Radiator not getting hot but car not overheating

Joined
Jul 2, 2020
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24
Location
United Kingdom
I decided to change the coolant on my little Suzuki Jimmy 1.5L (M15a), it hasn't been changed in over 2 years. The engine has never overheated and the temp gauge is always rock solid at around 185F when warmed up. On the workshop manual Suzuki recommends running the engine up to operating temp, draining the coolant and then fill again and repeat 2 - 3 times to flush it out. The problem is the engine get's to operating temp but the coolant doesn't flow through the radiator, and the radiator pipes are stone cold. I tried after driving on the highway for an hour but when I drained the coolant it was ice cold. It's like the thermostat is stuck closed but the engine does not overheat. The heater core works fine though. Any ideas on what the problem could be, I can't flush the coolant because what I drain / add to the radiator doesn't seem to actually get to the engine?
 
your heater core is pulling out more heat than the engine is putting into the coolant. Make sure the fan is off, put some extra revs on the engine and wait till the hoses to the rads are both getting warm. If the fan is controlled by a viscoous coupling, make sure that is working as it should aswell
 
How sure are you the engine isn't running hot? Just because the temperature gauge says one thing, doesn't mean it's accurate.

Pull the thermostat and put the housing back on, then remove the upper radiator hose and start the engine. Does it pump the water out? Could have a worn water pump and/or a lazy thermostat as well.
 
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I decided to change the coolant on my little Suzuki Jimmy 1.5L (M15a), it hasn't been changed in over 2 years. The engine has never overheated and the temp gauge is always rock solid at around 185F when warmed up. On the workshop manual Suzuki recommends running the engine up to operating temp, draining the coolant and then fill again and repeat 2 - 3 times to flush it out. The problem is the engine get's to operating temp but the coolant doesn't flow through the radiator, and the radiator pipes are stone cold. I tried after driving on the highway for an hour but when I drained the coolant it was ice cold. It's like the thermostat is stuck closed but the engine does not overheat. The heater core works fine though. Any ideas on what the problem could be, I can't flush the coolant because what I drain / add to the radiator doesn't seem to actually get to the engine?
Depending on where the temp sensor is located it may not be immersed in coolant and will not give an accurate reading. You can vacuum fill it or depending on the location of the thermostat you can try pulling the upper hose off the radiator or a heater hose to block and pouring coolant slowly down the hose.
Once you are sure it has coolant in reconnect the hose and start the engine, blip the throttle to 3K a couple of times and feel the hoses for signs it is getting warm. Let it idle until you see movement of the coolant then shut it off and let it cool, the level may drop, if so fill to the full cold mark if it has one.
 
I decided to change the coolant on my little Suzuki Jimmy 1.5L (M15a), it hasn't been changed in over 2 years. The engine has never overheated and the temp gauge is always rock solid at around 185F when warmed up. On the workshop manual Suzuki recommends running the engine up to operating temp, draining the coolant and then fill again and repeat 2 - 3 times to flush it out. The problem is the engine get's to operating temp but the coolant doesn't flow through the radiator, and the radiator pipes are stone cold. I tried after driving on the highway for an hour but when I drained the coolant it was ice cold. It's like the thermostat is stuck closed but the engine does not overheat. The heater core works fine though. Any ideas on what the problem could be, I can't flush the coolant because what I drain / add to the radiator doesn't seem to actually get to the engine?
OP, welcome! I have more questions than answers for you.

1. What sort of winter temperatures are you experiencing in the UK at present? It's possible your cooling system is fine, and the small engine, likely with an aluminum(i)um block and head, is radiating enough heat such that the thermostat is not opening. If it's designed to start opening at, for example, 190 F, then all's working as it should.

2. Which ls to my second question - what are the thermostat's ratings (initial opening temperature and fully-open temperature)?

Edit: I'm going to stop here - others have posted excellent suggestions. Lots of things for you to check!
 
All of the service manuals for my cars have you elevate the RPM to get the thermostat to open. Otherwise it can take a very long time especially when the weather is cool or cold.
 
