Problem I have is that the radiator fan ( single fan version ) will not turn on even if coolant temperature goes up to 115 C.

I did the usual stuff like check all the fuses which were all good. Both under the steering wheel and on top of the battery.
I checked the radiator fan module ( 5J0 919 506 ) which already did go bad few years ago and replaced it with few years newer one from the junkyard. I even got a new one just to make sure junkyard one did not give up. Did not change anything.
I checked if the fan was even working and both slow and fast speeds work fine if I give it power directly from the coolant temperature switch plug.

I replaced coolant temperature switch ( 1J0 959 481 A ) with a cheap chinese one and radiator fan did kick in at around 102 C and turned off at 99 C which seemed oddly high to me so I got a slightly more expensive Mahle one but now the fan will not turn on at any temperate just like with the original. Putting the chinese one back, fan turns on again at 102 C and off at 99 C. This baffles me. I have no check engine codes besides 00819 ( High Pressure Sensor ( G65 ) Open or Short to Ground ) which shouldn't be related??? And even if I turn on AC ( Not working currently ), radiator fan does not turn on.
Chinese switch is supposed to turn on at 95 C and off at 85 C. Mahle  version should have slow speed turn on at 97 C and off at 87 C. Fast speed should turn on at 102 C and off at 97 C.

I did recently rebuild my engine so I have filled cooling system with new coolant, I installed new thermostat etc. I checked radiator hoses, upper one gets hot and the area around the coolant temperature switch also gets hot. Lower radiator hoses get quite warm so clearly coolant is flowing through the radiator. I did my best to get any air out of the system.
Now I'm wondering if I have faulty thermostats or perhaps I left something in the radiator hose which blocks coolant flow. But then again why does the cheap chinese switch work somewhat?
 

Did you check car computer for codes?

 

Also, depending on your engine it might be impossible to get all air out of the system without pulling vacuum on cooling system and using refilling modes with a diagnostic cable.

Yes, only code is 00819 ( High Pressure Sensor ( G65 ) Open or Short to Ground ) 

 

I have 1.2 CBZA. Ideally you'd want to use vacuum to remove air but it seems to get air out of the system just fine. Heater blows hot air. Coolant flow into expansion tank is nice and smooth. When there was air in the system I would rev the engine and I would see how temperature drops untill there seems to be no more air in the system after which temperature starts rising.

I checked everything I could think and noticed that lower radiator hose is completely cold even when temperature goes up to 110C after first run. This would also explain why the temperature switch does nothing as it's also cold. Only after screwing around for the whole day and constantly running coolant up to 105-115C is when the radiator fan finally kicked in.

So either one of the thermostats is sticking  ( New cheap Topran thermostats with thermostat housing ) or there is a blockage in the area.

I think your cheap Chinese thermoswitch may be working OK, depending on where and how you are measuring the coolant temperature.

The thermoswitch is sampling the output temperature of the radiator, not the engine coolant temperature as it is at the temperature sensor. Clearly the point of the radiator is to reduce the coolant temperature, so you should expect the output temperature to be significantly lower than the input temp.

 

It does seem like you may not be getting good flow through the radiator though; coolant pump, thermostat or blockage all could be involved, I guess. 

 

 

If G65 isn't giving a sensible pressure reading,  A/C will not be operating.  The fan should not be expected to work via refrigerant pressure increase, if there is no such increase, nor any signal telling the HVAC unit about it.

 

 

Lower radiator hose and lower half of the radiator were completely cold after first run. Only part of the lower radiator hose that started getting some warmth was the end on the thermostat housing. I guess that means thermostat was opening. Otherwise warm coolant shouldn't get to lower hose at all. I'll be inspecting cooling system with endoscope and see if I can find blockage.

I drained whole cooling system. I checked everywhere for blockages but I did not find any. Radiator is clean.

I swapped new thermostats to old ones and nothing changed. Though lower radiator hose did get slightly warm after a while which is good.
Coolant pump is new and seems to be fine. There are no leaks.

Coolant does not leak into combustion chambers, there is no smoke at all from exhaust, no misfires etc.

Combustion gases do not leak into cooling system, coolant is clean and flows nice and smooth. CO2 tester showed negative.
 

I was thinking perhaps I hooked some hose in the wrong place but as far as I could tell everything was exactly where it is supposed to be. Near impossible to get stuff mixed up.

 

So now I am wondering if there is air stuck in the cylinder head. I did change coolant once in the past but I did not drain the whole system like you do with full engine rebuild. So perhaps I do need to have air vacuumed out like mentioned before. I did my best to get air out by squeezing various hoses. I had heater running at max. I ran the engine with the expansion tank lid off. I took the car for a spin and revved the engine to get coolant flowing faster etc.

Also I have VCDS and I checked temperature through it as it shows more than typical OBD II.
After hitting 115C coolant temp, Exhaust showed over 500C, turbo was over 320C and oil temperature was just over 90C. With ambient at 23C.

There are all kind of other odd values that show impossible numbers like "Coolant temp at radiator outlet" which shows -48C. Pretty sure there is no sensor for this. There is also "Coolant temp at radiator output" showing constant 70.5C. Pretty sure this means nothing aswell.
Coolant fan 1: activation-radiator fan 1: 9.8%. Coolant fan 2: activation-radiator fan 2: 0%. 

