Who Can Diagnose This: Heater Keeps Blowing Cold

[Mad Scientist 106]

On the way back from a track day on Anglesey yesterday, my heater kept blowing cold. It was on full hot as it was freezing so this became quite irritating!

 

Symptoms were:

 

Heater would start blowing cold, at the same time water temp would rise from 1/3 to 1/2. After a while, the water temp would drop and the heater would warm up again. This happened over and over again.

 

Car never got worryingly hot engine wise, nowhere near overheating. It was also still running fine.

 

No water in the oil. No oil in the water.

 

Did notice that the header tank had coughed out about 200ml into my catch tank during the whole 380 mile trip, including 100 hard track miles.

 

 

 

i'm thinking airlock, but the car has been run up to temp with the cap off, the bleed valve on the stat housing has been opened. Haven't done anything with the heater pipes though.

 

 

Essentially, car is a GTi6 running the BBM T piece hoses.

 

Any thoughts?

 

Thanks,

 

Pete

[allye 530 3 Cars]

My intial response would be air lock too, I just flushed through my XS and took about 30/40mins to fully bleed it (I know tu's are bastards though) did you bleed it for a good while?

[smithy 1]

as the car has been ran up and air blead out i cannot see it being an air lock,i would be more inclined to suspect either iffy thermostat or head gasket issue

[steve@cornwall 100]

as the car has been ran up and air blead out i cannot see it being an air lock,i would be more inclined to suspect either iffy thermostat or head gasket issue

 

Have you retained the bleed point on the heater hose?

[Mad Scientist 106]

The heater hoses are aftermarket silicon so no bleed point. I was worried about it being head gasket but it doesn't overheat, just gets hotter, then back to normal again.

[Mad Scientist 106]

Come on guys! I need inspiration here!

[Daviewonder 533 3 Cars]

Thermostat sticking open?

[marksorrento205 80]

I have an almost new thermostat for a 6 here if you want to try it?

[Mad Scientist 106]

Cheers Mark.

 

However, it had a brand new oe pug stat with the engine build. Plus, if the thermostat stuck, would it not be cold all the time and the engine overheat, or alternatively take ages to heat up then stay hot? Why would it be opening and closing?

[smithy 1]

Thermostats do open and close to maintain optimum running temp,they do not heat up to temp open and then remain open until switched off.

It could well be the stat is closed when cold so warms up ok but then remains open all the time so not maintaining optimum temp hence your fluctuations.

Failing that I would say get head gasket checked out,just because the parts are new does not mean

They are ok so don't rule them out straight away.

[rallysteve 22]

Might seem a stupid suggestion but are you sure the temperature dial/cable is operating properly?

 

Steve

[Mad Scientist 106]

OK, thanks for the ideas. Here's my thoughts now.

 

Thinking more about how a thermostat works (thanks for the prompt) I think this is the issue.

 

My car runs quite cold on the motorway as the cooling system is very efficient (larger, deeper alloy rad). I also used a 205 thermostat. Looking at the part numbers on service box, it was an 89 degree stat. The 6 engine uses an 83 degree stat.

 

I know the temp gauge and sender are correct (as I can compare the actual reading on my DTA through the laptop) and I know it settles at around 89 degrees temp when under normal conditions.

 

Therefore, I think the stat is too high opening temperature. With the extra cooling of the -1 temps the other night, plus motorway airflow and not pushing the car, I think the coolant kept getting cold enough to shut the thermostat...........hence blowers went cold until the stat opened again and the temp gauge reflected this.

 

I seem to remember only a 205 stat fitting in the alloy elbow, but i'll whip it out and look. In any case, service box lists a later stat on the 205 that was 82 degrees, so this will do nicely.

 

Just for peace of mind I will sniff test the coolant and run a compression test.

 

I can't see it being a head issue as the engine is 1800 miles old. I used a Pug 3 layer steel head gasket and new OE bolts. Also the head had been skimmed and the block decked so the fit "should" have been perfect. Its still a possibility though as my engine runs 12.2:1 compression ratio and gets thrashed to 7500rpm.

