406 HDI 110 air lock in cooling system

[Peugeot Keith]

Hi, I'm new to the forum and need some help please!
406 HDI 110 (A/C) 2001 on 51 plate. (76000 miles)Had car for 2 years & 25000 miles, never any problems at all (and never had to add coolant!)
Low coolant light came on. Put more coolant in.
After 100 miles or so same thing.
Have now replaced coolant level switch in header tank, thermostat, radiator cap, all new genuine Peugeot items.
Car starts & runs perfectly but chucks water out of filler cap then the low level light comes on.
Takes about 6-7 miles for the temp gauge to reach 90 degrees, it used to get to this after 3- 4 miles.
Car has never overheated
I have used a large funnel sealed to the header tank inlet and maintained a good head of water whilst bleeding which was done;
Heater was turned up to 30 degrees (maximum) and the heater fan switched to "off"
opened bleed valves on thermostat housing and pipes to heater matrix and on top of radiator. Closed these when no air comes out starting with the rad screw and finishing with the heater matrix bleed cap.
Ran engine at 1500 rpm until temp gauge at 90 degrees.
Water level in the funnel attached to the header tank does not fluctuate whether at idle or engine revved.
Drive car 40-50 miles and water has been expelled through the rad cap resulting in coolant level light comming on and no water visible in the header tank.
Local garage has used a Carbon monoxide sniffer and no CO eveident in the water(he's a good mate of mine and assure me it will work on a diesel). Also pressure tested the system, no fluctuation whether revved or at idle.
Noteworthy is that the engine can be left idling or revved at 3000 rpm for 15 mins once 90 degrees is showing on gauge but gauge will not go any higher and the fan doesn't cut in. The fan does come on at low speed when I activate the A/C
I have now removed the thermostat, driven over 100 miles and everything is perfect, except that obviously the temparature takes an age to get up to about 70 degrees.
My conclusion is that there is (with the stat fitted) an airlock in the head below the thermostat. The air expands as the system gets hot but because there is no water at the base of the stat it won't open. I believe the sender for the temp gauge is also in this airlock so that's why the gauge doesn't go over 90, also maybe it's the same sender that switches the fan on.
The thermostat has no bleed valve but neither did the old one.
I have tested the stat in boiling water and it opens and closes correctly.
If you guys think I am right, when I refit the stat, how do I get rid of the airlock?
Sorry it's such a long post for my first one , but I thought that by giving all the info it might enable someone to give me the answer without asking more questions

[Welly]

Hi chap

My first thought was that your 'new' thermostat is not quite the right fitment and is causing trouble (allowing the engine to overheat periodically). However this does not explain why it spewed out coolant in the first place

To be honest you've done a good job there with the bleeding and there is no other bleed points to help. Your makeshift header tank was the right way to go and you've done it all right.

I have had a new thermostat fitted by the Peugeot Dealer and have never had any problems. The only thing I did do once (stupidly) was top up the already quite full expansion tank before a long run and it didn't like it and spat it out. But it did not loose enough to bring the light on.

The other thing it could be is a partially blocked radiator, we had this on a Premera at work and the rad was partially blocked and eventually the build up of pressure due to the poor cooling capacity blew the heater matrix and squirted coolant all over the ECU/Immobiliser (not to mention the dash removal to replace matrix).

I would disconnect the main rad hoses and flush water through to check whats in there otherwise, I'm a bit stumped here for you

[Peugeot Keith]

Thanks for your reply.
I don't think there is a problem with the rad as its never been any problem before this recent turn of events.
I am considering refitting the funnel to the header tank & and putting the hosepipe on it to keep it filled up, getting water all over the place when it overflows.
Then I will remove the stat housing and refit the stat ensuring that the pipe under the stat is filled with water whilst I do it. The head of water should cause the pipe in the head to overflow and expel air whilst I refit the stat housing.
Then bleed as before.
By doing this the water under the stat should cause the stat to open when I run it & it heats up and any remaining air can be bled out using the bleed nipples.
What do you think?

[Welly]

I'd still check the rad.

I really don't think you should have too much trouble bleeding the air out. I think you may be clutching at straws a bit on the air issue to be honest it really shouldn't be as difficult as you describe to vent the system.

We had no indications that anything was wrong with our Premera, not even on the temp gauge. But when we inspected the rad it was full of scaley cruddy soft crap blocking some of the flow. The system pressure was running too high and the Matrix let go in the end.

Something else that springs to mind is the water pump, sometimes the pump impellors can come off/break up and stop it pumping enough water. I know they leak on 406's but not sure if they suffer impellor damage like the VAG pumps do.

[Peugeot Keith]

I'm off to get some rad flush then

[Welly]

Ooo-er nobody's ever taken me that seriously before.

Hope I was right

[jokes]

I have the exact same problem on my v6 coupe

its doing my nut in!!

[bigstevemex]

hi the fault is probably the water pump.if the impeller is slipping it will not circulate and heat will build up.removing the stat just masks the fault.when pressure tessting a running system if there is pressure showing on the guage(ie you have pumped it up to 5psi or you have let it run and build up some pressure)you should see the needle move when the engine is revved as the pressure changes.its an easy one to miss.regards steve.