A quick check before replacing the thermostat again

wnrsm · Jan 18, 2017

Hello All,

I have a 2005 that has about 105K miles on it. My cabin heat is very weak and MPGs are low. I read the engine coolant temperature while driving and found that it never got above 60C with city driving. Highway driving saw 70C tops. No error codes are present or pending. Obviously a thermostat, right?

I replaced the thermostat last year. I used OEM per forum recommendations. I'll try another one but wanted to see if it could be something else. I had low cabin heat and replacing the thermostat fixed it. I have low heat again. My fresh/recirc servo damper is stuck on "fresh" - that could be limiting cabin heat performance but the low engine temperature concerns me. The temperature doesn't increase if I turn off the heat - in case the cabin heater core and cold fresh are doing too good of a cooling job (where the thermostat doesn't limit flow)

HVAC is throwing errors for fresh/recirc actuator and sensor (obviously) but is not throwing codes for anything else such as hot/cold temperature control stuff. Here's the history to my best recollection.

June 2014
- water pump replaced (worn out, rattling, no leaking)
- replaced coolant as a maintenance item while replacing the water pump

January 2016
- Lost all cabin heat, got code, replaced coolant control valve
- The pump to the thermos tank got noisy - ignored per forum recommendations

Sometime 2016
- fresh/recirc servo jumping all around. I tried disassembling to fix it and left it stuck on "fresh"

March 2016 - Low cabin heat experienced - replaced thermostat with OEM Toyota. I bled it properly. Cabin heat returned.

January 2017 - Low cabin heat again. Another thermostat? Coolant level is fine.

TWOPrius · Jan 18, 2017

I would check coolant level and bleed off air. Either the thermostat is defective , or air is trapped on coil side. But then it would be shut? Yry blocking the front of the radiator if its a very cold location?

Posted via the PriusChat mobile app.

Mendel Leisk · Jan 18, 2017

Did you check the performance of the old vs new? It's good to suspend both in a pot of water, with a thermometer, bring the the water temp up to just under a boil, and watch the behaviours: how much they open, at what temp, and the same going down.

dolj · Jan 18, 2017

Maybe check and/or replace the engine coolant temperature (ECT) sensor. While it may be working, as shown on whatever you are reading it with, it may be inaccurate. If the ECM is not getting a correct temp reading it won't be mixing the air/fuel correctly, contributing to poor fuel economy.

wnrsm · Jan 18, 2017

Thank you all for your replies! I appreciate your ideas to consider before I blindly replace that part again.

TWOPrius:
Past years' heat performance in colder times have been good... (I have had this car since new) I'd be more hopeful that blocking the radiator would help if I didn't have several years of good history in colder conditions. I'll double check the coolant level and redo the bleed procedure. If that clears it up, I will be very worried about how bad it was for this engine over the summer!

Mendel Leisk:
I didn't compare old vs. new performance when I made the swap. I did get my heat back after I put in the new thermostat. Maybe I'd be better off had I put in an aftermarket thermostat because I'd be less hesitant to replace it! If I replace it again, I'll test them both. It's looking like I am going to end up doing that.

dolj:
I'm reading it with a Bluetooth OBD2 dongle and a smartphone app. I'm inclined to trust the temperature sensor's low reading because it coincides with my very poor heater performance. When I shut down, I see the coolant temperature drop 20C as the (noisy) pump swaps out the contents of the thermos... so at the very least it is reporting a difference in temperature. The sensor ranges in price from $14-$50 at rockauto.

Mendel Leisk · Jan 18, 2017

Also inspect the gasket of the old one. Still, this time, I would think if it's only a year or two should look like new. When I went through this exercise (with another car, way back) the gasket of the old thermostat, that is the gasket that does the sealing against coolant flow, was brittle and flaking away, had chips missing. Also, it's opening/closing was such that it began opening sooner, at lower temp, comparing to the new one.

wnrsm · Jan 21, 2017

Thanks Mendel. I got a new thermostat and compared it to the existing one. Gasket was in perfect shape.

First, I got a Stant brand this time and it is perfectly identical to the OEM one I bought before. I can't links until I have four posts. My former account evaporated :-/ (been away a few years)
OEM purchase: search amazon for B00KTKWFAO $20
Stant purchase: search amazon for B001FKBEAY $15
(note, both were sold by Amazon. The OEM shows a third party seller now but the FEB2016 sale was Amazon)
I'm more inclined to believe that Stant is the OEM for Toyota

Second, I compared their performance on the stove. They opened and closed at nearly the same time but behaved at different rates. The existing one "behaved colder" where the opening was noticeably smaller when opening and closing. It seems that using the new thermostat will result in even colder engine temperatures.

