TD5 blowing top hoses *fixed*

Hi everyone. Sorry, this is a long one, but please read.

I know there are loads of posts about this and coolant loss in general but I am truely stumped and I need some new ideas.

I bought this 2004 TD5 with a known overheating problem but I was happy to fix it myself.
Based on the symptoms of pressurised hoses, I went straight in with a new head gasket, water pump, tank cap and thermostat. The old gasket looked ok but the shop that did the pressure test said there was a very fine crack between two valves IIRC and that it shouldn't be a major problem. There was also a leak from the bung under the exhaust manifold which was re-sealed.

The engine was rebuilt and seemed fine, no more pressure in the hoses and no coolant loss to speak of. I took it to southern France and bar a small coolant top up there were no problems at all.
Back home and a couple of months later I was towing an empty trailer for an hour when the top road hose popped. I believed it was due to a degraded hose and replaced it. I tried the same trip again and at nearly the exact same location, after an hour, the same hose blew again.
I replaced the hose again and checked all other hoses. I found some weeping and cleaned and refitted them. I did a sniffer test for exhaust gasses in the coolant but got nothing on any of the 3 tests.
I have still been driving it whilst trying to diagnose the fault and checking for a warm lower rad hose (60 deg, I think, hard to measure exactly), is the heater working etc.
I found that the heater sometimes blows cold and I can hear bubbles rising behind the dash - probably air (steam?) in the system rising through the heat exchanger causing the loss of heat.
I have to ask, what pressure would be needed to pop a top rad hose? If I have a pressure leak or blockage and the coolant boils, surely that steam is not sufficiently high enough pressure to pop a hose? The coolant cap was also replaced and will squirt coolant out when too hot.
It's tempting to blame the fine crack on the head but I'm thinking more about the rad. Is it possible that it is blocking coolant flow to such a degree that coolant in the block is getting superheated, turning to steam and can pop a hose before the headertank can vent the pressure? The temp guage stays exactly at half way until I'm about to breakdown, no pre warning from that.

Any help from people who know how these things are supposed to work is appreciated.

Hi, something quite similar happened to me and unfortunately was a crack in the head...new AMC head was the cure for me

Seems amazing to me you'd be advised its ok to rebuild with a known cracked head??! I'd be amazed if the problem is anything else unfortunately ........ I hope I'm wrong..... A

I think it was because the crack was between two valves so if it did migrate into the metal it would go into the airways and not the coolant cavities.

If it is the crack that's responsible I don't understand why I'm not getting exhaust gasses on the sniffer test.
And why would it take an hour's towing to blow the top hose?

Alpinejim said:

I think it was because the crack was between two valves so if it did migrate into the metal it would go into the airways and not the coolant cavities.

If it is the crack that's responsible I don't understand why I'm not getting exhaust gasses on the sniffer test.
And why would it take an hour's towing to blow the top hose?

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a crack between the valves is the most common and as all cracks do eventually lead to the water jacket,hence pressurisation ,a non functioning coolant tank cap can cause similar

if your block tester isn't getting a sniff of combustion ..rule out craked head etc

sounds like thermostat could be sticking or a water pump not turning and water is boiling as its not circulating

run engine do you see the expansion bottle circulating coolant ?

It is notoriously difficult to get a true positive reading from a sniff test on a diesel engine. You know you have a cracked cylinder head. Until you address that issue, your chasing your tail.

Yeah, I agree Rigsby but I was hoping to pin point the fault without going through the expense of a new head. Of course, if it is the problem then I'm losing time & money.

Deano, the thermostat, cap and water pump were all changed with the head gasket. However, I do feel that the water isn't circulating properly and is then boiling so I suspect a blocked rad. It looks original so is maybe due a change anyway. If that doesn't cure it, I'm back to looking at the head.

IMO a blocked rad or thermostat would lead to overheating too and the gauge would have climbed above the middle then also that bubbling noise is caused by the crack in the head as well, you should bite the bullet and go for a new head

or a the coolant system was not bleed correctly
I over heated my Td5 a few times but the ecu seems to protect the engine.... and the engine will shut down (while doing cooing flushes ) /air lock
oh ****
time to do a head or gasket I over heated 3 times

I have an update for this problem...

I changed the rad since it was the last item I could change before going back to the head. It made no difference of course but, since it was the original part, it was time for a change.

So, I found a replacement head and had it pressure tested. The engineer wanted the original head back to check again because I told him it was still overheating. Both tests came back good and the cracks that the engineer had mentioned on the original head turned out to be on the glow plug holes.
He said again that the cracks won't be a problem and that he thinks that the reason for the overheating is because the head was not torqued properly.
I doubted this but, as he explained, I began to understand why it might have been the cause.

He advised me to tap the holes for the headbolts, clean the oil out and use an assembly paste under the bolt heads to reduce friction. If there is anything making the bolts tight then your initial tightening will come too early and then the angular tightening will not get the bolt tight enough for this engine.

I couldn't tap the holes, they are too deep. Instead I had to blow them clean with air. With all of the oil out of the holes and assembly paste under the bolt heads, I torqued an initial 30nm, then 65nm, then angular turns of 90*, then 180*, then a further 45*. The torque at the end is around 150-160nm.

So far I have had no problems, it's not pressurising any more, the water level is stable so it appears to have worked.
I am still a bit cautious because I have previously driven 2000 miles before a problem came back but that may have been because the incorrect torque allowed the head to slowly lift and allow pressure out.

So, the moral of the story is to clean your bolt holes, clean all surfaces and ensure you are torquing bolts to the correct values.

With new bolts i presume...cos it's compulsory for the Td5 to not reuse the bolts.

Er, yes of course. I don't understand how some people think it's ok to reuse stretch bolts.

Great to hear a result, and file away for future reference - and glad I was talking rubbish as usual....... A

sierrafery said:

With new bolts i presume...cos it's compulsory for the Td5 to not reuse the bolts.

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Whilst I have never reused stretch head bolts it is common practice in many African countries as they just dont have access to new ones in the bush. When I worked in Tanzania a company mechanic stripped the head off one of our TD5s and replaced a leaking head gasket (plastic dowel syndrome) then put everything back using the original head bolts. That particular Defender went on to cover more than 75,000km with no engine issues before it was destroyed in a bush fire. The problem with reusing stretch bolts is you have no idea how they have fatigued or how many times they have been used and risk them snapping whilst tightening (normally where the rolled thread meets the shaft) which makes getting the remains out of the block a big and awkward job.