I'm a big fan of pulling the thermostat, putting the empty housing back on, open the block drains if accessible, pulling the lower radiator hose off the block (or radiator), sticking a hose in the thermostat housing opening, and running water through the block. Let everything drain, close the drains & put the lower hose back, put in 1/2 of cooling system capacity full strength antifreeze in the block, put the thermostat back in with a new gasket or o-ring, top off with distilled water. Bleeding the system via the highest point in the cooling system (usually a sensor on top before the thermostat) is a good idea. If there's no "jiggle valve" in the thermostat, drilling a tiny hole (less than 1 mm) in the 'stat will also help get the air pockets out.
 
OP, welcome! I have more questions than answers for you.

1. What sort of winter temperatures are you experiencing in the UK at present? It's possible your cooling system is fine, and the small engine, likely with an aluminum(i)um block and head, is radiating enough heat such that the thermostat is not opening. If it's designed to start opening at, for example, 190 F, then all's working as it should.

2. Which ls to my second question - what are the thermostat's ratings (initial opening temperature and fully-open temperature)?

Edit: I'm going to stop here - others have posted excellent suggestions. Lots of things for you to check!
The temps are about 28F at the moment so your theory is probably right. I just got back from a 20 mile drive on steep road with some 20% inclines with the heat off and when I got back the radiator was hot and the coolant I was using to flush had mixed. I think in the future I'll do this in the summer as the engine struggles to produce enough heat to get the thermostat fully open in cold weather.
 
Might have to partially block the radiator to get the temperature warmer, we have to do that here in the states & Canada sometimes with something old & with a weak thermostat to get decent heat. Hard to believe the UK gets that cold!
 
Is it an aluminum engine? Aluminum engines don't retain heat well. Especially if it's cold. My Subaru and Focus definitely have a harder time getting to temperature in the winter than my cast iron engines do. Very likely the heater core is pulling heat out of the engine quicker.
 
Many years ago, my friend Tim had an old-even-then Ford Anglia, a British import. It would not warm up enough in the harsh Edmonton winters to throw much heat. His uncle fashioned a removable steel cover for the engine. After 20 minutes or so, the engine would get up to temperature, and Tim would stop the car, get out, lift the bonnet, remove the steel cover, and stash the cover in the boot.

That was an unusual solution. Many of us put cardboard in front of the radiator instead, and a few of us dared to remove the mechanical cooling fan for the winter. The latter is a good solution until you get stuck in traffic - even on a very cold day, an engine will overheat with no airflow through the rad.
 
For next time round, you may want to get one of these funnels and run the engine at idle to clear out any air pockets. These are about $18 in the U.S., so I can't understand how it's over double the price at £42 in the U.K. but they are worth it if you can't vacuum fill.
 
I decided to change the coolant on my little Suzuki Jimmy 1.5L (M15a), it hasn't been changed in over 2 years. The engine has never overheated and the temp gauge is always rock solid at around 185F when warmed up. On the workshop manual Suzuki recommends running the engine up to operating temp, draining the coolant and then fill again and repeat 2 - 3 times to flush it out. The problem is the engine get's to operating temp but the coolant doesn't flow through the radiator, and the radiator pipes are stone cold. I tried after driving on the highway for an hour but when I drained the coolant it was ice cold. It's like the thermostat is stuck closed but the engine does not overheat. The heater core works fine though. Any ideas on what the problem could be, I can't flush the coolant because what I drain / add to the radiator doesn't seem to actually get to the engine?
Sounds like you drained the radiator with the cap still on.
 
Take the radiator cap off first after the engine has cooled then drain your radiator. If your radiator is still cool that means your engine has not reached operating temperature yet. Which means the thermostat has not opened yet, holding the coolant back until it has actually reached operating temp. Make sure your correct operating temp is 185F and not 210F. Your radiator and hoses are still cool because your thermostat has not opened yet to let your coolant flow.
Sounds like you drained the radiator with the cap still on.
 
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