So the heat still is very weak even when driving at near freezing temperatures or a bit above freezing temperatures. BTW TwoPrius The coolant level was fine before I pulled the thermostat.

My next guesses are

Check out that auxiliary pump for the cabin heater core. The coolant (both old and new) has always been clear without any signs of precipitate or drop-out so it is more likely to have failed than be blocked up.

Flush the core and see if it has flow.

Try to do some diagnostics on the coolant control valve. That caused a no-heat situation before.

Move to a warmer climate.

Keep holding out until my Tesla Model 3 preorder comes out, but it is a shame that my only annoying problems with this car has been cabin HVAC.

So, in your experience, what is normal engine temperature when it is 30-40F out?

wnrsm · Jan 21, 2017

I appreciate your help! I decided to get more data before going after anything else. I replaced the thermostat and put it into inspection mode as part of the coolant bleeding process. I believe my next step is to go after the fresh/recirc servo.

"A watched pot never boils" may apply when I say that it took a long time to warm up.

I took temperature measurements using these conditions/steps:

Engine continuously running in inspection mode

Climate: HVAC not in "AUTO," blower at maximum speed, "face vents" selected for output, set to MAX COLD

Thermometer placed in the face vent (a pen/spike type)

Waited for stable temperature readings and took "cold" measurements

Set climate to MAX HOT, waited for stable temperature readings and took "hot" measurements

Repeat #4/#5 twice more, then restarted vehicle to exit inspection mode.

Drove at 45MPH at MAX COLD and then at MAX HOT for measurements

Note that this all took place during the first warm-up cycle after coolant refill and bleeding.

GARAGE |exit insp mode| DRIVING
heat(max)setting-> cold hot cold hot cold hot | off on | cold hot Data Source
intake air temp 21 23 25 22 26 26 | 30 31 | 16 15 (OBD2)
coolant temp 85 81 93 91 94 91 | 78 90 | 85 76 (OBD2)
vent output 30 68 32 76 36 80 | n/a n/a | 38 68 (Thermometer)
ambient air temp 10 10 10 10 10 10 | 10 10 | 10 10 (CLIMATE screen)

My guesses/observations/questions:

When set to cold, should my face vent output be 20C-28C above ambient and 7C-22C above intake air temp?

I'm concerned that an incomplete COLD setting might be related to an incomplete HOT setting.

I heard dash servo movement when switching between hot and cold (and experienced the temp difference)

Guess: The fresh/recirc locked to "fresh" is providing a good radiator that bypasses the thermostat and slows warmup time... but I'm not getting that heat!

Inspection mode allowed the engine to run to full operating temperature and cabin heat works!

Does this (and the 30C-44C temp difference) mean that the cooling system is flowing and without air?

Heat was great from 2004 to 2015 in the same city.

Guess: Cooling system is free flowing and without air.

Exiting inspection mode caused the heat recovery pump to bring in cold coolant from the thermos, lowering coolant temp by 13C.

Does this (and cabin heat reaching 80C/176F) mean that the coolant control valve is operational? (I believe it does)

Patrick Wong · Jan 21, 2017

1. If you want maximum cabin heat, you should set the air vent to recirculate. Why would you let freezing air into the cabin at the same time that you are complaining of inadequate heat?
2. Many Prius owners who live in frigid climates will partially or fully block the air vents below the bumper (using a pool noodle, for example) to restrict the flow of cold air and allow the engine to warm up more quickly.
3. The fact that the engine coolant reaches 80 degees C shows the engine thermostat is fine. The coolant heat recovery system pump should operate upon initial startup and upon shutdown. I don't believe it has any reason to run when you exit inspection mode assuming the car is still READY.
4. The Prius by nature has a highly efficient gasoline engine. This means that there is relatively little excess heat thrown off. The more excess heat produced, the poorer the fuel economy. Hence you should expect the cabin heater may not perform as quickly as in your prior driving experience with other vehicles.
5. Your use of fresh vs. recirculate has no impact on the engine cooling system. The impact is whether or not fresh air is allowed into the passenger cabin.

wnrsm · Jan 22, 2017

Thank you for your reply, Patrick. Unfortunately I still believe I have a problem and it doesn't seem to help me get any closer to resolution.

