Water temp instability in Toyota MR2 with F20C swap.....

[sir_smeghed]

CAR: 1990 Toyota MR2
Modifications: F20C powered

Issue: Am I correct in assuming that the water flows from the radiator, into the thermostat on the cold side and there is a bypass valve in place with the hot water feeding out of the gearbox end of the block? When the car is moving at speed the water temp goes quite low coming out of the engine (around 60c) but when the car is idling the water temp goes up to just over 100c! This has got me stumped as the temp gauge sensor is on the gearbox end. This is my track day car so I am desperate to get this water temp stabilised ASAP.

Things to keep in mind: The engine is in the rear with radiator up front so radiator heat soak is not an issue. Airflow through radiator is good with no obstructions. Radiator has even heat distribution, no hot or cold spots.

I'm stumped! I reversed the flow through the radiator to check the water temp going into the engine (not exactly scientific but it worked), and the water going into the engine was stable at around 40c.

If you know why I'm having this issue please let me know, it's doing my head in. Car is in Brisbane.

[ChargeR]

Not super familiar with the F20, but it appears they're similar to the K series, in which case yes you're right, the water pump pushes the coolant in through the lower timing cover end of the block, and outlets at the other (gearbox) end of the head. 60° is quite cold coming out of the head, will it stay at that temperature even after some time driving?

Where exactly is your temperature gauge sensor located? In the stock location in the pipe heading out of the gearbox end of the head?

F20C cooling hose.jpg

Have you modified any of the coolant hose routing on the F20? What water pump are you using? I'm guessing electric if you were able to reverse the flow.

[sir_smeghed]

Sorry, I should have been a bit more clear. I didn't reverse the flow through the engine merely through the radiator (reversed hoses at the engine) so the water temp sensor was on the return side.

While I'm driving at speeds of over 60 km/h it pretty much sits at 60c or there abouts, the sensor is about 40 cm from the outlet on the engine (unless there is another that I'm not aware of). If I'm stuck in traffic the water temp keeps creeping up and up, I have a manual radiator fan switch in conjunction with the auto and this makes NO difference to the water temp at all! I reversed the flow to the radiator so I could get some indication of how effective the radiator is, this seems to be quite good so the engine is getting 40c water.

Why would the water temp be going up at idle? Where would I mount a water temp sensor for an accurate, useful and constant reading? Is there anything internally that could be wrong and causing this?

Thanks

[ChargeR]

I would mount the water temp sensor in the factory location, indicated in the pic I posed above, but from the sounds of things that's basically where you have it.

60° is too cold. So there is some underlying issue. A typical Honda thermostat is fully open at 85-90°, so the temperature coming out of the head should be in the ballpark of that most of the time.

The fact that having the fans switched manually makes no difference to the issue suggests the problem is coolant flow related, not air flow related.

Can you give a bit better picture of how the cooling system is configured? Particularly any modifications from the factory F20C routing on the block/head. S2000s have a an adequate cooling system, so IMO whatever the issue is it has to be something related to how the cooling system has been modified to work in the MR2.

[wpg81]

You need to let the car warm up and use a temp gun or feel the coolant inlet hose and the coolant outlet hose. It should be warm to hot on the outlet hose and cool on the inlet hose UNTIL the thermostat opens. The inlet hose will then warm up and become just alittle (10 degrees approx) cooler than the outlet hose. This maybe a little slower to happen due to the length of hose and pipes for front to back of car.

It sounds like the thermostat has been removed or stuck open. This allows the coolant to flow free at all times causing low temps. At idle the water pump is not moving a lot of coolant allowing the temps to get up to correct levels. Traditionally rear engine car will run a little hotter due to lack of air flow to the engine. If you replace the thermostat look into one that open a fue degrees earlier.

Just for interest why a f20 and not a k20 or k24? What gearbox is the f20 mounted up to and dose it use a adaptor plate? I would love to see a build thread or some photos!

Hope this helps with up problems.

[sir_smeghed]

Thanks guys. I didn't do the conversion but purchased the car with it already done, the previous owner starter a build thread on the MR2 website: https://www.mr2australia.com/mr2play/tm.aspx?m=96882

I think he chose the F20C because it was different, well.... he was right! The K20 or K24 would have been a lot easier, there are complete conversion kits available for them to suit the SW20 MR2. But I recognized this as special, which is why I bought it.

It has the toyota MR2 standard S54 gearbox (standard in the NA model), with an adaptor plate and a custom flywheel machined out of a single piece of chromoly. I have also got a bit of info on this forums introduction http://forum.clubitr.com.au/showthre...ht=#post294599

It is running a factory ECU and I have mounted an Apexi AFC-NEO just to get the fuel nice and smooth (all I could afford, I'm on a tight budget). With the factory ECU the block still has the factory sensor, hmmmm, I wonder if the Apexi has the ability to show water temp as it is running directly from the ECU? I will look into this.

It definitely has a thermostat and due to the MR2 setup I have replaced the factory unit with one that comes fully open at 75C. I'm starting to wonder if the previous owner removed or modified the bypass valve, would this cause these issues? When next I have the chance to put it on a hoist I will check it out more thoroughly.

The conversion has been done beautifully, and it is a real mover which turns heads! And that exhaust note..... WOW! Anyway, it's obviously still got a few of those niggly little things that you need to deal with when a conversion has taken place, I'm trying my best to make all rectifications with off the shelf parts and have a maintenance diary listing part numbers so next time I won't have to go through this.

I'm located in Brisbane wpg81, so if you're ever out this way and want to check it out let me know.

[wpg81]

I think it would be an awesome car, I love the f20 its possibly Honda best ever engine! It's a feet just getting a project like this running. I would love to catch up with you and your car! Hope you get this sorted soon and get the car out and about.