[MR2ManiacJC]

Whats up ya’ll? So I've owned my MR2 since May of 2015.
I’ve done a lot of stuff to it over the years(mainly aesthetics) and a 5 speed manual conversion. I love these cars and can’t get enough of them. Unfortunately, for the past 10 months I’ve gotten into an overheating battle with this car and I’m starting to throw in the towel. This is exactly why I’m creating this thread. I hope those more experienced MR2 owners could help guide me into the right direction and hopefully, talk some sense into me before I throw in the towel.:crying:

The issue:​

Back in February of this year I was on my way to school (46 miles) and I noticed my temperature gauge started rising when I came to the first stoplight after exiting the freeway. I immediately turned on the heater to try and bring the gauge down but had no luck. Luckily, the gauge stayed only slightly above the half mark so I figured I could make it to school. Then suddenly it began rising a little more and during this time the stoplight turned green. Once I got the vehicle moving I noticed my gauge went back to normal. So I thought to myself hmm that was odd? Then came another stop and my gauge began rising once again. So again I got moving and the gauge would go back to normal. If I slowed down the gauge would rise slowly. But if I slowed down completely the temperature gauge would just keep going up at a relatively fast rate. I ended up making it to school but did not want to risk driving the car home since I figured it would overheat in traffic, so I got it towed.

Unfortunately, It is now December and I still have not managed to fix the issue. For the past months I’ve been battling with this issue. I’m mainly battling because I want to figure it out without taking it to any local scam mechanic. Each time I think I’ve found the solution. . . I quickly realize that I haven’t. My car is 100% drivable. It’ll get on the highway or go as far as I want it to. The issue is once I come to a stop after a good drive I know for a fact my gauge will begin rising. There are days where I will drive the car around the city and have no issue. But I also know that if I hop on the highway and hit a 20 minute drive or so the car will overheat once I come to a stop. It’s been doing this since February. Anyways, here is what I’ve done to try and fix this issue from February to now. 1991 5S-FE

 

[MR2ManiacJC]

The Basics

1. 
   I checked for coolant leaks. NO LEAKS
2. I also checked for coolant in the reservoir. COOLANT PRESENT
3. I inspected all hoses in the engine bay. NO BAD HOSES SINCE I REPLACED ALL OF THEM BACK IN LATE 2016.
4. I pinned the ECU to check for codes. FOUND A CODE FOR THE COOLANT TEMP SENSOR. CODE: 22 >

So I began by addressing the Coolant temp sensor
I also replaced my radiator cap during this time just for the hell of it.

 

[MR2ManiacJC]

The Basics Continued

I went down to my local auto parts store and bought myself a new coolant temp sensor. Pretty cheap little thing. I know many of you will say it is a sh*tier quality if it came from the local auto parts store but I figured it would get the job done. After all, it was only about $10 :grin2:

5. I bought a new coolant temp sensor and popped it in. I also bled the system after doing this. CODE WAS GONE BUT THE OVERHEATING WAS STILL HAPPENING AT A STOP

For those of you who do not know . . . the coolant temperature sensor is the green sensor in your engine bay. Sits on the water neck near your distributor. Below is a picture of the green sensor. Ignore the grey sensor that this person is pulling off in this picture.

 

[MR2ManiacJC]

A bit beyond the basics

6. I noticed my temperature gauge would read slightly under the half mark so I looked into a new thermostat since I figured mine was stuck open.
BOUGHT THE LAST ONE FROM MY LOCAL TOYOTA DEALER HAHA.>

7. Pulled out the old thermostat and tested it alongside the new one. TURNS OUT THE OLD ONE WAS STUCK OPEN.

8. I put the new one in and bled the system again. AFTER A 20 MINUTE DRIVE THE CAR STARTED OVERHEATING AT A STOP AGAIN:frown2:


Below is a picture of my thermostats being cooked :grin2: This is just a random picture I took but basically from the very beginning the old thermostat was slightly open even when the water was still cold. It also did not shut completely anymore.
Top: Old thermostat Bottom: New Thermostat

 

[MR2ManiacJC]

Intermediate Basics

9. Started looking through the BGB to figure out if I could test my radiator fans. FOUND A FEW WAYS TO TEST THE RADIATOR FANS

10. I used my multimeter to test the No. 1, No. 2, and No.3 fan relays according to the BGB. EACH RELAY PASSED THE TEST

11. I tested the Fan Main Relay. IT ALSO PASSED THE TEST

These steps require you to know how to use a multi-meter. They are not hard test to perform so if you are interested in doing so I will provide a screen shot of the steps provided in the BGB. You are performing these tests at your own risk

 

[MR2ManiacJC]

Still Intermediate Basics

12. Moved onto testing the fans themselves. :surprise:

First Concern:​

Basically the BGB says,

• “Connect the battery and ammeter to the cooling fan connector”
• “Check that the fan rotates smoothly and check the reading on the ammeter”

According the the BGB:
An automatic transmission cooling fan connector should read: 8.8 - 10.8 Amps
MINE DOES NOT READ THIS

A MANUAL transmission cooling fan connector should read: 5.8 - 10.8 Amps
MINE READ CLOSER TO THIS BUT STILL NOT THERE

So based off this I figured maybe my fan motors could be bad but honestly was still not convinced because there could be a lot of other reasons why I was not reading at least at 5.8 amps . PLEASE RECALL THAT I DID A 5 SPEED AUTO TO MANUAL CONVERSION TO THIS CAR BACK IN 2015.

