Overheating problem!

[nfn]

When I change my coolant, I test for air in the coolant system several times to ensure that all - or most - of the air has been purged. If anyone wants to try this, here is what I do and is based on the fact that air in the system will be compressed when the coolant level rises in the expansion tank and creates pressure.

Start with a cold engine. Remove expansion tank cap and add or remove coolant so the level is exactly at the MAX mark and then replace the cap. Turn off cab heating and air conditioning and start engine. Run engine on a fast tickover until you are sure that that the coolant temperature is hot and stable, then switch off the engine.

The coolant level should have risen by 12 to 16 mm - depending on temperature - and created some pressure in the expansion tank. Mark this level with a permenant marker. Wearing a glove to protect your hands, slowly release the expansion tank cap and jiggle it about until you can no longer hear any air escaping, hence no pressure. The coolant level will rise in the tank indicating how much air is left in the coolant system. Mark this new level.

My experience is that a rise of level of between 1 and 4 mm is about the best I can hope for, but I do try and get it down to around 2 mm. In theory, a completely purged system should remain at exactly the same level when the pressure is released but maybe old coolant hoses expand slightly under pressure or there is a small amount of air which is impossible to remove.

For me, this test works extremely well and I would expect it to show up any head gasket failure or lack of purging.

This is an ingenious way to check for air in the system. Given people's problems with airlocks in the bongo cooling system, this process should always be performed after the cooling system has had work done on it. One question: should the engine be warmed up all the way to operating temp? -- i.e., Mason gauge at 1 o'clock or normal bongo temp gauge at 11 o'clock.

[bongoben]

nfn:

My calculations are based on an average engine temperature of around 70 degrees C. Usually I do this on the front drive to avoid any overheating problems on the road through excess air in the cooling system. I simply heat up the system from cold using tickover and an occasional burst of 1500 to 2000 rpm until the temperature remains stable at close to 90 degrees on the flow side. This is an estimate as it's a hell of a job to calculate an instantaneous average temperature.

A few facts that might help:
The manual says the system holds 13 litres. As most specifications are for the standard Bongo - the rear heating is an optional extra - so having a rear heater I base calculations on 14 litres and also assume the cold level specification is exactly at the Max. or Full mark.

The curve for water expansion is neither linear or simple logorithmic, but doing the maths, the expansion from 10 degrees C to 70 degrees C is about 2.8 percent. This equates to approximately 0.39 litres and 0.39 litres is a rise of approximately 15 mm from the Full Mark in the expansion tank at the average temperature of 70 degrees C. Incidentally, the total volume above the Full mark on the expansion tank is approximately 1.2 litres, so knowing the expansion you could simply calculate what the running pressure should be for any temperature.

When I change my coolant from a known healthy working system, I always do it when the cooling system is cold and I always measure the total liquid volume that has been drained off. When I re-fill the system I make sure that the same volume of liquid goes back in the system so I'm pretty sure it's close to being right before I do any tests. I have to say that I have never managed to drain more than about 9.5 litres of coolant, the rest seems to be impossible to drain unless you remove ALL the hoses and blow down the heater ones. I don't do this, nor would I suggest anyone else does, I use a flushing technique which ensures that all the old coolant is flushed out.

If you know your cold coolant level, and you do this test immediately after a drive with heavy engine loads, be aware that your average coolant temperature will be 10 or 20 degrees higher than that mentioned above so the expansion may be in the order of 20 to 25 mm in the expansion tank. Also, allow the system to cool down for 10 or 15 minutes before loosening the expansion tank cap.

As I am a really old fart, you may need to check the maths and other data in case I'm wrong.

[Peg leg Pete]

I agree with the back flushing, I did this with our bongo and was dismayed to see how much crud was in the system, 2 years on and did another back flush when re doing the coolant, not as much crud, but enough to warrant the little extra time taken I would definatly check the expansion bottle cap as I recently had a small leak of coolant from the tank, cured it with a new cap