Sierra heater working on and off
 

I have a Sierra 2.0 twin cam and when the car is idling the heater starts blowing out cold air and the temperature gauge starts to go into the red, but when I start to rev the engine the heat starts coming out and the temperature starts to come back down.

Friend said that it could be the thermostat sticking....just need some advice or if anyone else has experienced this problem.

Your comments would be greatly appreciated.

Sounds much more like an air lock, try bleeding the heater/water system, make sure the de-gas pipe is nice and clear, this will be the top most small pipe returning water to the expansion tank.
tabetha

I looked to see if there is water coming from the upper most pipe to the expansion tank, and when you rev the engine the water starts flowing in, but when the car idles there is no water feeding back in....is that normal!

this happened to my brothers and it was a air lock

my mates cossie is constantly on..any ideas what it might be?

It should be feeding water all the time through the de gas pipe, even at idle, a strong flow, maybe your matrix could do with bleeding first, take off pipe(outlet), run engine till water gushes out(need helper), clamp pipe near held up end, so that air can't get in, then reconnect to where it was before you took it off.
Do this with a cold engine saves burning yourself, ideally the stat will have a 4-5mm hole in the highest point to aid ridding air locks when refilling etc, these used to be common and had a "jiggle pin" in them, but 4-5mm hole is fine.
Let the engine idle with the cap ON up to full temp, let fans come on then go off, then turn off the engine, after around 10 minutes you will hear it suck water in as the air is dispersed, due to the water super heating(gets hotter after switch off), this draws out 100% of the air if everything as it should be.
There is nothing to stop you from taking out say the temp sender till water flows nicely without air then plonk sender back in, use any bleed points you can.
tabetha

Scrap it mate lol

Your appreciation is not required ;)

Thanks for your help tabetha, I changed the thermostat and now the heater is working ok and the car is not overheating...thermostat must not have been opening fully.