Cooling Help Please! At the end of my tether :{

[Manxda]

Morning All
Well, my 2.5TD has been overheating (again). Thought it had sorted itself out a few months ago after a thorough bleeding session. Apparently not. It started again, out of the blue, a couple of weeks ago. Running fine over the Mountain Road at 60-70mph but as soon as I slowed down to 30 the temp gauge exited right. Pulled over and switched off. Fans on and water initially pouring out of header tank. Anyway, managed to get home (eventually) and this is the order I did stuff to rectify the problem:
1. Topped up bled the cooling system as per proper procedure. Worked for about 5 miles with the gauge at 11 o'clock and then went ballistic. Turned on the heater - cold. Water out of header tank
2. Checked the bottom hose - cold. Replaced the thermostat with genuine Mazda part (what a ball-ache!). Top hose was looking a bit tired, so I replaced that as well, bled system. Bottom hose got hot (thermostat open) - ran okay for a few runs, thought I'd fixed the problem then, boff! - overheated again, seemingly at random, not doing any hard work, 40mph. Water out of header tank but not empty, as usual.
3. Checked everything over again and noticed that the new radiator installed about 10,000 ago had a pressure cap rated at 0.9. Changed that for a new 1.1 and did a couple of trips to town and back (5miles each way). Wahay! seems to be working. Went out this morning and overheated again after a couple of miles. Oddly the gauge went to about 1 o'clock and stayed there.
So, what have I missed or am doing wrong. Water pump was replaced a couple of years back (8k miles). Any ideas gratefully received or the Bongo is going to have an appointment with the Wicker Man!
Cheers

[mikeonb4c]

Sorry to hear of your woes. Have you considered changing the main and possibly the front heater radiators. Did you find the old top hose crusty with scale inside? Have you got an engine temp block gauge fitted so you can see what temp the engine is getting to?

[g8dhe]

Could it be a pinhole leak in another pipe that is drawing in air when it cools down (pressure reduces and sucks air in) which is building up over several cycles and then you get an airlock and the overheat occurs ?

The pressure cap on the radiator should matter as it doesn't have a vent and the pressure release part never gets used as such.

[rita]

It would probably be worthwhile having a Sniffer Test done.

Good Luck

[Manxda]

Sorry to hear of your woes. Have you considered changing the main and possibly the front heater radiators. Did you find the old top hose crusty with scale inside? Have you got an engine temp block gauge fitted so you can see what temp the engine is getting to?

Not come across that solution before, so thanks, I'll look in to it

[Manxda]

It would probably be worthwhile having a Sniffer Test done.

Good Luck

Thanks, what's one of those?

[Manxda]

Could it be a pinhole leak in another pipe that is drawing in air when it cools down (pressure reduces and sucks air in) which is building up over several cycles and then you get an airlock and the overheat occurs ?

The pressure cap on the radiator should matter as it doesn't have a vent and the pressure release part never gets used as such.

Hmm? I've had this before - small pipe coming from the head. Curious thing is why it's not going the whole hog with the gauge setting at 1 o'clock, anymore. Cheers.

[Manxda]

Sorry to hear of your woes. Have you considered changing the main and possibly the front heater radiators. Did you find the old top hose crusty with scale inside? Have you got an engine temp block gauge fitted so you can see what temp the engine is getting to?

Crusty on the outside, didn't check inside, but it had ballooned a bit. Ta.

[mikeonb4c]

Sorry to hear of your woes. Have you considered changing the main and possibly the front heater radiators. Did you find the old top hose crusty with scale inside? Have you got an engine temp block gauge fitted so you can see what temp the engine is getting to?

Crusty on the outside, didn't check inside, but it had ballooned a bit. Ta.

Unless the head/gasket has already gone (sniffer test needed and if like mine I'd expect noticeable steam smoke from exhaust, and plus hoses pressuring straight from cold could also indicate trouble), it does sound a bit like obstructed coolant circulation. I'd be tempted to consider new main radiator and, if budget will run to it, new front heater radiator, if they are the originals. Do you have any sign of rust staining in the header tank?

[Bob]

Sniffer test checks for combustion gasses in cooling system.

A garage can do this and it indicates a head/gasket problem.

[g8dhe]

Curious thing is why it's not going the whole hog with the gauge setting at 1 o'clock, anymore. Cheers.

Sometimes the force of the boiling is sufficient to force the trapped air along and out, and hence the temperature varies wildly!

[haydn callow]

Not what you want to hear but I reckon your heads-goosed.

[Manxda]

Sorry to hear of your woes. Have you considered changing the main and possibly the front heater radiators. Did you find the old top hose crusty with scale inside? Have you got an engine temp block gauge fitted so you can see what temp the engine is getting to?

Crusty on the outside, didn't check inside, but it had ballooned a bit. Ta.

Unless the head/gasket has already gone (sniffer test needed and if like mine I'd expect noticeable steam smoke from exhaust, and plus hoses pressuring straight from cold could also indicate trouble), it does sound a bit like obstructed coolant circulation. I'd be tempted to consider new main radiator and, if budget will run to it, new front heater radiator, if they are the originals. Do you have any sign of rust staining in the header tank?

Header tank nice and clean. Rad was new about 10K miles ago. I've had the head go before - about 3 years ago. It was like a steam engine at the back! Nothing to indicate head trouble.

[Manxda]

Not what you want to hear but I reckon your heads-goosed.

Hmm? that would make it the third one in the 30k miles I've had it - went not long after I got it, then again 2 years ago. If it's gone again it's going to the crusher. No steam at exhaust, no loss of power. Any other tests for head gone? Cheers.

[Manxda]

Sniffer test checks for combustion gasses in cooling system.

A garage can do this and it indicates a head/gasket problem.

It would be great if I could find a garage that would touch it. They don't appear to interested in doing any work on old stuff, here on the rock. I have to do work on it myself and, as I have arthritis in my spine, it's not the best solution for me. Found a mechanic to replace the head last time and he piped it up all wrong! Managed to sort it myself.
Thanks for the advice.