Perhaps new temperature sensor is wrong? I am going to swap old one back in. Other than this and getting air vacuumed out of cooling system, I don't know what else to check or do.

 

 

Edit:
I just realised one thing. Even after car has been sitting over night, coolant temperature sensor shows 40C which can't be because ambient is 15-25C. This could be a good indication that my new temperature sensor is wrong?

 

No, wait. I am wrong. Coolant temperature shows ambient temp like 22C but after starting engine it quickly climbs to 50C, takes roughly 1 minute, and then suddenly it starts climbing at much much slower rate upwards. from 50C to 60C took 2 minutes. Normal behaviour right? I assume this is because after cold start engine revs high to get temps up?

Edited 25 June, 20241 yr by Koreenium

I found this site: https://skoda-fabia-israel.blogspot.com/2013/04/cooling-system.html

It explains a lot about the cooling system and how it works in this engine ( CBZA and CBZB ).



Based on this having air stuck in the cylinder head is probably unlikely?

If coolant temperature sensor is not wrong, which I doubt because it is obvious that engine gets quite warm even when you stand next to it. Too warm.

Secondary coolant pump behind the engine is working. You can hear it running after you shut off the engine and you can see coolant flowing into the expansion tank. Main water pump is new and pretty simple so not much can go wrong, right? But I did not realise that coolant pump is controlled by vacuum.


There is a small check valve and solenoid valve N492 which controls vacuum to turn water pump on and off. And apparently if this solenoid valve is stuck open then it causes coolant to get hot because coolant pump is not working. This could also explain why lower radiator hose stays cold for a long time after coolant goes over 110C. Simply coolant is not flowing because pump is not puping. But this should trigger a engine warning code, right? I have none.

[ SOLVED ]

I can't believe this stupid little thing is the sole cause for my problem.

 

It was stuck open which prevents coolant pump from pumping.


This is most insane and utter useless piece of garbage "smart" technology you can have on this car. When this thing was broken it did not take very long for coolant to warm up to 80-90C. With this thing working it takes me good 10 minutes with a hot 24C weather to get up to 80-90C. Insane. I am 100% convinced this is also main cause for my engine failure as this thing not working, does not register in the ECU as broken and thus it does not give you a warning code. And because this car does not have a temperature gauge you'll never know that this thing is broken until your engine has grenaded from running at constant 110-120C. Stupid overheating warning kicks in at 130C.

I actually wanted to comment about the vacuum system of the pump, but thought that it won't be an issue since you are getting heat in the cabin... On my car when the pump is closed, you don't get cabin heat from the coolant.

 

My CUSB diesel engine also has the same issue though it's not operated by vacuum, pump stops pumping and everything overheats.

15 hours ago, Kostas123 said:

I actually wanted to comment about the vacuum system of the pump, but thought that it won't be an issue since you are getting heat in the cabin... On my car when the pump is closed, you don't get cabin heat from the coolant.

 

My CUSB diesel engine also has the same issue though it's not operated by vacuum, pump stops pumping and everything overheats.

Does your car also have a secondary coolant pump? I don't know about diesel variants but secondary pump is pretty much on every gasoline engine and in my case it was probably only thing making some coolant flow which prevented coolant from reaching 130C and thus did not drigger overheating warning. This also helps coolant circulate into the heater core which probably explains why I got heating in cabin.

35 minutes ago, Koreenium said:

Does your car also have a secondary coolant pump? I don't know about diesel variants but secondary pump is pretty much on every gasoline engine and in my case it was probably only thing making some coolant flow which prevented coolant from reaching 130C and thus did not drigger overheating warning. This also helps coolant circulate into the heater core which probably explains why I got heating in cabin.

Mine even has 2 aux pumps (due to a third coolant loop for charge air). The cabin heat comes from the cylinder head. From the EA288 self-study manual:

 

Quote

The coolant is not circulating because the coolant pump is off, and the engine reaches its operating temperature faster. When the coolant pump turns on, it provides coolant to the cylinder head and the EGR cooler during the engine warm-up phase.

 

It seems I wasn't getting cabin heat because I needed the main pump to function properly too.

 

When I started my car in the winter, the electric heater provided the heat for cabin while the engine was completely cold. When there was enough heat in the engine, the pump should start circulating the coolant round the block, but not thru the radiator since the thermostat was still closed. However the pump mechanism was stuck, so it would not start the circulation. Engine temp would go up and the heater would go cold (because the electric heat believed it was no longer needed), then later (after 10-15 minutes) the pump shut-off mechanism would get unstuck and both the engine temp would go down as well as cabin heat would return.

 

The whole coolant system is extremely overengineered, with million one-way valves which prevent "hose squeezing" bleeding. I still have a bubble in my coolant system since the shop that replaced the pump didn't even know that you need to run special bleeding programs on CUSB engine. It doesn't interfere with cooling very much, but the level in the overflow tank can get below minimum when it's cold outside and then spills coolant out the overflow when the bubble expands.

Haha, I guess it could always be way worse.