In my OP I stated that the servo is stuck on "fresh" and that the system is throwing error codes for the fresh/recirc servo. Just to be thorough and avoid assumptions, I'll confirm that I had all the windows in place and closed.

My first response stated that I have had this car since new. This is my 13th winter with this vehicle while living in the same house and climate. I have 11 years of satisfactory heat performance and two years of unsatisfactory heat performance. My commute has been unchanged for three winters. I'm now in the early part of winter here while temps are still above freezing and am already experiencing unsatisfactory heat performance.

My OP is asking what to try besides another thermostat. In the third sentence of my OP I stated that coolant temps were not exceeding 70C and were usually 60C. This caused me to suspect the thermostat. I tested the existing and new thermostats so I already know it is not the thermostat. Also, the engine took a long time to warm up in inspection mode while the vehicle was stationary.

I exited inspection mode by turning the car off, which then caused the vehicle to run the recovery system pump.

I heard the recirc pump run. I'm rather sure of that since my OP states that it is noisy and that noise is being ignored.

OBD2 readings showed a temperature drop concurrent with the noisy operation of the recirc pump.

I'm sharing this because I'm wondering if the recirc pump running and temperature drop indicates proper operation of the coolant control valve.

Again, the heat was adequate for 11 years and global warming has been helping.

Here's why I'm wondering if the fresh/recirc being stuck at fresh is a contributing factor.

Stationary warm-up: My initial testing was done in inspection mode without the vehicle moving. Being stationary eliminated wind chill for the engine and gave it the opportunity to warm up past 70C. I believe that blocking the grille will only serve to mask symptoms. My road test began only after a full warm up while stationary. I'm doing a start-from-cold road test tomorrow.

Excessive cooling: I suspect that the following are working together to prevent or delay warm-up:

Cold air that will soon get much colder here.

A moving vehicle providing abundant cold air (as opposed to a stationary vehicle during inspection mode) to both the engine itself and the heater core.

A heater core whose flow is not controlled by the thermostat, unlike the radiator under the hood.

A failed servo stuck providing fresh cold air to the heater core. I'm not sure if the damper has intermediate positions.

...and my troubleshooting has yet to turn up anything else.

The only confirmed malfunction is the fresh/recirc servo so I can only do two things right now - keep troubleshooting and fix the servo. The servo failure is complicating troubleshooting. I'm still rather convinced that I have a problem with low coolant temps and insufficient heat so I'm open to any and all ideas.

New question: I'm still trying to find root cause and am open to suggestions. Does anybody know if a normally operating fresh/recirc damper has "intermediate positions" or does it only have fully-closed/fully-open positions? A use case for an intermediate position would be "everything's cold" (cabin, ambient, engine) where a full-open would simply be too much cooling performance for a bypassed thermostat.

I'm hesitant to take it to a mechanic/dealer because troubleshooting involves cold road test conditions and I'm dreading the runaround caused by them being unable to repeat the malfunction or telling me I'm not used to weak/efficient hybrid heat.

andrewclaus · Jan 22, 2017

Pardon me for coming late to this party and not reading every word so far. I compliment your troubleshooting and reporting. I feel the same as you do about paying a mechanic for this kind of troubleshooting, especially if both mechanical and electrical systems are involved. It's hard to find someone with talent in both areas.

I don't have a definite answer for the fresh/recirc servo question, but from my observations on a working Prius, the blender door is two position, bi-stable. Nothing in between. Consider that the system should not operate for long periods with the door in recirc mode, thus the dash light alerting you to its position. It should only be used for short periods of maximum demand, heat and cold. It doesn't make sense to use the door as part of a temperature control system.

You've mentioned the heater core water pump. Do you still suspect that, and have you tried troubleshooting it yet? Unfortunately, the relay is in one of the integrated units in the engine room relay and junction box, not the easiest to access. The pump itself looks very hard to access, too--it may require inverter removal. An easy first step would be to remove the relay and bench test it. Then jumper 12 V power to the motor lead, see if you can hear it run.

Do you have a complete wiring diagram? I found one in the Library section on this forum.

Patrick Wong · Jan 22, 2017

Thanks for a very complete summary of your history with the vehicle and your troubleshooting efforts.

