COOLING ISSUE AFTER FITTING LT THERMOSTAT

Morning all. Not sure if anyone can help but I have a strange cooling fault after fitting a low temp thermostat to my 2003 C4S.

The car will get up to the new slightly lower temp OK (just under the 80c mark) and when driven gently at slow or motorway speeds the needle will remain fairly steady, even at 70 mph for a long period.

The problem I have is when the car is driven hard and RPM is above 4k and beyond the coolant temp will rapidly drop. When you bring the RPM back down but still maintain the same speed it will quickly climb back to normal temp.

So I'm thinking faulty thermostat or more likely an air lock in the system. I've done around 400 miles since fitting it and the coolant level has not moved since originally bleeding the system.


For info the LT thermostat is a design 911 one and I fitted a new -4 expansion tank cap at the same time. Car was running perfectly before fitting the new thermostat.
If I have got an air lock. Best advice to remove it?

Thanks in advance,

Phil.

Did you refill the coolant with the cars arse end up in the air? This can help to get air locks out quickly.

Also keep the burp valve open (located ontop of the expansion tank) for a while while running the car at idle upto temp.

Have you noticed the rad fans kicking on and off at all since the swap?

Did you fit just a thermostat or that and a new housing as well?

Hi Harv,

I reverse up onto small blocks which are admittedly only a few inches high so no not that high. Yes I did keep the expansion tank valve open for a while and during bleeding. My high speed fans are working but I have just ordered new ballasts as the low speed are not. I didn't run the A/C during bleeding to bring the fans on manually, maybe I should of but didn't think It would make that much difference.

Yes It was the complete housing/thermostat kit.

Harv said:

Did you refill the coolant with the cars arse end up in the air? This can help to get air locks out quickly.

The rear of the car should be as high as possible (I raised mine by 60cm, but don't forget just a nice slope will give you a good starting point)

Although this is more about preventing airlocks than removing them - you don't wish to drain any coolant forward of the block - ie forward pipework and rads as they are a pig to remove air locks from without recourse to the radiator tapping points - which means front PU access and/or front under cladding access - oh, and contrastingly, raising the FRONT of the car!

Also keep the burp valve open (located ontop of the expansion tank) for a while while running the car at idle upto temp.

ABSOLUTELY DO NOT IDLE UP TO TEMP - this will do nothing to remove air locks and will just risk hotspotting an airlocked engine.

Whilst the rear is still raised, lift the burp (bypass) valve and fill.

LEAVE THE RESERVOIR CAP OFF

The engine should be run and rev'd (pulsed) to 4K rpm for a good minute (idling will do nothing useful). The reservoir should be at the point of erupting frothing coolant everywhere (it will look almost white with foam).

Drop to idle and top up as necessary (doing this with someone else to keep an eye and top up whilst you control throttle and monitor heating vents and temp gauges is the best option - if the full heat ventilation/temp gauge suddenly chills and does not respond ASAP, turn off for a while and resume after 30 mins)

Repeat 4K pulses until no more can be topped up in this regime (and always brim, the MIN/MAX are irrelevant at this stage - there will be a further 2-3L you can get in once driven hard.

Once car can't be filled any more in the raised position, REPLACE RESERVOIR CAP AND LEAVE BURP VALVE UP - go out and drive it like you stole it: much braking, accelerating and aggressive steering inputs to shake the system out - monitor temps and vents continuously and stop every 5 miles to carefully remove reservoir cap and top up.

Stop immediately if temp gauge/vents goes cold and does not recover ASAP - retry in 30 mins.

Repeat until no more 5 mile stints require top ups (an exact measure of how much dropped out and how much went back will tell you how much more top ups you have.

Park for the night with burl valve up.

Drive the next day still expecting to top up.

Repeat until there are no more mornings without top up

Now drop burp valve and drive normally.

Have you noticed the rad fans kicking on and off at all since the swap?

Not relevant to removing air locks (nor is AC) - in fact it is the full HEATING circuit that needs to be on to monitor coolant circuit health, although it make no difference to the the actual circuit as they are isolated via a heat exchnager

Did you fit just a thermostat or that and a new housing as well?

Not relevant in this instance as the thermostat is working, it is the system that is airlocked

Click to expand...

I would also review again the steps outlined in the 996 and 997 FAQs LTT threads

We are trying to help honest

Stands to reason if you have heater circuit to fully on and engine is "coming" upto temp with no heat from vents then certainly there's space in those lines.

Personally I'd raise the rear a lot higher and see how you get on with additional filling, pump will need to be turning significantly to get fluid round the system. Agree you don't want it boiling over but needs to be at 80deg or so with stat open.

When I drained and filled mine I kept careful note of how much went back in via 5 litre bottles and found this was a good trick to knowing how much of the 20-22 or so litres the system holds was back in there. Mine you that included fully draining it and fitting new front rads.

Just don't do the idle thing!!!

Thanks for the replies and Advice Harv/GT4, much appreciated.

Going to jack the rear up much higher, drain the system down at the engine drain plug and repeat the bleeding process again with your advice and tips.
Hopefully this will clear it.

I've got a track day booked at Silverstone on Monday so just want to make sure its 100% for then. Will give it a go tomorrow.