Hi there,

I have a problem with my car and was hoping someone could tell me what the problem could be.

Sorry for long post, I'm just trying to be as informative as possible. If you don't want to read, skip to "symptoms"

[back story]

It started loosing coolant every 1200 miles, so I (stupidly) poured a little Radweld into it. And that's when problems arose.

It all foamed out the expansion tank, car heater fluctuated from mega hot to freezing cold and all sorts of stuff. Also the car got to temperature VERY quickly.

I took it to the garage and they flushed out the system, said there was no blockages and they thought it could be the head gasket

Took it to another garage and they changed thermostat and said everything working fine, there's no holes or leaks or blockages (they also flushed they system). They said they couldn't duplicate the problems I was having. They also suspected the head, they said it will get worse if not fixed (they think the coolant was escaping through combustion chamber)

ANYWAY, I decided to just run the car as normal, carry 2x 2litres bottle of water in the boot and top up as required and stop if car temp is about to rise.

This is where the problems changed

My car suddenly stopped firing out coolant, heaters suddenly started working as they should, great my car fixed itself...or so I thought.

It still got to temperature a lot quicker than normal, but I didn't see that as a problem, more as a bonus really, especially with it being a diesel and normally taking for ever to warm up.

Everything was normal apart from when I came to junctions and traffic lights, 10 seconds later my heaters would go cold, I mean freezing cold! As soon as I applied to revs around 1300rpm, it came through hot again.

From 900rpm (tick over) to 1200 ish, thats where it would stay cold, any higher, then my heaters would start blowing out heat again.

I drove it like this for around 1000 miles and then symptoms changed again!!

All of a sudden, heat came out the heaters at traffic lights and junctions and behaving normally. The car went back to taking a while to warm up (bloody diesels). Occasionally having to still top up coolant every 1500miles (the light comes on dash) all seemed great.

But this is how it is now and these are the symptoms that I have:

Symptom 1

Right, my cars fine apart from going up hills, a normal step hill. There's one hill I go up and the heaters turn cold around halfway up, 5 seconds later my car temp starts shooting up, I've never let it reach max, its only just touched around 110 then goes straight back down.

If i'm in top gear and go up the hill, the temperature shoots up a lot quicker than it would if I dropped down to 4th.

Symptom 2

If i'm on the motorway and start doing 80mph, then after a minute or so, the heaters turn cold, then as expected the temperature starts to rise. I slow down, the heaters start blowing warm air and the temperature goes down.

Now you could say "well don't do 80mph, that's speeding" but my car still shouldn't do that and if I need to get past someone quickly I need to know my car is going to hold up.

I try to drive around it by making sure my car stays bang on 70mph or slightly under and not really overtaking anyone, but after half an hour I get the same symptoms as I do at 80mph. I have noticed if I put my foot flat to the floor the heaters turn cold for a few seconds.

I try to go back roads a lot of the time as my car is completely fine at 60 and I get no issues, but I cant go back roads and avoid hills forever

So yea these are the problems i'm having and would really appreciate if someone could help point me in the right direction as to what it could be as its really annoying me.

Cheers

Two things you need to check first. First is the thermostat, I know you said it's been replaced but it's not uncommon for a new thermostat to be faulty out of the box. Second, is the radiator fan kicking in as it should?

Also you need to find the cause of your coolant loss.

Two things you need to check first. First is the thermostat, I know you said it's been replaced but it's not uncommon for a new thermostat to be faulty out of the box. Second, is the radiator fan kicking in as it should?

Also you need to find the cause of your coolant loss.

Yea the new thermostats been checked, the garaged checked it as they thought it may be faulty. The fan is also kicking is as it should.

Yea thats the thing, I cant figure out where the coolant it going, I wasn't sure if I had a micro leak somewhere, that's why I put a drop of Radweld in it to begin with.

I've since then thought the head gasket going through combustion side but that doesn't seem to add up with cold heaters depending on RPM.

Couldn't be water pump at all could it? Like one of the fins cracking, so at low RPM it cant flow round the coolant, therefore heaters go cold, and the coolant sat in engine gets hot, so car temp goes up? Just a theory.

Edited 16 March, 201511 yr by fivepointtwelve

The water pump isn't very effective below 2000rpm on the petrol engines, So I'd imagine its similar on the oil burner.

Have you had the water system pressure tested to see if it's leaking? What colour is the expansion bottle cap? as the black ones were replaced by blue ones due to loosing pressure

The water pump isn't very effective below 2000rpm on the petrol engines, So I'd imagine its similar on the oil burner.

Have you had the water system pressure tested to see if it's leaking? What colour is the expansion bottle cap? as the black ones were replaced by blue ones due to loosing pressure

No I didn't get them to check the system pressure, they did however mention that the pipes seemed very pressurised. The expansion bottle cap is blue.