P38 oveheating

Hi. I have a diesel auto p38 which keeps on overheating. I have changed the thermostat, water pump, and, viscous fan. Still overheats I’ve added rad weld and holts coolant system seal in.
No obvious water leaks. No sign of water in the oil and it’s been pressure tested. It’s fine around town and sitting in traffic but over heats after around 40 mins. Any ideas???

You need to have a sniff test done on the coolant tank to check for exhaust gasses in the system. The heads are 50/50 to skim or not

I really don’t think it’s the head but I have ordered a sniff test should be here tomorrow.

All that junk you have added may well have partially blocked the rad. As both hoses go in the top, the rads are prone to collecting stuff in the bottom which impedes flow.

Datatek said:

All that junk you have added may well have partially blocked the rad. As both hoses go in the top, the rads are prone to collecting stuff in the bottom which impedes flow.

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I have some rad cleaner I was planning on draining it running the cleaner through and refilling unless there is a better way to get the stuff out of the bottom of the rad? I have a donor car that had no problems re the cooling system.

Wayne890 said:

I have some rad cleaner I was planning on draining it running the cleaner through and refilling unless there is a better way to get the stuff out of the bottom of the rad? I have a donor car that had no problems re the cooling system.

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Taking the RAD out and inverting it is the only real way to clean it out. Even then it is not always possible to clean it out.
Check the Rad is evenly hot both sides.
Check also for airlocks.

Also check that the baffle is in place in the top of the rad, do the marble test if it can roll from one side to the other its no good.

brianp38dse said:

Also check that the baffle is in place in the top of the rad, do the marble test if it can roll from one side to the other its no good.

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Well said, another common failure.

Datatek said:

Well said, another common failure.

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So I’ve replaced the radiator with one I know is good. It’s from my donner car which never overheated in the 4 year I ran it. It got 15 miles out of it driving with the windscreen demister on pulled over and turned the heating off and it instantly overheated. So far it’s had a new rad thermostat and water pump no leaks in the cooling system. Any ideas???

Wayne890 said:

So I’ve replaced the radiator with one I know is good. It’s from my donner car which never overheated in the 4 year I ran it. It got 15 miles out of it driving with the windscreen demister on pulled over and turned the heating off and it instantly overheated. So far it’s had a new rad thermostat and water pump no leaks in the cooling system. Any ideas???

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Wayne890 said:

So I’ve replaced the radiator with one I know is good. It’s from my donner car which never overheated in the 4 year I ran it. It got 15 miles out of it driving with the windscreen demister on pulled over and turned the heating off and it instantly overheated. So far it’s had a new rad thermostat and water pump no leaks in the cooling system. Any ideas???

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I was told when I bought it that the heater matrix had been reconditioned recently. Could there be a fault with the heating system causing it to over heat?

Is it over heating or boiling over? I suspect the latter.
The heater matrix has nothing to do with it, the flow is always on even when you ask for cold air

It boils. It will idle for hours no problem and it’s fine around town to and from work etc. only seems to be a problem when I turn the heating off though.

Wayne890 said:

It boils. It will idle for hours no problem and it’s fine around town to and from work etc. only seems to be a problem when I turn the heating off though.

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Cools down very quickly though

That sounds like a flow problem causing an isolated hot spot
How do you turn off the heating?
Do you have a viscous fan, does it work?

The viscous fan was replaced a few weeks ago definitely works you can feel it on your feet if you stand next to the car. I turned the heating down to lo and turned the blower right down on the control unit on the centre console.

kermit_rr said:

That sounds like a flow problem causing an isolated hot spot
How do you turn off the heating?
Do you have a viscous fan, does it work?

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Where would you start looking for this cold spot it’s a different radiator and has the same symptoms do you think it could be a pipe? I’m not loosing any coolant and it appears to be flowing as it should

Hot spot, not cold spot.
I don't know the p38 diesel variety, but i know the coolant should always flow through the heater matrix whether your select cold or hot.
If you have no problem when the cabin fan is blowing then there has to be a flow problem that's limiting the flow to/around the radiator (does it feel hot?) or the viscous fan isn't blowing enough volume when idle
If the aircon is on max cold then the electric fans should come on i think, does that help?

Where exactly should I be looking for this hot spot. I have all the same symptoms even after changing the radiator

It's just a guess, if it's cooling down quickly then it must be localised boiling. Are you sure the expansion cap is good and holding pressure?

kermit_rr said:

It's just a guess, if it's cooling down quickly then it must be localised boiling. Are you sure the expansion cap is good and holding pressure?

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The cap is good I have tried a couple of different ones