Heater goes cold on idle. Hot when revs rise. Water pump on the way out?

[Prawn]

Guys, really strange issue I noticed for the first time tonight on my AUM 20vt

Parked up in a carpark, waiting for my GF to pick her up from town, engine on to get the heater going, and the heater wouldn't blow hot.

Checked coolant temp on climate 49, and it was up to 107 degrees, despite 0 degrees outside.

Held the revs up to 2k for a few minutes whilst static, and the heater came through hot, and temp dropped back to 94 (remained at 90 on the dash the whole time)

Let it drop back down to idle again, and within 30 seconds the heater cooled all the hot water in the matrix, the air went cooler again, and the temp on climate 49 rose back to over 100.


Anyone else had this happen before? Seems to be like perhaps the waterpump has issues and is no longer circulating enough water on idle. Once moving, the car seems fine.

Car has done 111k currently, and last had a cambelt done at 55k. Unsure if teh water pump was done at the same time. So it's due a cambelt and waterpump in 4k anyway.

Your thoughts?

 

[hartey]

Hi my 1.8t engine does the same, if I stop at traffic lights or wait for someone the heater blows out cold!
Hope someone has an answer!

My cambelt and waterpump were done not 2000 miles ago!

 

[Emmo]

I've had the same issue on my B6 A4 1.8T earlier on in the year, but its now fixed.
When it first came about I thought it might be the waterpump, the car had done over 150k anyway so I had the cambelt done and waterpump replaced.
After bleeding the system the problem didnt go away.

After doing a bit of research and considering the mileage on the car, I found out that the heater matrix could be blocked up with gunk.

I undid the rubber pipes going to the heater matrix in the engine bay and attached two 1.5 metre lengths of rubber hose I got from B&Q and put one end in a bucket and attached the other end to a hosepipe. I turned the hosepipe on and let the water run through into the bucket it was filthy, bits of crap and dirt came out, I let it run for a while until the water ran clear.
Then I switched the hose pipe around and flushed the water into the bucket going through the opposite way, again lots of dirty water and crap came out.

After the water was running clear again I reconnected the cars rubber hoses. I drained the coolant out and refilled with fresh coolant. I left the cap off the reservoir and let the engine run to let any air out of the system. Put the cap back on and went for a nice long drive and opened it up a bit, I turned the heater on while I was driving around and the nice tropical heat had returned. Even sitting on idle it was giving out nice hot air!

Doing this saved me £500 for a new heater matrix.
Try this first it only cost me about £15 for coolant, rubber hoses and a couple of jubilee clips.

There was a guide floating around somewhere but I couldnt find it.

If anyone has a picture of the engine bay I can even point out which pipes to take off.

 

[bengal]

I've had the same issue on my B6 A4 1.8T earlier on in the year, but its now fixed.
When it first came about I thought it might be the waterpump, the car had done over 150k anyway so I had the cambelt done and waterpump replaced.
After bleeding the system the problem didnt go away.

After doing a bit of research and considering the mileage on the car, I found out that the heater matrix could be blocked up with gunk.

I undid the rubber pipes going to the heater matrix in the engine bay and attached two 1.5 metre lengths of rubber hose I got from B&Q and put one end in a bucket and attached the other end to a hosepipe. I turned the hosepipe on and let the water run through into the bucket it was filthy, bits of crap and dirt came out, I let it run for a while until the water ran clear.
Then I switched the hose pipe around and flushed the water into the bucket going through the opposite way, again lots of dirty water and crap came out.

After the water was running clear again I reconnected the cars rubber hoses. I drained the coolant out and refilled with fresh coolant. I left the cap off the reservoir and let the engine run to let any air out of the system. Put the cap back on and went for a nice long drive and opened it up a bit, I turned the heater on while I was driving around and the nice tropical heat had returned. Even sitting on idle it was giving out nice hot air!

Doing this saved me £500 for a new heater matrix.
Try this first it only cost me about £15 for coolant, rubber hoses and a couple of jubilee clips.

There was a guide floating around somewhere but I couldnt find it.

If anyone has a picture of the engine bay I can even point out which pipes to take off.

My heaters dont seem to be blowing that hot any more i will give this a try. Great info :icon_thumright:

 

[Welly]

A bit of coolant flush would be worth using as well to aide in getting rid of the crap...

The above would make good sense - the water only gets into the matrix properly when there is more pressure from the higher revving engine.