Still running cold.

Toyota curren 3sge.

Ran cold from buggered thermostat.
Replaced thermostat with toyota genuine last year got warm quicker and was great.
Except when you drove hard or coasted down hills with mid range rpm but no throttle it would cool down to below 1/4.

temp would jump back up after a few mins of normal driving.

this stunk to me of being a stuck thermostat as at higher revs more water flow to over cool the engine.

pulled the toyota one out and it opens at the right temp but if you push it open manually it had a tendency to stick open and required a wiggle to shut properly.
So i put a new tridon thermostat in and it holds temp when coasting etc

Only issue i still have is it takes a while to warm up but if i idle it to nearly 1/2 on the gauge then give it a hard run for a min or two it cools off back down below 1/4. however it only does this within the forst 10 mins of driving.

while racking my brain for a cause i came up with the idea of the oil cooler.
oil takes longer to heat up so if the oil is cold and theres good flow at higher rpm could the oil going through the water/oil cooler be enough to cool down the coolant?

would make sense as to why it doesnt cool the engine once properly hot.
any other thoughts?

I'd change the temp gauge next.

more like your new thermostat is also faulty, use a infrared temperture gun and mesure befor and after the thermostat to see when it has opened and what the tempertures are you can also mesure on the oil exchanger to test your theory but i extreemly doubt that your theory will be proofen, correct but who knows cant rule it out untill you test things
also check for missing thermostat seal and possible a bypass that is faulty , vw have had a aftermarket bypass fitted to stop heater core blowouts on the mk 2 golfs years ago so knowing how it works for testing is a must even on the cooling system

Ive got a water and oil temp gauge to put in the car so might get them in before i do anything rash.

intrade i agree the theory of the oil cooler works but i doubt there would be anywhere near enough water flow to have the effect i was thinking.

is the radiator clean.and also the water pump impellors. even though both point to over heating problems. not over cooling. plus whats the 3sge in?

toyota curren.

water pump is 24 months old.
radiator and hoses were replaced with new ones when the first thermostat went in last year

coolant was and is clean

temperature bounce.
sounds like you have a sensitive gauge. i would fit a decent gauge to confirm.
whats happening is thermostat is opening quickly, that forces the stone cold water in the radiator into the engine and cools it suddenly. thats your temp drop. thermostat closes till water heats up and continues the cycle until the water in radiator has increased.

depending on if its iron block, alloy head etc, manufactures try to avoid to much sudden temp change as it causes head gasket failure.

one thing to watch is triton thermostats are often rated at fully open temp, while toyotas ratings are sometimes starting to open temp. so with both being rated the same, tritons will open sooner and run cooler than toyotas thermostats.

Check fan comes on when it should. Fan usually for in stop start traffic shouldn't run on open highway situation.

I take it the genuine thermostat has a bleed valve in the outer part of the thermostat body and the Tridon one hasn't? If this is the case try drilling a 3mm hole in the flat part of the outer body in the Tridon one, refit it and then see how it behaves.

Both valves had the bleed hole in the top, fans function as normal.

From looking online it looks like toyota put the temp probe in a shit spot, right by the heater return which means with the heater on its getting colder water flowing past and its also in the best spot to get a air bubble by it if theres an airlock somewhere.

I dont think theres an airlock but apparently these engines are renowned for it so might chuck a better gauge in and have a play bleeding it till my fingers turn blue.