Scirocco Register

As title suggests having trouble with engine temperature,,,, guage all the way to top and flashing,, water seems to be building alot of pressure, had the head pressure checked and all good,, no contamination in the water,,, is it possible it's the instrument cluster or the water temperature sendor, ive had the thermostat out and is still out, thought that was issue but no change,, if I leave cap off and run engine all water boils over, its a mk2 rocco 1.8 running webber carburettor, changed the temperature sendor today as old one wasn't working and this is my findings

What colour is the cap, blue or black? The black ones are known to fail!

Is the new temperature sender you fitted a genuine VW one and is the wiring in good order as it could be shorting and making the gauge read to the top?

Also, what coolant are you using G11 (recommended!) or G12/G12+?

Is the water pump operating as intended and has the system been bled?

Its black cap, its a brand new after market sensor not genuine,, and.just water atm. As it keeps dropping fluid

How about the other questions? I suspect the sensor or wiring!

No the wiring is all good all tested with a.continuity tester,, and its a aftermarket sensor

To test the temperature gauge you can get the car warm so the gauge reads something, stop the engine and leave the ignition on so the lights on the dash are on then pull the wire/spade connector off the sensor and the gauge should drop to cold then put the wire/spade connector onto the engine block and the gauge should go to max, then put the wire back onto the sensor and it should read where it was before you stopped the car (my be a little cooler.)

Do all the water hoses in the engine bay get hot?
How is the engine timing as if that's out it will make the car run hot.

Running the car with the cap off will make it boil over as the water system need to be pressurized to work at it's best as pressurized water has a higher boil point.

I know it is obvious but does the fan work? How long does it take to get the needle to the top and the light flashing?

I had a similar problem a couple of years ago, turned out to be the water pump, replaced it and everything was back to normal.

Got a German after-market pump from VW Heritage for about £30, was satisfied with the quality.

Don't forget the thermostat too. If you are going to change the water pump, it makes sense to change the thermostat too.

The thermostat has been removed, the temp goes from flat to top in about a minute maximum all the water pipes get hot everything, I was thinking water pump but dont know.how id test

You can see if the water is flowing by looking in the header tank and seeing if coolant is being pumped from the small pipe from the radiator to the header tank, you obviously need to have the cap removed to do so.

Have you changed the black cap for a blue one?

as Ian says they are often the culprit.

Does all of the rad get hot?

Have you had it sniff tested to confirm hg ok?