 

[MR2ManiacJC]

More intermediate basics

Second Concern:​

The BGB also has a test for the radiator water temperature sensor which I never suspected could be the issue. After reading through the BGB I’m thinking maybe it is an issue.

The BGB says:

• “Disconnect the sensor and check that the fan rotates if not replace sensor”
  ROTATED AT HIGH SPEED WHEN I DID THIS
  
• “Raise coolant temperature between 185 - 194 degrees fahrenheit and check that fans rotate on LOW speed if not replace sensor.
  IT DID ROTATE AT LOW SPEED
  
• "Raise coolant temperature above 194 degrees fahrenheit and check that fans rotate on HIGH speed if not replace sensor".
  I COULD NOT GET MY COOLANT TEMPERATURE PASSED 194. THE CLOSEST I GOT WAS 190

So I figured that maybe my radiator water temperature sensor is the issue but I need to perform that third test in order to verify. My thoughts are that above 194 degrees fahrenheit the car should technically be “overheating” and my high speed fans should kick in. I know cars naturally tend to run hotter when sitting at idle. Therefore, it is possible that the high speed on the fans is not kicking in when at idle(during a stop) above 194. This would mean that my water temp sensor is bad after all because it only allows my LOW speed to kick in. However, I'm not convinced yet but I'm sure hoping this is the issue. I went ahead and ordered a new OEM water temp sensor. Sadly, this one was a bit more expensive than the coolant temp sensor. It costed me a good $70 :grin2: .

Once the sensor arrives I will rip out the old one and perform the next tests listed on the BGB just to make sure it is the bad. Even if it is not the issue I'm going to replace it anyway. But I have my fingers crossed that this is what's been causing my overheating. I provided a screenshot of the BGB for the tests I will be performing on the radiator temp sensor.

 

[MR2ManiacJC]

In the meantime

The sensor should be here by next week which is when I will update this thread. Feel free to post any suggestions or give any feedback. I know many of you will probably say it is my water pump but I'm really hoping it isn't :frown2:

Anyways, I'm open to any ideas. Hopefully someone who has experienced this same issue comes across my thread and lends me a hand. In the meantime I will share some pictures of the beast. Enjoy>

 

[merryfrankster]

I've scanned through and a few things:

a. you should be aware that there are two versions of the radiator temp switch/sensor. One is for cars with a/c. The second is for cars without a/c. They have different part number, different spec, different behavior, and there are different sections of the BGB that apply to each one of the two.

b. the real functional test of your fans is do they come on when you come to a stop after your 20-minute drive. So pull over, get out of the car, and check whether the fans are on.

c. replace your thermostat with a new OEM or just remove it entirely.

d. Use a pressure tester and check for leaks. If the coolant system does not hold pressure it will show symptoms of overheat. Recently I did a complete overhaul of a customer's 1994 5sfe coolant system that was overheating. It was leaking out of the heater hoses, the radiator hoses, the cap, and, worse of all because of the labor involved, it was also leaking out of the oil cooler hoses - in other words, everywhere. With all the leaks fixed there was no more overheat.

 

[MR2ManiacJC]

a. you should be aware that there are two versions of the radiator temp switch/sensor. One is for cars with a/c. The second is for cars without a/c. They have different part number, different spec, different behavior, and there are different sections of the BGB that apply to each one of the two.

b. the real functional test of your fans is do they come on when you come to a stop after your 20-minute drive. So pull over, get out of the car, and check whether the fans are on.

c. replace your thermostat with a new OEM or just remove it entirely.

d. Use a pressure tester and check for leaks. If the coolant system does not hold pressure it will show symptoms of overheat.

a. Yes I am aware of the two versions. Thank you for that though.
It took me a while to locate a radiator temp sensor for cars with A/C. In the end I finally found one though.
b. Yes that is what I have failed to do. Everytime it starts to overheat I panic and just get driving so that my gauge goes back to normal. I keep telling myself to step out the car and check if the fans are on or not but I haven't grown the courage to pull through with it.
c. I did replace my thermostat back in August with an OEM Toyota one.
d. The only reason why I haven't done a pressure test is because I have no leaks but I guess it isn't a bad idea. All of my hoses are about a little over a year old since I replaced them during the end of 2016. My radiator cap is also relatively new. (13 psi)

 

[rmeller]

...The only reason why I haven't done a pressure test is because I have no leaks...

Well, you don't know that you have no leaks until you do the pressure test. An invisible leak will still cause you to lose coolant. That is because the system goes to vacuum when it cools off. That is normally when it recovers coolant from the overflow bottle, but if it is not hermitic, it will recover air instead. So the overflow bottle will fill up and your cooling system develops an air bubble.

