Clogged radiator on a 2007

Hey folks,

I just wanted to give a summary of an issue I've solved in case it helps someone else.

TL;DR a little bit of RTV can block the radiator a little bit and dramatically reduce the effectiveness of the cooling system. The Gen2 Prius cooling system doesn't use RTV anywhere normally; RTV was used in conjunction with an inorganic gasket when my water pump was replaced, and I discovered it by backflowing my radiator into a bucket. My fix was to replace my radiator and clean out my coolant system of RTV.

I pulled a 2007 Four out of a field in early May; it had 180K on the clock and a bad hybrid battery (it had been sitting since Dec 2021, bar one drive to a dealership in 2023 for an inspection). I replaced the hybrid battery with cells from a front-end totaled 2017 with 41K, and drove it home, then cleaned it up quite a bit in the intervening time.

Since I got it, I noticed the engine coolant would run very hot; I thought to check because I noticed the fans were coming on more often than I was used to when I was in town. I went through some tests, and found that I was resting at 105C on the highway, and that letting off the gas on the highway would bring the coolant sensor on the outlet of the head as hot as 108C with the cabin heater on low, or 112C with heat off. Vermont has lots of medium-sized mountains on highways, so letting off and suddenly going downhill is very common, but that had me concerned as nobody else reported temps near these without ultimate HG failures.

Throughout the following I ran combustion leak detector tests: all passed without issue.

I bled the cooling system, using https://automotivetechinfo.com/wp-content/uploads/2020/09/Mastering-the-Toyota-Gen-2-Prius-Engine-Cooling-System.pdf for tips and tricks, and going further in that article by letting the system cool between 20-30 minute burping runs, so the ECU would decide to run the CHSS pump in case there were bubbles there. I noticed that with a pedal depressor running the engine at peak parked speed (2500RPM), the outlet of the radiator just barely ran at 40C, and the inlet ran closer to 85C.

So, I figured the thermostat was dead and constricting flow (or not blocking off the bypass correctly as it does when fully open). I removed the thermostat and inspected it. I learned that thermostats open slow and close slow, but when this one was held at 82C it just barely began to open, and when held at 95C it was really open. I replaced it anyways.

While in there, I inspected the water pump. It wasn't clear at the time that it had been replaced, but I confirmed that the impeller blades all turned fine.

The coolant itself looked absolutely fine - I didn't notice any debris in it. At that point, my theory was that the issue was probably a bubble in the cooling system. I triggered a P1121 at this time -- my 2007, per that article, can tell TechStream that the CHSS valve is not following the commanded position -- and replaced the CHSS valve. I figured at this point that perhaps there had been a bubble left in the system due to an intermittent P1121, and then it'd been messed with repeatedly because of that.

While bleeding the cooling system, I noticed the coolant would just barely begin to boil (~111C) when not under pressure, particularly when the system decided to charge the battery (it drops to 1200 RPM at that point, reducing the coolant flow). (Note that you can get around this slow-down by shifting to D with your foot firmly on the brake, and then charge the battery with both brake and accelerator fully depressed; this does increase engine load substantially which will probably cause the boil to continue anyways).

**Lesson at this point for the future: if under these circumstances your coolant is boiling, you do not have enough flow.**

Anyways, at this point I pulled my radiator. I live in a state that uses road salt, so I had a very difficult time disassembling the radiator / AC condenser mounts. Once I got them apart, I placed all of them in Evapo-rust, then heated to disassemble, cleaned and applied antiseize to all threads and loosely reassembled them before dipping them in boiled linseed oil.

To test the radiator, I poured hose water through a new CSF radiator and the old radiator. I noticed no real difference in flow, which was disappointing. I then blew with my mouth through the radiator, and noticed the backpressure was noticeably higher on the old one; high pressure hose water wasn't going to show this difference.

Finally, I concluded the old radiator must be blocked somehow. I didn't see this when scoping the radiator: my endoscope showed the radiator looking very clean.

As it turns out, when backflowing the radiator with hosewater into a bucket, there were maybe a couple of grams -- hundreds of tiny smudgy pieces -- of RTV, including a few curved pieces that must have come during the water pump replacement.

To reassemble, I installed a new Aisin water pump (with metal gasket), installed the new radiator, reassembled the whole car and went for a drive.

Before this work: I'd end drives at around 101C, and highway peaks were as high as 112C. Flat terrain temps at 75mph were about 105C.

After this work: I end drives around 85-91C, and highway peaks are as high as 102C after letting off (and hold 95C on flat terrain at 75mph).

 

Not mentioned:
- 90476-33006. Buy new radiator clamps and cut the old ones off if you're having any trouble getting them off. They aren't worth your time if they're already rusted and bent, you're not going to be able to get them to "clip" to themselves.
- A tiny bit of polyurethane grease on the coolant outlets will get you a long, long way.
- Remove everything you can before you start, and while you're in there clean up as much as you can. I replaced my HIDs (if yours have a cloudy quartz enclosure inside they're failing or failed; used Philips HIDs D4R HIDs are fine and very cheap, as long as they're actually the German made ones); I also properly closed the HID enclosures, and even considered doing something about the knock sensor lead that a rat chewed off.