 

Does this seem to make sense?

 

Thanks,

 

Pete

[Mad Scientist 106]

Compression test done. As expected and exactly the same on all cylinders.

[trogboy 17]

Hmmmm... Not sure if I have interpreted your last post correctly but the thermostat closing should make the heater matrix hotter not colder.

 

I'm not 100% sure on the XU but on the TU the supply and return to the heater matrix is independant of the thermostat which just controls flow through the rad. Water flows through the heater matrix all of the time.

 

I think it sound more like a partial air lock with periodic coolant flows through the matrix causing the pulses of heat through the blower. Are the pulese of heat related to burst of high revs i.e. could the increased water pressure from increaded pump speed be partially overcoming an air lock in the matrix?

 

Just my two pence worth!

 

Ad

[Mad Scientist 106]

Hmmmm. You are of course correct and my thinking was wrong! There is a chance there is air in there again. Hard cornering and a less than full header tank may have introduced some air. Where is the maximum level on the phase 2 tank?

 

I'm going to drive it tomorrow night and see how I get on.

 

At lower revs off the motorway, the problem dissapeared. Very annoying this one!

[Peetypug 5]

To sort mine I just loosened the clip off the top matrix hose and allowed the air to escape from there.

Surely due to the t piece and the way the cooling system is set out on a 6 engine it's possible for the water to circulate while cold until the water in the head being held by he stat is hot enough to open, then you get your hot blast, the cooler water then enters the head again closing the stat and the whole process starts again?

 

Or are these the incoherent ramblings of somebody that has had less than 5 hours kip in 36 ?

[smithy 1]

Whilst the heater matrix runs separately to main coolant when cold basically through head and matrix bypassing rad once stat opens the coolant then flows through complete system until temp drops again with airflow stat then closes to maintain constant temp around engine.

[kyepan 291]

Whilst the heater matrix runs separately to main coolant when cold basically through head and matrix bypassing rad once stat opens the coolant then flows through complete system until temp drops again with airflow stat then closes to maintain constant temp around engine.

i was going to say, how is your set up configured?

[C_W 3 1 Cars]

Didn't some thermostats have a double element, almost sounds like it's directing water flow to the radiator, then back through the heater (if one goes cold when the engine gets hotter!)

 

I think if the thermostat was stuck open the engine would never warm up in this weather and the heater would never really get warm unless you stood it still for a while.

[Mad Scientist 106]

The set up is as follows:

 

Bottom hose from rad to S16 water distribution on back of block.

Water distribution to lower matrix pipe.

Upper matrix pipe to thermostat housing (engine side of stat) "T" off to expansion tank

Stat elbow (radiator side of stat) to rad top hose.

Expansion tank to top of rad.

 

Its the BBM T piece style plumbing essentially, just using S16 distribution lock instead of the Gti6 metal pipe.

[kyepan 291]

well that sounds fine, as the heater matrix is on the engine circuit not the coolant circuit, the only bit that might cause issues is the expansion tank, but the flow of water is not going that way.

 

new matrix's are fairly cheap.. perhaps flush it through with a hose pipe and see if it's blocked.

 

but airlock would also be my first port of call.

[Mad Scientist 106]

The matrix is a brand new Valeo one. Maybe its air.

 

I'll run it with the cap off for a bit tonight then take it to the Staffs monthly meet and see what happens.

[Mad Scientist 106]

Sniffer test passed (cheers Mark) and it ran fine tonight. I think it must have been air. Just out of interest does it matter which pipe goes in and out of the matrix? Mine goes from water distribution block to lower matrix pipe. Is that correct?

[welshpug 1,627]

Mine goes to the top port and has a bleed point on it (samco kit)

 

[Mad Scientist 106]

I might try swapping them then and see if it makes any difference. Could do with a bleed point there to be honest as its the highest point.

Thanks.