Overheating, Thermostat or water pump???
 

A friends sti hawk 2.5 temp gauge keeps jumping up to 2/3 when he boots it (i.e. on motorway slip roads etc) but doesnt do it when driving normally, been trying to sort it, and bleed the system ,,,, is this a sticky thermostat or failing water pump ?? or anything else ???

Thanks guys :thumb:

Possibly a faulty thermostat but given what car it is, the head gaskets are likely to be blowing.

Thermostat - I had similar problem one day , after replacing thermostat and coolant problem disappired

Good luck

Cheers guys thermostat will be the cheapest to replace so will get him to do that first .. If the heads were blowing then it would be using loads of oil And coolant ?!?! Would a compression test answer that ???

A compression test and a cylinder leak test should answer most of it. Any coolant or oil loss or gain would add more info too.

Bring the engine up to normal working temp and check the top and bottom hoses are hot - this would tell you if the stat is opening correctly.
In my experience, it's normally the head gaskets which fail on the 2.5 hawks and ring land damage to No. 4 + 2 pistons.

Hopefully your mates problem is a simple/cheap one to fix.

Mick

My guess would be headgaskets, then a coolant issue, (airlock) and only then would I think of thermostat.

I have been working on car engines for over forty years and have NEVER seen a thermostat stick shut. They stick open, which causes the car engine not to get warm enough.

And lastly, there IS NO WAY of effectively bleeding Scoob engines.

If it gets airlocked, drain it down and start again, S L O W L Y.

Cheers guys .... Why would a blowing head gasket make the temp increase for a short time though ?? Obviously one of the liners is crossing in effect but just curious for the cause of temp increase ??

I wonder how many head gaskets have failed due to the thermostat not working properly?

OP also you might want to flush out your rad whilst you are at it

Not many, given that MOST thermostats will stick in the OPEN position, causing the car to be BELOW working temperature?

The heads are lifting at high boost and letting air into the coolant system.
The coolant loop then can't cope with the extra heat load in combination with increased heat transfer inefficiency due to bubbles or air pockets forming, so the temp gauge rises.

Off boost, the heat load falls and can be dissipated even though the coolant loop isn't working at its most efficient, so the gauge falls back to normal.
If the bubbles or air pockets merge then airlocks can form and stop flow through the radiator.
The temp. gauge will then either stay high for longer or not even fall back.

Are there any black smudges collected under the rad. caps or in the overflow bottle?
Does the overflow bottle spill over regularly?

It may be a fault with the early 2.5 heads or bolt clamp pressure easing off, rather than an HG issue.
However, once the heads start lifting then the HGs are free to flap and lose their sealant - hence the black smudges.

No this is the thing it isn't pushing coolant and the coolant is clean ... I thought an air lock or heat surge as it were would prevent the heater matrix from heating effectively and blow cold air ( as it happens the matrix sometimes blows hot and sometimes cold, when gauge increases) ... Is there any other ways to establish the cause apart from replace it and error lol ????

Do the drain down and S L O W refill first, cost is minimal if you collect the used antifreeze.

Then come back if the fault persists.