Yamaha TRX850 Enthusiasts Forum

Morning all. I've a cooling issue that, from researching the site, I can see others have encountered too but I still have a few questions if anyone can assist.

My issue relates to coolant being expelled from the expansion tank when hot. I've a modified engine (878cc high compression pistons) but thermostat, fan etc all kick in at correct temp and, while it maybe running hotter, temp seems to be being managed fine (apart from blowing coolant). For info the expansion tank is smaller than stock and relocated inside the front faring.

My main question is related to a suggestion to increase the pressure rating of the rad cap. From what I can see the standard cap is 0.9 bar and the next logical step would seem to be to go to 1.1 bar but am I just likely to create issues elsewhere? For info all hoses are new samco replacements on a standard TRX rad/thermostat.

I have also seen other topics related to coolant ratio and an R6 rad upgrade (straight replacement?) but clearly the rad cap is a (potential) simple fix.

As always any thoughts/advice most welcome.

Cheers

Any gas bubbles indicating a gasket or crack, blowing out the coolant ?

alextrx850 wrote:Any gas bubbles indicating a gasket or crack, blowing out the coolant ?

Thanks for the quick reply. None that I've seen. The engine has been rebuilt and only done 180 miles from completing the full bike overhaul. The coolant simply fills most of the (admittedly smaller) expansion tank and blows through the air hole in the tank cap. Not that desirable given it's over the front wheel!

Have you changed the rad cap to a new one? Might be worth a try first.

Silver wrote:Have you changed the rad cap to a new one? Might be worth a try first.

No but I have got loan of a new 1.1 bar cap to try as this was the recommendation. To be honest I wasn't keen to do it until raising the issue on the forum as it has been invaluable in the past. I was going to get the original cap tested to make sure it was working within range but they want the rad as well. Yeah, I know, I'm being a bit lazy but given the general view was that more heat will equate to more pressure, the 1.1 bar cap seemed a requirement whatever the old cap was doing. Just thought I'd raise the question among those that may have had similar issues - tho' I've not seen any post specifically related to cap pressure ratings.

My money is on the radiator cap, had the exact same problem on my FZS1000. New cap fixed it straight away, had a look at the old cap and there was a small split on one of the rubber seals.

have the same problem tuned engine - ported head, higher compression, FCRs and big bore exhaust and my coolant blows out of the rad into the header tank. Can't see any isssue with the rad cap, but it is very old, so spring may be weak. Out of interest, where did you get youur 1.1 bar rad cap?

You'll find your have a lot more heat in the motor with the 878 bore and if you've upped the compression again from the 11.5 JE piston it adds a little more..
All that fluid get a lot hotter a lot faster, it all equates to loss of power due to heat...
A few here have gone for larger radiators so would be a good investment for a little more a little more cash...and a lot more gains in power and longevity from your motor..
I think they where an R6 radiator or some kind of quad bike...

I have a suspicion changing the ratio of coolant is not really the way to deal with this as such.
It's to do with the way the coolant is used to transfer the heat so thats why a specific ratio is set.
But I did find information on the fan switch as the hyabusa boys have overheating issues too and have found a switch that will cut in 10* c cooler than std.
http://www.ebay.com/sch/sis.html?_nkw=1 ... 240%3A1318.
Weather these are the same size as the trx or values I have not gathered that info yet.
ZXR750r radiators where used as an option on a few of the TRX track bikes back in there haydays.

OK here is a little more heads up on the temp values..
Hyabua is around the same as the trx at 107*c
These switches will cut the fan in at 93*c

Now all we need to do is find out if the switches are interchangable with the TRX.
The two pin connection is an easy one to get around if the plug doesnt fit.

What this will do is..

It doesn't make the bike run cooler. The engine coolant temp is regualted by the thermostat.

What this switch does do is turn the radiator fan on sooner at a lower temp and shuts off at a lower temp as well.
It can help keep the temps better regulated.
The OEM sensor waits until the coolant temp is higher before the fan kicks on which means it's got to work that much harder to get the temp back down. Then the OEM sensor also shuts off soooner which in turn means it'll have to turn on again sooner to control the temps.

So in a nutshell the thermosat value (78*c) is set around the optimum running temp.
Buy the time the temp rises close to 107*c your prob have lost a couple of HP.
By the fan cutting in at 93*c your haven't really lost any HP so it's keeping you close to optimum..

Interesting, like your thoughts Cobba can you keep us posted? be good to get a simple solution as am pissed of about having to change my gearbox Grrr!