Coolant boiling in expansion tank

Hi, I have a 4.6 HSE which has started to overheat. The coolant seems to boil in the expansion tank even before the thermostat opens. The tophose is not pressurised and there are no blockages although once the thermo opens I only get a few bubbles out of the radiator return pipe, no flow as expected. Air lock or weak pump? Thx

Are you sure it is boiling water and not airbubbles from an airlock or Head Gasket Failure??

Get a sniff test done...and get a thermometer (not a medical one) and measure the coolant temp in the expansion tank....the system is designed to run 85-95degC.

If it is starting to boil, also check the coolant/antifreeze concentration...the correct mix will increase the boiling point of the coolant to beyond 100degC.

My money is on an airlock or HGF.

HG done 2000 miles ago! But will et sniff test done just in case thanks.

Low flow rate could be done to knackered pump...

It has been known and well reported on here that the impellor comes loose on the shaft reducing flow drastically.

The venerable Rover V8's were on the limit cooling wise...a small change in flow rate will make big issues!

Its done 97000 so probably due major cooling system surgery, I think its time to change the whole shebang together. Thanks again.

RonHarvey01 said:

Its done 97000 so probably due major cooling system surgery, I think its time to change the whole shebang together. Thanks again.

Click to expand...

Get, at least, a sniff test done before you start spending money just to eliminate the HG. You're going to feel a prat with a new cooling system if it's still bubbling.

My 50p is on a blocked radiator.

If its boiling but not presurising i would guess the rad caps not holding the presseure as water under presure dosent boil

does anyone know how much it is to buy a sniff tester myself. been looking online, but cant seem to find any. very odd?
anyone recommend a place that sells them?
would be a useful bit of kit i think

oops, just found them. been searching under "sniff tester". silly me.
head gasket tester of course

Most garages will have them and would imagne they wouldent charge much

As the others say; get sniff test done first (local garage) to eliminate hg failure. After I did my heads I was still getting hot pressurised water pi**ing out of the expansion tank. Turned out to be a worn out viscous fan coupling. Changed that (& water pump while I was there) and all sorted. Good luck.

over heat /boiling before thermostat opens /sticking thermostat me thinks.

Thx for advice chaps, massive airlock in rad, running great again.

did you just squeeze the hose to push air through, or drain and fill again?

Drained and flushed the whole system through then refilled rad from tophose and backfilled through tophose to expansion tank. Left nowhere for air to hide! Worked a treat first time. Drove it a good distance and temp never left midway whereas before, I couldnt get out of the drive.

was just wondering as i have never had an air lock before, so wondering how it happens to begin with

get sniff test done with exhaust gas analyser- put it in engine bay engine running- if hc jump over open header tank- headgasket or liners.

liquid sniff tests are ****e

New expansion bottle cap cured this one for me, not on the RR though, but it was a Rover.