Unimog 411 cooling problem

[RichardT]

Finally got round to trying to sort out the old mini mog I bought off Fleabay a few months back.

 

Poor / non starting solved with a new set of Westfield plugs (thanks whoever suggested that on an earlier thread), new rocker cover seal stopped the oil loss, but it's consistently overheating, and I'm damned if I can work out why.

 

I've drained & flushed the system; had the pump off and the impeller seems fine; got the header tank off and the thermostat out, it opens ok in a pan of water (don't know at exactly what temp, but certainly before the water's bubbling); and checked water and oil for x-contamination of which there seems none, nor any obvious external head gasket issues.

 

The thermostat still seems the obvious candidate: the block gets hot, the cab heater loop gets hot, the cast elbow and hose between block and 'stat gets hot, the rest stays cool - ie header tank, top hose, rad and bottom hose.

 

Basically coolant doesn't seem to be circulating.

 

Anyone got any ideas why a thermostat would work 'on the bench' but not in place - eg could back pressure build up in the 'closed' (starting) circuit and stop the thermostat opening when it reaches trigger temperature?

 

Failing that, what else could be going on?

 

Any ideas welcome. My default next step is a new thermostat but they ain't cheap and I'll be pig sick if that doesn't do the trick.

[Robert_S]

run it without the thermostat in see if its the same or it could be the water pump not circulating water or cores on the radiator blocked. series of elimination mate, hope you get it sorted. stick some pictures up we like mogs here

[combined tree services]

personally i would flush the coolant system through including the radiator , often if an engine is sat for a while the coolant can degrade and cause blockages and the cappilaries of the rad etc .

[niftysteve]

at very worst head gasket may have a pin hole in just enough to make an air lock in the system my 406 had this happen lasrt week ive now got the engine apart and going to do a full rebuild of engine

[RichardT]

That's exactly the worst-case scenario I'm worried about. (Well, the worst-worst case would be a cracked head or the like...).

 

Given the odd layout of the 411 cooling circuit (in-cab header tank a good 20cm above top of rad and 15 above block) I'm wondering if filling from the tank could leave a big air gap immediately below the thermostat (which is fitted to the tank). Would this affect the operation of the 'stat, or would hot air open it just as well as hot water?

 

The rad is definitely not blocked - it may be a bit mucky but water poured in the top comes freely out of the bottom.

 

Pump turns freely, impeller looks clean enough.

 

Tricky to run w/out 'stat as it's integral to the tank, would need to make a loop with 2 hose gauges...

Edited June 19, 2011 by RichardT

[RichardT]

So, an update: I've bodged up a temporary test bypass by the simple if messy expedient of boiling the t-stat (which for those not familiar with older OM636 setups is in an unopenable housing which in turn is compression-fitted into the header tank), poking a couple of pieces of copper wire through the expansion gap and hooking the ends over the housing rim.

 

The result is that: coolant is now definitely circulating through tank and rad; block and attached cast ancillaries are cooler to touch; and the gauge hasn't got above 60C even after flooring it in 2nd up a steepish 1km hill and with the cab heater circuit shut off. (From which I conclude that I've got the t-stat wedged further open than it would ever get under normal conditions....)

 

I think(?) this eliminates a catastrophic blockage anywhere. Still doesn't indicate whether the issue is a temperamental t-stat (or coolant not filling all the way up to it?) or air getting into the coolant. The amount of vibration on the thing makes spotting bubbles in the tank all but impossible, am I right in thinking there's kit available to test for gas in the header tank?

 

Here's a couple of pics for anyone interested:

[skyhuck]

Thats a nice litte bus!!!!!!!!!!!!

[Robert_S]

very cool mate

[niftysteve]

think you can get a chemical that changes colour if exhaust gas is leaking into the system im sure b101 will have an idea what s up