F78 overheats when towing, any suggestions?


I recently upgraded from a F75 to a F78 the old F75 pulled lovely and only ran a little hotter when towing. However the new F78 (80K miles on clock) sopmetimes runs very hot when towing (especially in hotter climates).

The full story: Upgraded to F78. Towed caravan to south france no problems. Pulled over mountains into Spain (ran very hot). Once in Spain I was unable to exceed 50mph as this would cause the temp to continously rise. At no time did I allow the engine to boil over, I simply throttled right back and put the heater on full blast (no fun in Spain). Please note that I have run the exact same route in hotter weather whilst pulling the same caravan with the old F75 without a problem.

The following may be relevent.
1).There is no oil in the water or water in the oil, I am not loosing any oil or water and the water stays in the radiator when the engine is running, thus I assume the head gasket is intact.
2).When I open the fuel cap to refill after a run there is a noticable suck cause by a vacume in the tank. Would this cause the engine to run lean? If so what effect would this have on a turbo desiel?
3).There is a knocking comming from the front of the car on right hand corners. I assume this is the RH CV joint. However I assume that this shouldn't be turning in 2WD mode. Thus the auto locking hubs must be stuck. Could this cause extra drag and thus overheating?
4).The oil light has been on for the last 3000 miles. As far as I can tell this is an electrical fault as I have been unable to get it to turn off by shorting out the connection on the oil pressure switch. The oil is circulating around the engine as I have a oil cooler that warms up shortly after the engine has started also the oil filter warms up after a few minutes running. Does the oil light also indicate oil levels in the difs and gearbox? If so this may be the cause....

Any ideas as to the cause of the overheating?

if the oil light has been on

if the oil light has been on that long i would assume its a wiring short somewhere , the engine would be knackered in a couple of minutes if the oil pump had gone. the light only shows the presence of engine oil pressure.
if the tank vacuum is worrying you drill a small hole throug the cap to let air in, or yor could try running with a loose filler cap when the tank is low to see if it affects the cooling problem , but its unlikely.

water pump, thermostat or radiator are the obvious places to start... is the water circulating to the rad , check how soon it warms up and that its warm all over.. cold areas indicate that the rad tubes are blocked, taking it out and stuffing a hose in the bottom may clead it out.. it could be the airways are blocked with mud, insects dust etc.. we often have to pressure wash the front of tractor rads cos they get clogged with grass seeds... a hose directed against the front should see water going straight through, if it all bounces back the air cant get through either.

the other thing to check is the cooling fan viscous clutch actually locks when warm

re the oil light, for the

re the oil light, for the light to be on the switch must be on so shorting the wire out to the block would keep the light on.. the oil pressure turns the swich off, so dissconnecting the wire and holding it in mid air should turn the light off,

I would test the rad as

I would test the rad as already stated. Check that the thermostat is working(opening at the correct temp) and check that the viscous fan is engaging.

Thanks for all the suggestions

I have removed the fan as this seems like the most likely culprit. At room temprature the fan 'clutch' is quite stiff (can be turned by hand but doesn't spin). After heating with the misses hair dryer for five mins it remains the same. The center pin doesn't apear to turn. The four screws that hold the fan 'clutch' together will not budge so I may have to lop the heads off with an angle grinder and drill out the studs :(.
With respect to my locking free wheel hubs is it posible to replace the auto hubs with the manual hubs of an old F75 or will I have to buy the manual hub kit from Milners?

I am pretty sure that the

I am pretty sure that the manual hubs can replace the auto ones, but you would need to speak to someone whos done it.

The best way to test the fan is to run the truck till its overheating(or about to) stop the truck, jump out and see if you can turn the fan. If you can't its engaged, if it turns like it does when its cold, is broke Smile

If it is the fan, I've seen second-hand ones for about £10 on Ebay.

thermostat problem?

maybe you should check the engine thermostat. is it still there or not? is it working properly?

I ran a different diesel 4x4

I ran a different diesel 4x4 with no fan and no thermostat for 3 years before fitting a thermostat and had very few problems, Fans are good but sling the thermostat for 3 months and see if anything changes. it still has not got a fan.


Thanks for all the comments:

Thanks for all the comments: I fitted a new radiator before the holiday so it is clean and clear both inside and out. While on holiday I removed the thermostat and cooked it up on the stove. I don't know what temp the thermostat opened up at but it was fully open before the water boiled. Finally the oil light remains on whether I short the lead or open circuit it.


Hi. I suggest you look up 'viscous fan' in the site search. In my opinion your solution lies there. Many of us have had this problem. Initially try locking the fan up as mentioned in Nev's posting. Regards.

Hi, I have a f78 and am in

Hi, I have a f78 and am in Australia... I am having the same problem but I have replaced everything... I have a brand new radiator, thermostat, fan clutch... Don't know what else to do.... Mine is fine driving around town but I live on a mountain and driving up it everyday from work.. well it is about 3 miles up a steep drive.. and every day it has filled the overflow bottle and pushed 2 liters of coolant out the overflow... I am just wondering if Daihatsu did not make the cooling system effective enough for the td 2.8 under tougher conditions.. let me know if you solve yours... also mine does not seem to run hotter just gets t a point and blows the coolant out.. yes it has a brand new cap to... anyway good luck