proper temperature like it's supposed to?

It's possible that your temp sensor is high resistance, giving the low gauge reading. Have you measured the temperature by other means?

IIRC, there's no need to specifically bleed the GT200, it's self-purging. If I'm wrong, hopefully someone with more recent experience can advise.

mostly at or below 1/4 on a cool day is normal for this bike.

My gilera runner VXR200 has the same engine as your GT .
There's no bleeding of system needed just fill up slowly then let it run then top up As for the temp my VXR has 12 digi bars on a cold day below 10 deg C ( 50 F ) it won't go above 3 bars , On a warm day ( 20 + deg C 70 F )it will run between 4 & 6 bars depending on riding conditions ( town or open road ) , Even on a very hot day it won't go above 7 bars where the fan cuts in without sitting on idle for a while .
your scoot sounds normal to me
Its over cooled for the normal uk weather really

I'm looking at testing the sensor next. Would this condition cause it to read normal at idle and low while riding?

The thermostat should keep it at 'normal' (whatever that is) while riding. Idling for a while it should read hot (but not off the scale) and the fan should kick in for a bit now and then.

When riding temp go's down abit because cool air is going through radiator .

That is exactly what happens at idle. When riding, the temp goes down.

OK.

Keep an eye nostril open for the petrol in the oil - if it no longer happens (assuming the engine gets hot quick enough) then job done.

That'sy plan, assuming the engine will stay at temp to evaporate the gas in the oil. So far, not the case.

I'm beginning to suspect there is some kind of bypass for the thermostat that is over-active and allowing too much coolant to bypass the thermostat.

Also, just tested the gauge in the instrument cluster - ground out the temp sending unit lead to the engine block - needle rose to "H" so the gauge is good.

Any other thoughts? How do I, or do I need to, bleed or "burp" the GT's coolant system?

After riding in really hot weather today - 90's and down to mid-80's - the temp gauge never once rose above 1/3 even after idling for 15 minutes in the driveway. Fan never came on once. I am beginning to suspect the sender unit is faulty.

I tested the sender unit with my multi-tester and it showed the same resistance as the leads do when the tester is on and the leads are not touching.

The engine was off and cold. With the key off, one lead on the contact point for sending wire, the other to various points near the unit including the brass housing that screws into the cylinder head. Did the same tests with the key and headlight on as well. Some instances gave a crazy result on the meter like 193.4 or 183.9 momentarily, then would fall quickly to null.

Any thoughts? Jimc- you've been a good source for me thus far...

If the fan never came on then I still suspect the thermostat - even if new! Did you check the old one by immersing in heated water and monitoring the temperature?

The temperature sensor is just a thermistor in series with the gauge, I have no figures for its performance.

The fans are controlled by a separate thermal switch.

Actually, I did check it out though I'm not so confident im my results - I used a meat thermometer and the thermostat sat in the bottom of the pot of water. It seemed to stay closed until about 158 degrees according to my test.

Looks like I'm going to need to crack open the system again and test the new one, too...

That temp is spot on according to the GTS workshop manual 69.5 to 72.5 degrees C.


Harvey

You may just be chasing nothing abnormal. I have a 2006 GT200 with about 16,000 miles on it. I bought it new and have only had the fan kick on 3 times. Once when idling in traffic at about 103 degrees, and the other 2 times also stuck in traffic at about 100 degrees. If it is under 75 degrees out the temperature gauge reads between 1/4 and 1/2. In 85 to 100 degree weather it reads right on 1/2 unless I get caught for a long time in stop and go traffic. When it reaches a little over 3/4 (which is almost never) the fan kicks on and cools it right down. I have a friend with a GT250 which is similar although his fan does come on slightly more often than mine. I just assumed the fuel injected GT250s ran a little hotter. I hope this is of some help to you.

Agreed, absolutely.

So the petrol in the oil is either far too many short journeys or worn rings.

MichaelR: Thanks for that anecdotal evidence. I may be chasing nothing after all. Hoe hard is it for a couple GT riders to eyeball their temp gauge a few times and report back? I appreciate the input.

Jimc: It very well could be from short trip, too. I do a fair bit of in-town riding when perhaps I should be taking my wife's LX instead.

Anyhow, I have more info. I removed the new thermostat and tested both the old and the new side-by-side and they both check out. Both opened at required temp (155-160 degrees) in the pot of hot water and continued to open further as the water got hotter. Then the closed as the water cooled. No bunk units. I ended up installing the old one, running the bike to temp - it took 10 minutes to reach thermostat operating temp in 80 degree weather - per the procedure in my Haynes manual.

And, Jim: I am an American, and I weigh less than 200 lbs.

Hmm...

I'm actually very sorry if I sent you on a wild goose chase. BUT with luck it will prevent others from enduring what you have done to cover some basics.

What remains:

The temperature gauge shows low(ish). This may be no problem except for a duff gauge or a model-specific characteristic.

Petrol (gas) in the oil. Now this IS a problem. Regular short journeys with no long hot blasts will exacerbate this - but still it shouldn't really happen - these scoots are designed for urban life FFS.

Thinks - do you let your scoot 'warm up' before riding? That could account for the gas in the oil - these machines are supposed to be ridden, not cossetted!

Start-up is as follows: start bike and let idle while gearing up, locking house, garage, etc. Jacket, helmet, gloves. Check signals, lights, tires. Leave.

Total time is around 5 minutes, sometimes more, sometimes less.

Agree with Michael R, however for me it's in celcius:

If it's 20c to 30c my gauge is on about 1/4 of the way to H
35c-40c the gauge is at half way - if I get stuck in traffic then the needle goes up to hot, but then the fan kicks in. Gave me a bit of a fright the first time, but it seems to work as intended.

Oh and by the way, above 40c is to be avoided unless you like bathing in your own sweat lol!

May I suggest getting ready first, then starting the bike and riding straight off?