Since you have a full history with the car and can attest the cabin heater performance has deteriorated, I offer four areas to consider:

1. Fresh air door: Repair the issue with the servo controlling the recirc/fresh air flow. I agree with Andrew that this likely is either open or closed, with no in-between status.

2. PTC electric heaters: The Prius has two positive temperature coefficient electric heaters which provide supplemental cabin heating until the engine coolant temperature can rise to a satisfactory level. Check the 30A fuses to the heater elements: HTR1 and HTR2, as well as the relays with the same names. Also check the 50A fuse PS HTR and the relay of the same name. The first heater has HTR1 and HTR2 providing power to it while the second heater has PS HTR providing power.

3. Heater core pump: As Andrew stated, check the operation of the cabin heater core coolant pump and replace if it is not functioning.

4. Cooling system obstruction: What was the condition of the drained coolant when you replaced the engine thermostat? If you found the drained coolant to be clear then there is not a high likelihood of a cooling system obstruction. If the coolant contained debris then perhaps an obstruction is an issue.

VFerdman · Jan 6, 2018

I would like to know if there was any resolution to this problem since last year. I am having a similar problem of low cabin heat, but I am in a different position than the OP. This car is my very first hybrid ever and I bought it in July 2017, so have n experience with what is "normal" for cabin heat in these vehicles. It seems inadequate and monitoring coolant temps on the OBDII dongle supports the fact that it is sub-par. In the summer/fall my coolant ran around 187F. Now it fluctuates between 156F and 168F. I have not monitored it for long and it is very cold outside right now (9F). I definitely feel like there is a problem with the heat. I set the system to the highest setting on auto and the most I get from the air ducts is slightly warm air. I've been driving Volvos since 1998, and those cars have amazing heat no matter how cold it is outside. I understand that hybrids are very different beasts and this is why I am trying to figure out if there is anything wrong with my Prius or it's just the way these cars are. I installed seat heaters and that makes the car a lot more comfortable, but without those heated seats this car would be borderline dangerous (fall asleep from hypothermia behind the wheel) to drive. Again, it may be normal, I have no reference point, but it just seems I should get air a bit warmer than just slightly warm out of the heater. Also the coolant tems seem to not ever climb into 180F range. The engine runs a lot in the cold weather, which also may be normal, my fuel economy is down to low 40s - high 30s. In the warmer months it was as high as 50s to high 40s.

Please describe to me the mechanics of this heating system. I understand that like almost every other system in this car it differs from the conventional ICE vehicle. Is there electric heat? Some sort of storage tank for when the ICE is not running? How does this system work?

Thank you in advance.

ILuvMyPriusToo · Jan 6, 2018

Just to make sure you checked, since car is new to you, that the cabin air filter behind the glove box is not dirty and clogged. That would restrict airflow and reduce heating, and would be a super easy fix . . . (instructions are in your manual)

VFerdman · Jan 6, 2018

ILuvMyPriusToo said: ↑

Just to make sure you checked, since car is new to you, that the cabin air filter behind the glove box is not dirty and clogged. That would restrict airflow and reduce heating, and would be a super easy fix . . . (instructions are in your manual)
Click to expand...

Thanks the suggestion. The filter was fresh when I bought the ca in July. I have not checked it lately, but my problem is not air flow, but rather air temperature. I get only slightly warm air coming out the vents, never anything really warm and the temperature of the coolant seems to be low as observed via OBDII while driving.

Mendel Leisk · Jan 6, 2018

I don't think it's abnormal for engine coolant temp to plateau lower in winter; that's what I've seen on our 3rd gen. 156~158F is still pretty good. Anyway to check if coolant flow in and out of the heater radiator is ok? Maybe just by feeling the hoses?

VFerdman · Jan 6, 2018

Mendel Leisk said: ↑

I don't think it's abnormal for engine coolant temp to plateau lower in winter; that's what I've seen on our 3rd gen. 156~158F is still pretty good. Anyway to check if coolant flow in and out of the heater radiator is ok? Maybe just by feeling the hoses?
Click to expand...

Today we had a high of 9F (-13C), so feeling anything other than my wood stove is out of the question The cold snap is supposed to break in the next few days and I will try feeling things under the hood of the Prius. I imagine it's pretty simple, like the thermostat, which is most likely original as lots of things on this car. The air coming out of the vents is just barely warm and I don't think that's right. But I may be wrong. This is my first hybrid.

