Over heating on motorway

Got a defender 90 300 tdi. I can drive around it all day laning or down country lanes and temperature is fine but as soon as i hit the motorway and put it under constant load it overheats and then when i slow down temperature drops. Its got a viscous fan. Could this be the problem not working properly as all seems fine under normal working.

could be rads on its way out or flush it through see if that helps !

It sounds basic but have you checked the fins are clear of mud/debris, If you use it off road and don't wash the radiator and inter cooler through they can clog up over time

Where does your gauge sit when at normal operating temperature? And when it overheats how high does the needle go?

the_wolf said:

Where does your gauge sit when at normal operating temperature ?

Click to expand...

proberly in the dash next to the fuel guage

Mine was doing this the other day.
50mph no problem (92 deg), 70mph temp had got up to (110 deg)
Think (& hope) it turns out to a small air lock on the top bleed bolt, not been on the motorway since.
I think cos the water pump is up the top it don't like having air in it but I may be wrong.

Have a look at the fins on your rad.
Mine's doing exactly the same thing,My rad has huge sections with no fins,so I've just ordered a new one this morning.

If the cooling system is right you should be able to tow a heavy trailer along a motorway inc hill climbs without a cooling fan fitted so it sounds like either your rad or water pump is failing to me

My rad doesnt look to bad. Maybe i shall get it reconditioned. still seems strange it cools down when you slow down.

When it has been running hot feel the bottom pipe. Hot or cold. Could be a sticky thermostat. Few quid

Just enquired about a recon radiator. Bloke said they do a special core for off road use. Has anyone heard of these. Its £200.

Not sure i'd payout that much til i knew that was the fault. Run it up to temp and check bottom hose and whole radiator.

Have you checked for air lock

chunkymunky said:

Not sure i'd payout that much til i knew that was the fault. Run it up to temp and check bottom hose and whole radiator.

Have you checked for air lock

Click to expand...

Will do that tonight.

I got mine refurbed at kempston radiators, and it cost £200, it's got haven't duty fins which allow you to pressure wash it with no problems!

Why pay £200 for a refurb????
When a new rad is £100.

Rawly86 said:

I got mine refurbed at kempston radiators, and it cost £200, it's got haven't duty fins which allow you to pressure wash it with no problems!

Click to expand...

Same place i got quoted from. Does it look that different.

Farrar said:

My rad doesnt look to bad. Maybe i shall get it reconditioned. still seems strange it cools down when you slow down.

Click to expand...

not strange at all, the system is not flowing enough water to dissipate the heat build up when under more load

either rad is partially blocked, not working efficiently due to blockage externally, thermostat may only be partially opening, waterpump fins may have corroded away or airlock causing cavitation

first thing I would do is remove stat n try again.... no cost involved and will confirm or eliminate that common issue

cheers steve

Farrar said:

Same place i got quoted from. Does it look that different.

Click to expand...

Not the best photo took it through the grill.

Rawly86 said:

Not the best photo took it through the grill.

Click to expand...

Cheers i see its a lot different. Much better for cleaning.Ive checked mine and all hot where it should be.thermostat opens as it should. Washed radiator and intercooler and some mud came out. Need to drive it now on motorway. Lets hope it was just a bit of dried mud and it been warmer thats been causing overheating otherwise i guess its a new radiator.

I have a new rad,water pump, and 88 stat ( not checked it with thermometer) and viscous fan with no cowl and mine runs hot when pushed and hotter when towing.