Cracked head? Overheating? Help...

[topchef]

Hi all, spent last couple of hours searching and reading as much as possible about overheating, cracked heads, testing for cracks, radiator blocked with gunk, thermostat u/s et al...hoping to find a clue or two to my latest problem. To explain problem...always had a bit of an issue with the 3l getting a wee bit hot, so, using tips from the forum, had the fan out and found no oil in clutch, re-filled and fitted and what a difference!! Massive airflow and a bit of a result with the overheating (or lack of) so thought I'd solved it. However, when pushing the beast a bit on hills it drifted towards red on gauge, slowing down brought temp down (obviously) not massive speeds or strain tho, up to about 70mph, 2,750rpm or so on a reasonable gradient ( 1:15) Today, towing a 1300kg horse box with 650kg of horse and we're in the upper end of gauge from the off (flattish roads with only shallow gradients, no more than 60mph) Slowed driving right down for temporary relief until we shot right into the top red. Hard shoulder and steam from header tank..bow locks! Header tank going like a coffee machine, loads of steam and water boiling away like a good un. Got rad cap off carefully and managed to get over 5ltr of water in, again, carefully to avoid damage to block. Covered last 30 miles very carefully! Water levels seemed ok on last checking.
So, having read so much I need to clarify a few things...one point says about gases getting into coolant system and forcing water straight out of rad cap. If I do this when engine is cold, and water comes over hole, is that exhaust gases pressurising system and hence shot? How much water would come out?
Could it be the rad is partially blocked and not cooling efficiently and leading to overheating? Shall I get the rad out and flush through, see if there is gunk in it?
Is this a problem that only manifests itself under load as normal/steady driving is fine?
Anything else anyone thinks I should check/test?
Sorry this is a bit long winded, but needed to explain I've done search etc and explain problem in a bit of detail so the more knowledgable might know what's up
Many thanks...

[shokenore]

Get the coolant tested for exhaust gases.

[Tankerman]

Long shot, is the thermostat working properly? And is the radiator cap sealing ok.

[topchef]

Get the coolant tested for exhaust gases.

Hi, on my list of things to do but noticed when engine cold and rad cap off, fire her up ~ give it a couple of minutes, and there is no water coming out of the top. I am guessing that if there was a crack in the head allowing exhaust gases into the cooling system then the water should be getting pushed out? Or am I barking up the wrong tree?
Thanks for advice nonetheless, regards

[topchef]

Long shot, is the thermostat working properly? And is the radiator cap sealing ok.

Hi there, thanks for reply, rad cap seems ok, no rust, spring allowing free movement etc. Took her out for a bit of testing today, normal driving is fine (temp wise anyway) but on hills and giving it a good bit of gas the temp goes up a bit quick. I would expect a reasonable bit of a rise because of high loading to engine but I would also expect it to be able to handle it without going into the red. Keep coming back to radiator not able to dissipate heat efficiently, except under moderate conditions, which seems to suggest partially blocked in the core? All fins look good ~ clean, straight.
Regards,

[BUSHWHACKER]

..........
So, having read so much I need to clarify a few things...one point says about gases getting into coolant system and forcing water straight out of rad cap. If I do this when engine is cold, and water comes over hole, is that exhaust gases pressurising system and hence shot?

No, thats just the excess coolant 'falling' out thats been sucked back into the cooling system from the expansion tank as the engine cools.

Could it be the rad is partially blocked and not cooling efficiently and leading to overheating? Shall I get the rad out and flush through, see if there is gunk in it?
Good idea to flush the rad anyway if it has never been done before.

Is this a problem that only manifests itself under load as normal/steady driving is fine?
Seems like you could do with an auxillary transmission cooler.
When towing or driving up long steep gradients, the ATF gets very hot due to the torque converter being unlocked as the transmission has most likey downshifted and that excess heat transfers into the engine's cooling system via the ATF cooler in the bottom of the radiator.

Anything else anyone thinks I should check/test?
Sorry this is a bit long winded, but needed to explain I've done search etc and explain problem in a bit of detail so the more knowledgable might know what's up
Many thanks...

If you can see bubbling and/or frothing with rad cap removed on a warm engine, then exhaust gasses are definitely getting into the coolant.