Patrick Wong · Jan 6, 2018

VFerdman said: ↑

...In the summer/fall my coolant ran around 187F. Now it fluctuates between 156F and 168F. I have not monitored it for long and it is very cold outside right now (9F). I definitely feel like there is a problem with the heat. I set the system to the highest setting on auto and the most I get from the air ducts is slightly warm air. I've been driving Volvos since 1998, and those cars have amazing heat no matter how cold it is outside. I understand that hybrids are very different beasts and this is why I am trying to figure out if there is anything wrong with my Prius or it's just the way these cars are. I installed seat heaters and that makes the car a lot more comfortable, but without those heated seats this car would be borderline dangerous (fall asleep from hypothermia behind the wheel) to drive. Again, it may be normal, I have no reference point, but it just seems I should get air a bit warmer than just slightly warm out of the heater. Also the coolant tems seem to not ever climb into 180F range. The engine runs a lot in the cold weather, which also may be normal, my fuel economy is down to low 40s - high 30s. In the warmer months it was as high as 50s to high 40s.

Please describe to me the mechanics of this heating system. I understand that like almost every other system in this car it differs from the conventional ICE vehicle. Is there electric heat? Some sort of storage tank for when the ICE is not running? How does this system work?

Thank you in advance.
Click to expand...

1. Cabin heating system: includes the normal heater core which is heated via engine coolant, plus three supplemental "positive temperature coefficient" electric heater elements.

2. Check the condition of the following three fuses: 30A PTC HTR1, 30A PTC HTR2, 50A PS HTR. Those provide 12V power to the three PTC elements.

3. I haven't driven Prius in temperatures colder than ~20 degrees F. At that temperature the cabin heater works fine in my experience.

4. As you know, the Prius easily logs 50 mpg in good conditions. Hence it is not unnecessarily wasting gasoline via production of excessive engine heat. It is reasonable that the mpg will decline in cold weather depending upon trip length and less efficient operation in cold temps.

5. Try blocking the vents around the front bumper, which would allow the engine to reach a higher operating temperature in the winter. Some owners use home plumbing water pipe insulation for that purpose.

6. The purpose of the 2G engine coolant heat recovery system is to enhance engine warmup for emissions control purposes.

Mendel Leisk · Jan 6, 2018

VFerdman said: ↑

Today we had a high of 9F (-13C)
Click to expand...

High was plus 8~9C here today, lol. It's just nuts right now on the east coast.

VFerdman · Jan 6, 2018

Patrick Wong said: ↑

1. Cabin heating system: includes the normal heater core which is heated via engine coolant, plus three supplemental "positive temperature coefficient" electric heater elements.

2. Check the condition of the following three fuses: 30A PTC HTR1, 30A PTC HTR2, 50A PS HTR. Those provide 12V power to the three PTC elements.

3. I haven't driven Prius in temperatures colder than ~20 degrees F. At that temperature the cabin heater works fine in my experience.

4. As you know, the Prius easily logs 50 mpg in good conditions. Hence it is not unnecessarily wasting gasoline via production of excessive engine heat. It is reasonable that the mpg will decline in cold weather depending upon trip length and less efficient operation in cold temps.

5. Try blocking the vents around the front bumper, which would allow the engine to reach a higher operating temperature in the winter. Some owners use home plumbing water pipe insulation for that purpose.

6. The purpose of the 2G engine coolant heat recovery system is to enhance engine warmup for emissions control purposes.
Click to expand...

Thanks, I will check the PTC fuses, for sure. I am new to the hybrids and am just discovering all the ways they are different from pure ICE cars. I love this Gen 2! The more I use it (already logged 8K miles since I bought it) the more I like it. It is really well thought out in most cases save for some silly things. I am not crazy about having to go through touch screen for some controls (like climate control), but in general this is an amazingly well designed car. It has so much leg room for all the passengers, countless storage spaces, great economy, very decent handling and acceleration (not a performance car, but more than serviceable for family car duty). Even at its worst economy I get around 40 mpg. I may do better if I find the source of the heat issue. It may or may not be effecting economy. ICE is more efficient at higher temperatures. If I have a bad thermostat keeping the ICE colder than normal, it may negatively impact economy.

A quick check before replacing the thermostat again

wnrsm Member

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A quick check before replacing the thermostat again

wnrsm Member

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