I have just changed the thermostat on my Skoda Felicia, 1.3 petrol engine, of 2000 vintage. Since I fitted the whole unit, which includes the temperature sensor, the thermostat and the housing, I presumed that all would be well.

As per the manual, when I topped up the coolant, I removed the temperature sensor as well as the cap from the expansion tank and kept the motor running with the heater at full. I then proceeded to add the coolant, noticing that a lot of air bubbles were surfacing. After a good half an hour, these bubbles appeared to cease. I presumed that any air in the system had been expelled and refitted the temperature sensor, leaving the top off the expansion tank through which I topped up coolant to the level marked on the expansion tank between the 2 lines. Throughout this procedure, the temperature gauge did not fluctuate, but rose gently until the fan kicked in, returning the gauge to normal.

I then took the car for a drive of around 20 miles, along a variety of roads, which included motorways and town centres. During this trip, which lasted over an hour, the gauge fluctuated to just below the red lines, dropping back to normal. This occurred on 6 separate occasions on both the motorway and normal roads. I noticed that the heating which was quite fierce when the gauge was at normal, suddenly cooled and the gauge would begin to rise to the red marks, dropping back to normal, at which point the heating would return.

When I got back home, I inspected the expansion tank to find that the coolant had been expelled through the cap and the amount lost from the trip, amounted to 800 cc's.

Any thoughts on the possible cause and solution to this problem, would be very welcome.

I wonder if the stat is faulty.

Hopefully a mod can move this to a more suitable area where there are more Felicia experts around. Welcome to Briskoda though.

It sounds like something blocking the coolant flow through the heater matrix - circulation stops so engine temp rises and heater goes cold. Not sure why it would be periodic unless it's somehow relieved by the opening or closing of the thermostat. Dislodged scale or debris from the old 'stat perhaps?

I don't think you read from a Felicia manual, but filling the system with coolant and bleeding it is not done like you did. That's why you had air pockets and expelled coolant.

Here is what Skoda Felicia Workshop Manual says:

- with engine NOT running and usually cold (doh), remove coolant temp. sender

- pour coolant into expansion tank till coolant begins to flow through temp. sender fitting

- replace temp. sender and secure it. Leave expansion tank cap off.

(at this point I usually squeeze a few times the big hose going to top radiator)

- START engine and let it idle till fan cuts in (heater switch position doesn't matter, coolant goes through heater matrix at all times); this takes about 10-15 minutes, all air is bled from the system, don't need to count the bubbles

- top up level in the expansion tank till MAX sign

- screw in expansion tank cap

- enjoy

Other than that, rather than just adding lost coolant, it is recommended to drain all fluid and do a good system flush to clean all French cheese from the pipes. I also got my heater out and cleaned its fins of all dead bugs, leaves and dirt stuck in the last 10 years.

Here is a supporting illustration from my vault

Edited 16 April, 201313 yr by adurer

Adurer, thx for the steps to fill the coolant system. including the temp sender procedure. I thought I was going mad because temp gauge was still reading low, even after swapping out the sender. problem solved.

 

Although, I'm curious about when to put the expansion tank cap back on.

 

My rational is, putting the cap back on whilst the coolant is hot and unpressurised would create the condition of partial vacuum in the system as it cools back to ambient temperature. This in itself is not a problem, hoses, seals and gaskets work in both ways to block a pressure differential.

However, upon heating up the coolant will only pressurise upto 1atm (the pressure it was capped at). approx boiling point of water/glycol. Would this not lead to dissolved gases coming out of solution to form an air lock???

 

My understanding (and please correct me if the physics is all wrong), is that it is best to pressurise a closed coolant system from a nominal 1atm cold (the point at which you put the cap back on) to it's operating temp ~90C at it's corresponding pressure to increase the boiling point therefore stopping dissolved gases coming out of solution and causing air locks.

 

Simply put, Cap-on hot or Cap-on cold. Or does it really make any difference at all ?

Although Skoda Felicia service manual doesn't say, it is exactly the same thing from the bleeding end result point of view. 99% of purging is done when you fill the cooling system and a constant stream of air-free coolant is flowing from the temperature sensor hole. The rest of 1% trapped air will be evacuated during the first trip into the expansion tank which is the highest point of the cooling system. I leave the cap off after filling only because the fan will kick in faster and I can get on with my life sooner. But after the engine is cold again, I take care to open the cap and put it back on even if there is no need to add more coolant up to half way between the MIN and MAX marks.

 

If the engine steel overheats after a new thermostat is put in, that is mainly because the system has't been flushed properly and debris are blocking cooling passages.

So basically, do your purging cap-off then allow cool (cap-on/cap-off any way you want) then equalise the coolant pressure to atmospheric pressure by unscrewing off/screwing on the expansion tank cap.

Not wanting to go too off topic and starting a new thread, but are Felicia's prone to coolant system blockages more than any other brand of car?

If you wish start a new thread about coolant flushing this might be helpful for all forum users.

Many thanks for your help on this not so obvious query.

Cap issue

Yes, you got it right.

 

Coolant system blockages issue

I call it 'blockage' when the coolant flow is restricted by dirt, flakes, broken blade from pump, glass-like deposits from heater matrix that come off, dead mouse etc. From this point of view, Felicia is no different from other cars having foreign objects inside the cooling system.

 

I call it an 'trapped air pocket' when air is trapped in highest points: heater matrix area or most often under the temp. sensor. The coolant flow is not restricted in this case, except the heater performance is reduced and/or temp. gauge indication is not accurate. Felicia is not more prone to air pockets than other if the bleeding is done correctly.

 

Tips & tricks to get rid of air pockets when bleeding the system

- park the car on a slight uphill road.

- remove the heater matrix from its place (still attached to hoses) and let it hang as low as possible

- post bleeding, do a short drive, let the engine cool down, open exp. tank cap, take out temp. sensor, pour coolant in exp. tank till coolant streams out air free, then immediately put temp. sensor back on.

 

Errata from previous message, last paragraph: "still" not "steel". Sorry.