Header cap left off query

[banjoboy1]

Thanx for a very comprehensive reply.What can you tell me about the operation of the scavenger and radiator fans.Is my understanding that the scavenger fan kicks in when the engine temp rises when driving and the rad fans are liable to come on when stationary?Probably not so simple?Anyway my temp of 110 dropped steadily down after about 10mins stationary and dropped down to 93 when I resumed my trip.Sitting there watching the temp rise when the alarm is sounding is a bit stressful but I am reassured by your reply that it will not boil over.

[Driver+Passengers]

The scavenger fan is controlled by a temperature sensor poked down through just behind/beside the handbrake - I can't remember at what (engine bay air) temperature it causes the scavenger fan to start but it's basically there to provide airflow through the engine bay when you're not driving - eg stationary in heavy traffic or after a long drive, etc...? The radiator fans are controlled by a temperature sensor low down towards the back of the engine block (the dash gauge is controlled by a separate sensor towards the front top). The rad fans are multi-stage - the thermostat is independent and starts to open low 80s and is fully open low 90s - I see my rad fans come on about 96 degrees coolant temperature, but that may not correspond with your own readings. When the rad fans kick in, I see the coolant temperature drop to mid 80s within a minute. These figures are what I see when bleeding the van up in the garage.

The rad fans will come on during steep climbs, in heavy traffic or otherwise stationary - they usually have the effect of dropping the temperature nicely back down to within the thermostat opening range - ie if the thermostat is already fully open and passing coolant through the radiator and the passive airflow over the radiator is insufficient to cause a satisfactory lowering of temperature (eg when not moving forward), then the rad fans kick in to provide additional airflow over the radiator - there are two fans and I think at least one of them is multi-speed.

I know that one or other of the rad fans will come on if the aircon is turned on (helping with airflow across the aircon condenser), but otherwise, when stationary if the engine is off, then the water pump will certainly not run and therefore not circulate coolant - I don't know if the rad fans will run if the engine is off or not... won't make much difference to the head temperature even if they do.

Basically, you want to extract heat from the head as efficiently as possible - if you're pulling off the motorway for example, then you want to let the engine idle for a minute, let the turbo cool down, let the coolant circulate, and either the rad fans will kick in themselves, or you can switch on the AC to help things along, but otherwise letting it drop temperature more quickly than it will if you just cut the engine. I cannot see that the coolant would then boil without additional heat input, and I think the temperature rise when stationary that you are seeing is the latent heat in the head moving around and effectively raising the temperature at the probe site.

If, in a worst case scenario, a head hasn't cracked by the time you turn it off, I think it is unlikely that turning off and leaving and overly hot engine is going to then cause damage, but I always feel better when I see the temperature dropping off slightly, before I switch off.

I may be missing something, but there should be some more theory of operation in there to go on.

[Simon Jones]

Excellent write up D&P (as always). I think the scavenger fan should kick in at 80 degrees based on that being the temp that the engine manual says to test the sensor resistance.

[banjoboy1]

Thanx again for all your info and also Simons input.I do understand that there are too many variables to be certain of everything but would you say that my experience of motorway driving in the Bongo is normal.By that I mean that although the thermostat and rad fans are operating within the correct temp range it seems that I run on a continual cycle of temp rise to 98ish fans kick in down to 93ish then back up again.As outlined previously whenever I pull onto the services the chances are the temp will rise to about 105.I shall of course do as you suggest and leave the engine running for a while to facillitate cooling .It would of course seem unlikely that I have a big prob with overheating if I never boiled over driving without the rad cap on but I'm cautious after having probs last year which involved fitting a new rad and having to do several bleedings of the system.

[mikeonb4c]

Thanx again for all your info and also Simons input.I do understand that there are too many variables to be certain of everything but would you say that my experience of motorway driving in the Bongo is normal.By that I mean that although the thermostat and rad fans are operating within the correct temp range it seems that I run on a continual cycle of temp rise to 98ish fans kick in down to 93ish then back up again.As outlined previously whenever I pull onto the services the chances are the temp will rise to about 105.I shall of course do as you suggest and leave the engine running for a while to facillitate cooling .It would of course seem unlikely that I have a big prob with overheating if I never boiled over driving without the rad cap on but I'm cautious after having probs last year which involved fitting a new rad and having to do several bleedings of the system.

Did you fit a new thermostat at the same time? Would it be worth experimenting with a replacement temp sensor? Overall though most of us have no idea what temps our Bongos (or any car) might be reaching around the engine block so it's relatively uncharted territory.