 

[MR2ManiacJC]

Well, you don't know that you have no leaks until you do the pressure test. An invisible leak will still cause you to lose coolant.

I agree, but visible or invisible leak should cause your coolant level to drop always. My coolant level has not changed since February. Which is why I'm suggesting that I have no leak.

 

[merryfrankster]

Is it possible that either the gauge temperature sender or the gauge itself are misbehaving?

 

[MR2ManiacJC]

Is it possible that either the gauge temperature sender or the gauge itself are misbehaving?

This is something I did consider. A while back in 2015 I replaced the coolant temperature switch which is that tiny little thing that looks like a ground and bolts into the bottom of the water outlet. I replaced it because when I purchased the car the temperature gauge was not working. Once I replaced it the gauge worked. But it was a crappy quality one since I bought it from autozone. I figured maybe it could be bad again but who knows. I can't find any way to test it though. Do you know of any methods?

Also, I find it important to mention that ever since I replaced my thermostat I noticed my gauge will read slightly under the half mark when driving. As far as I recall my MR2 would always read exactly at the half mark. When my thermostat was bad I was reading half way between the half mark and cold. So I replaced the thermostat but now I read slightly below the half mark. It's pretty strange to me because sometimes It seems to read at half but once I get going it moves slightly under half.
The image on the left is my temp gauge sitting at the half mark when at idle. The second image is my idle slightly below the half mark once I get moving. This has been happening ever since I replaced the T-Stat. But I don't think its normal. Sorry for the blurry second picture. I took it while driving >

 

[merryfrankster]

I've had the experience of parts store temperature senders bad out of the box. Sometimes they will peg the gauge immediately even when cold.

Obviously the temp gauge is not calibrated linearly and it is designed to jump when it reaches a threshold, but you can make a rough comparison to another known temp reading.

a. point an IR gun onto the water neck. If the gauge says overheat and the gun says 190F you know there's an issue somewhere.

b. measure the resistance from the ECT sensor (the two-pin sensor) and look up the corresponding temp. The temperature calibration of the sensor is available from a number of sources. This would be easier with an OBD2 car where the ECU spits out the temp.

 

[MR2ManiacJC]

I've had the experience of parts store temperature senders bad out of the box. Sometimes they will peg the gauge immediately even when cold.

Obviously the temp gauge is not calibrated linearly and it is designed to jump when it reaches a threshold, but you can make a rough comparison to another known temp reading.

a. point an IR gun onto the water neck. If the gauge says overheat and the gun says 190F you know there's an issue somewhere.

Yea I've had that happen to me before. That is why I will be testing the new part when it arrives before installing it. What is the recommended operating temperature for these cars? I don't know off the top of my head but my guess would be like 160-170 fahrenheit.

 

[merryfrankster]

Yea I've had that happen to me before. That is why I will be testing the new part when it arrives before installing it. What is the recommended operating temperature for these cars? I don't know off the top of my head but my guess would be like 160-170 fahrenheit.

I don't know for sure but that 160-170 guess is probably too low. I believe that the thermostat is rated at 180F and this means the operating temp is slightly above that say 190F.

 

[MR2ManiacJC]

The sensor arrived early

So last night I said I will be testing the new radiator temperature sensor next week but I was wrong. It was supposed to arrive next week but it surprisingly arrived today. I wasn't expecting it but it made my day I guess.

I went ahead and tested the new sensor just to make sure it was working. Recall that my radiator temp sensor is for an A/C equipped car so if you have no A/C these numbers will not work out for you. Also recall that the BGB says

• @ 85 deg celsius = 1.35 Kilo Ohms
• @ 90 deg celsius = 1.19 Kilo Ohms
• @ 95 deg celsius = 1.05 Kilo Ohms

Below are pictures of the thermometer reading for 85 and the resistance reading at 85. Also of the thermometer reading at just about 90(the camera guy got distracted hence the 91) and the resistance reading at 91.

As you can see the numbers are pretty close to those listed in the BGB so this sensor is good. I cant wait to test the old sensor. *fingers crossed* But I probably wont get a chance at testing the old sensor til Sunday. Til then friends.
1.35 BGB vs 1.31 Actual
1.19 BGB vs 1.20 Actual

 

[MR2ManiacJC]

I don't know for sure but that 160-170 guess is probably too low. I believe that the thermostat is rated at 180F and this means the operating temp is slightly above that say 190F.

You are right. I was thinking for a normal car. . . and as we all know the MR2 is far from normal haha. I would probably freak out if my scion was running at 190 but the MR2 it would make sense. After all, the low speed fans don't kick on until the temperature is between 185-194.

 

[merryfrankster]

You are right. I was thinking for a normal car. . . and as we all know the MR2 is far from normal haha. I would probably freak out if my scion was running at 190 but the MR2 it would make sense. After all, the low speed fans don't kick on until the temperature is between 185-194.

That 185-194 the radiator temp. It's lower than the engine temp.

I wouldn't be at all surprised if operating temp for your Scion (assuming tC) is also about 185-190F.