Yaris: Engine fan coming on prior to engine getting hot.

[chainsaw_masochist]

Toyota Yaris (2002) 1.0 petrol Mileage: 51,000

Problem: Engine fan coming on without reaching HOT

I am a bit bemused by this particular issue as the car is running well generally. The engine fan comes on about the time that the blue light (COLD) switches off. I thought originally that this could be caused by a problematic coolant temperature sensor but there is no EML light illuminating so presumably no OBD code is being raised. Also the engine runs very evenly throughout the range on a run up the motorway etc. (I understand the defective temperature sensors can result in the setting of a too weak mixture etc). The red light (HOT) never comes on.

Anyway, I intend to check the resistance of the sensor with a multimeter to see if it shows acceptable readings throughout the heat range. Looking at the pics below can anyone tell me the best way to pull off the sensor connector as it is very stiff? I guess I should try to squeeze the connector at its widest point and perhaps push towards the sensor and then pull it backwards to free it. (It has not been separated from the sensor for nineteen years so no surprise that it is stiff). Also I have a query about the bolt below the sensor and its purpose in text on the pic.

The car has air-conditioning which uses the same fan when switched on. Is it possible that this problem could be caused by a defective switch? It would appear unlikely, I guess, as the fan seems to cut in around the same time on each occasion. Maybe a relay could be problem here?

Thanks greatly for any help.

 

[Burnerman]

Does the fan still cut in with the A/C switched off?
I think that bolt just holds the sensor in place, often there’s an O ring in there too.
John

 

[chainsaw_masochist]

Does the fan still cut in with the A/C switched off?
I think that bolt just holds the sensor in place, often there’s an O ring in there too.
John

Yes, John, that's how I noticed it. The fan just cuts in before the engine is properly warm. I thought the sensor is just screwed into the cylinder head and then tightended with a 19mm spanner/socket - low torque, mind you, something like 15 lbs/sq in? Mine looks something like this though I think it is made by Denso.

 

[Burnerman]

In your photo, the sensor is a push fit into the casting, typically Toyota style yet the replacement is screwed in?
I’m not sure if that sensor actually controls the fan itself or more likely, the ecu fuelling but if you are going to change it I would recommend a genuine Toyota part. I always thought that Toyota screwed their sensors into the engine top hose anyway, so can you check this?
John

 

[Harry Bloomfield]

Have you any means to tap into the OBD diagnostics, to see what temperature the system thinks the temperature is?

 

[chainsaw_masochist]

In your photo, the sensor is a push fit into the casting, typically Toyota style yet the replacement is screwed in?
I’m not sure if that sensor actually controls the fan itself or more likely, the ecu fuelling but if you are going to change it I would recommend a genuine Toyota part. I always thought that Toyota screwed their sensors into the engine top hose anyway, so can you check this?
John

Oh dear, this could be embarrassing …I think you are quite right, John, the item I referred to is not the temperature sensor at all. Looks like I have jumped the gun here and thought that the round black rubber section covered the hexagonal section intended to be tightened with tool. Complete rubbish…as you say, it is for the fuelling. I will have to search a bit deeper so will get back on this one. Looks like you have saved me from a journey up Shіt Creek…not for the first time.

Have you any means to tap into the OBD diagnostics, to see what temperature the system thinks the temperature is?

Normally I do, Harry, as I purchased an OBD scanner about three months back. Stupidly I left at my folks and I have simply not been able to get there with the lockdown. I don’t think this looks to be a serious problem, hopefully, so maybe I can let it wait for a few weeks until the lockdown is lifted. The car is rarely used currently.