2002 Corolla Thermostat Replacement Guide (With Photos) - Toyota Corolla, Chevy Prizm (1998-2008)

My Corolla got a Christmas gift today: new thermostat. Below are photos of the step-by-step process.

Perform this action when engine is completely cool

First, if you want to reclaim the coolant in the system and want to save a mess of coolant flowing out when removing the thermostat housing and thermostat, remove the radiator cap and remove the radiator drain petcock. You can use a makeshift "baffle" to channel the coolant into the basin.

When no more coolant leaves the radiator, move the basin to an area in the garage where debris will not fall into it. Re-install radiator drain petcock. You can leave the radiator cap removed.

Now you have to loosen serpentine belt tension. I used a 17mm socket on the bottom bolt to loosen it (not take it all the way off). Then, used a breaker bar with a 19mm socket for the bolt that you pull forward to loosen belt enough to take it off the alternator.

Disconnected connection at negative terminal. You may not have to do this if you don't plan on unplugging alternator's electrical connector.

Lift the alternator out. The thermostat housing is exposed has two small 10mm nuts; be careful not to drop them upon removal.

Installed new gasket from Beck/Arnley on the new OEM thermostat.

Used a Brillo pad to remove debris and build-up in the area where old thermostat was.

Installed thermostat with jiggle valve at the TOP. This is important.

Make sure thermostat is snug, then re-install the two thermostat housing mounting nuts. Tighten each side equally (two turns on one, then two turns on the other) to help prevent leaks.

Re-install alternator and re-plug electrical connector to it (if necessary)

Use breaker bar with 19mm socket to bring belt back to alternator. Make sure the serpentine belt is correct on each pulley.

Re-tighten the 17mm bolt below the tensioner bolt.

Reconnect negative battery terminal connection if necessary

Remove radiator cap and gradually pour in a 50/50 mixture of Toyota red antifreeze with distilled water. I use 60/40. You can also use Toyota's pink antifreeze/coolant that is pre-mixed. If you reclaimed coolant prior to removing the serpentine belt, use that mixture to pour into the radiator.

When no more coolant can be fed into the radiator, start the engine (can leave cap off radiator) and let it idle a couple minutes. Again, add more coolant to the radiator until system doesn't accept any more.

With engine idling, turn the heater on with fan setting at high. I put my system on recirculate.

Check for leaks at the thermostat housing.

See if engine will accept any more coolant through the radiator. If yes, add a little. If not, leave car idling with radiator cap off so the engine can burp air out of the cooling system.

Re-install radiator cap. Shut off engine.

Install a coolant mixture into the recovery tank. (after a drive, the car will warm up fully, circulate coolant, bleed out any air) After a highway drive and the engine cools, the cooling system will draw any more needed coolant into the radiator via coolant in the recovery tank.

Check for leaks again at the thermostat housing

Before your second drive, check coolant recovery tank level and add coolant if necessary.

My Corolla pours out more hot air now, not just warm. Idle has improved. Fuel efficiency should improve. Doesn't take as long for temperature gauge needle to get to the half-way point in 20-degree temperatures.

Good write up.

I assume one of your first steps was to drain coolant at radiator drain and block drain, and also drained the reservoir... If flushing with distilled water, and refilling with distilled water the drain again, start refilling with a gallon of concentrated coolant first.

I flushed the entire system, including the block, in four months ago before I learned the thermostat was bad.

Didn't do any of that flushing. Had leftover red and distilled water, so just put some new in. Works great.

Yeah, that's what I thought, but didn't you drain the coolant to replace your thermostat? It's not mentioned in your first steps.

I just simply let it flow out when I took thermostat cover off and then thermostat. It would be wise to drain coolant out at the radiator petcock before starting this work in order to reclaim coolant. The coolant I caught in the basin had particles in it because it touched so many dirty parts before hitting the basin.

So, I am amending the steps. . .

So glad I didn't have to mess with the TPS and IAC regarding the idle issue.

Questions: When replacing the ECT sensor, is it required a person disconnect the negative battery terminal?

I replaced the ECT first, then replaced the thermostat a week later. Does the ECT react differently to a new thermostat (old was 180 degrees and new is 180 degrees). However, I don't know if the old thermostat is OEM and/or original. The newly-installed thermostat has a Toyota emblem on the rim. Here's a photo of the removed thermostat:

No need to reset anything with a new ECT. It's only reading engine coolant temperature. ECU doesn't know you replaced it, nor does it need to know. If you didn't compare their readings, you can only hope the new one is giving the ECU a more accurate temperature input... If you ever do want to reset ECU, you can simply remove blue 15A 'EFI' fuse in engine compartment for a while.

I can't tell where your removed thermostat came from, and if it's the original. Its design was quite different. Are there any other markings on it?

New thermostat also has a larger core and spring, much like an upgrade to a Stant Superstat (not available for Corolla) which allows quicker warmups and better temperature control and stability.

There are no other marks on it.

Is there ever a time where a person should reset the ECU after an engine part replacement or any other reason?

The new thermostat has done well so far and it does quicker warmups. Glad to get it done before it gets even colder next month.

And, fuel economy should improve a bit. . .

Yep after a maf sensor replacement.

The only time the ECU memory would need resetting is after replacing the ECU or engine if you have an automatic transaxle, or when replacing automatic transaxle... Otherwise, ECU never needs to be reset when disconnecting or replacing any sensor or any engine part at all.

MAF sensors are popular items to pull on Corollas I've discovered. Today at the pick-and-pull, I found a Corolla just arrived and the MAF was removed. Common issue?

Good point Dom.

Yep it's common auto.

The MAF/IAT sensor often tends to be unfairly blamed for air/fuel mixture and driveability problems. Some also claim to have fried it while accidently shorting out the battery on an aluminum CAI or SRI. It can also get contaminated with over-oiled air filter or dirt, and simply need to be cleaned properly... Mine is still fine with 130,000 miles on it. The Toyota mechanic gave me a free MAF just before my basic warranty ended, and let me keep my original as spare because I gave him hell for testing my Corolla like he had stolen it while bouncing off the rev-limiter. He was just impressed and surprised by all my power mods, header, exhaust system, etc.

You're one of those "modder" type of guys, huh?

I'll keep my eye out at the yards; can sell 'em on eBay

Now that idle issue is resolved, put the engine cover back on after six months removed. . .

Lows of 4 to 9 degrees this week, with highs around 15. Glad I got this done in time.

Thanks for the guide, autotech! I have a 2002 Prizm, and am going to replace the coolant soon, whenever the weather gets nice enough on the weekend. I think I'll do the thermostat too -- it takes a long time for the car to get up to temperature, and my gas mileage has been miserable of late (I know, winter gas blend + cold idle, but mpg is REALLY low). Was there any specific symptom that prompted you to replace your t-stat?

I'll post a pic of the replacement t-stat. Going w/ the ACDelco part, since nominally my car is a "Chevrolet" default_wink . A lot of the time it ends up being a Denso part in a Delco box.

If you have female torx and a swivel you can pull the lower stud after you remove the nut and take the housing off without pulling the alternator. You need a long 1/4 extension with a 3/8th adaptor, an E8 (I think) socket, a 10mm socket (10mm swivel socket preferred), and a swivel. It's a good way to save a little time on the job if you're comfortable working in tight spaces.

Didn't realize anyone else replied. I decided to replace it because I didn't feel the temperature blowing out of the vents was warm enough. At around 224,000 miles at that time, I wondered if it was original. I've noticed the needle on the gauge move quicker to the mid-point now (and when it was -4 out).