Can anyone give me some advice on my problem.

I have recently fitted a mishimoto radiator to my Subaru gc8 with a rcmhigh flow thermostat which opens at 70 degrees but then upon driving it I found the car struggled to maintain temperature and soon as I was on a motorway or duel carriageway it would plummet to cold on my gauge on the clocks, I swapped that one out for a high flow thermostat that opens at 78 degrees but it still can't maintain temperature on a motorway or duel carriageway it's better but not good enough, can anyone guide me what to do next?

Do I go back to a standard thermostat?

Get a smaller radiator? Or is my temp sensor playing up?

 

Altho the gauge reads cold does the air coming out of sir vents reflect this ?

If so I'd say standard stat or smaller Rad

 

The vents are lovely and warm, It's just the gauge reading it, I think il change for a standard stat don't really want to change the rad as I bought it for track days

 

If you got warm air and now the needle is lower, that to me is a plus, the alloy rads make a big difference to coolant temps which allows for when car is driven hard and coolant will but under control unlike with a smaller or old rad.

 

Mine Sits just above 1/4 and 1/2 when driving hard, you'll also find the more you drive with air getting to rad the cooler it will be for obvious reasons, but slow down into stop start town driving and will be upto to normal.
Don't worry about it

 

With the low temp opening stats, unless you are using the car on boost it will not stay at the temp stated on the stat, especially on light throttle openings to maintain speed at 70 to 80 mph, you wait until the winter temps start if your having issues now, look also at the oil temp, that struggles even more with these low temp stats.

 

If your vents give out hot air and you fitted a 78 degree stat then the sender in the alloy water pipe is at fault , my 95 Wrx did the same , at idle it showed correct reading as soon as I started cruising it dropped to pointing at 9 o clock .

 

Presumably it will over fuel too!!

 

Does sound like a faulty sender,won't over fuel as its a separate sensor on a classic.

 

This

 

But the op is now running a 78* stat . Same as oem isn't it?

 

Alloy radiator with better heat exchange will also compound issues.

 

mine is 84 deg.

 

Isn't the problem with the high flow performance stats?
I remember API modded my OEM 84deg stat by drilling 2 holes in it to increase flow, it worked but gave me issues with low temps when cruising on dual carriageways in winter.