Hijet - Faulty Thermostat?

Before I go through the endeavour of removing seats, central handbrake/gear bar and then awkward thermostat housing bolts (especially in the outdoor cold), I’d just like to get a forum second opinion please.

I’ve noticed recently that the temperature gauge needle settles a notch higher than before. (One o’clock position instead of twelve o’clock).  No big deal as engine runs smoothly. However, on a few occasions when I’ve let the engine continue to idle, I’ve noticed the fan doesn’t kick in and the hose from the thermostat remains cold. More often the fan will eventually kick in when the temperature is nearing high and the hose from the thermostat eventually opens to trigger the fan sensor.  Seems to point to a late engaging thermostat?

Key question is – Should the thermostat normally open on reaching normal engine running temperature?  I've also noticed the overflow reservoir level doesn't change between the engine being hot or stone cold?

air lock in the system is my first thought

buttondup wrote:air lock in the system is my first thought

Thanks Buttondup. Are you suggesting that the thermostat could be opening normally, but there is an air lock as indicated by the reservoir level not changing?  What would you suggest I do?  Thanks. Chris (1.3L 2005 Hijet Pickup)

If there was air in the coolant system, the header tank would show excessive fluid.

HighlyJetted wrote:If there was air in the coolant system, the header tank would show excessive fluid.

Thanks Highly Jetted for your thoughts. What would you suggest I do first?

have you checked the header tank level when it begins to overheat, wouldn't the excess in the header tank be drawn back in as it cools?

I'd have thought the best possible strategy to try first would be the usual advice - replace radiator cap, check good seal with rim and do Matt's thorough bleeding procedure - the cheapest option. If that fails to resolve it then maybe then think about the thermostat.
Don't take my advice as gospel, most of my mechanical knowledge has come through the helpful advice from Matt, Woodie and others on this forum.

Thanks Guys.  Hasn’t actually been overheating – just noticed on a few occasions when I’ve let it continue to idle that the thermostat hose wasn’t getting hot and consequently the fan sensor not kicking.  Cut engine before actual overheating.

UPDATE: Having removed radiator cap and hearing mild gurgling, I’ve found the thermostat hose heating as normal and fan kicking in correctly.  So, appears to show all back to normal. However, I’ve swapped the expansion tank pressure cap but level still hardly moving - a centimetre at the most.  How much should the expansion tank level typically change between hot (normal engine running) and cold?

I’ll keep monitoring. Thanks again guys.  Chris

UPDATE 2: Just been for few miles drive and with engine fully warmed up found thermostat hose still cold. However, letting continue to idle on the driveway afterwards noted the hose did eventually become hot and fan kicked in at usual higher temperature (not overheat). So fundamental questions are:

1). On a 5C degrees evening should the thermostat/fan sensor hose still be cold after a decent journey?
2). By how much should the expansion tank level typically change between hot (normal engine running) and cold?

Thanks again guys. Chris

All seems to have settled down now - probably was an air lock. Thanks again guys. Chris