Hi,
Odd one this.
A couple of days ago my heater suddenly started blowing cold air. 1st thing I thought of was blend door actuator (ACC) but checked that out & all was OK. Even tried removing RDO/Bat fuse to reset things but no difference. Can hear door moving from hot to cold etc.
Checked underhood today & was suprised to find one of the heater hoses hot & the other cold. Hot one was the hose connected to the water pump - cold one connected to the inltake manfiold. 6 months ago I drained, thoroughly flushed & refilled cooling system (and matrix) with 50% antifreeze (not Dex-Cool) but a good alternative. Old coolant was literally like mud. Heater performance improved instantly. However, I occasioanally have had a slight noise from behind the dashboard that sounded like a bit of air in the heater matrix - usually on kickdown or when the engine is at high revs. Have always lived with it as no other problems have been encountered. Now the noise has disappeared & coincidentally the heat has gone too.
I cannot believe that the matrix could have become blocked overnight? Is it possible that the air has moved to another part of the system & is preventing the coolant from circulating through the heater? Is there a special technique for bleeding the cooling system? Factory manual says to fill system to 1/2" below radiator filler & run engine until thermostat opens then top system up. This is what I did 6 months ago & all has been Ok until now.
Incidentally temperature gauge has always read normal & coolant has never dropped in expansion tank or radiator.
Any advice would be appreciated.
Thanks in advance.

 

Sounds like you have air in the system. It can collect in the heater core and cause an 'air lock' of the core. I would try to bleed the system again by getting the front of the vehicle as high in the air as you can (large hill anywhere close?) with the passenger side slightly higher than the driver. Then run the engine with the radiator cap off until it is up to full operating temperature. Shut it off and allow it to cool down. Fill the radiator and repeat at least two more times.

 

Thanks a lot for the quick reply. I will try the method you suggest tomorrow & let you know how I get on.
I have a right hand drive model but, as far as I can establish, the heater etc is a mirror image of the left hand drive models - even the radiator cap is on the opposite side so getting the passenger side up high sounds logical. My driveway is on a steep slope so it won't be a problem. I will jack up the passenger (right) side too to make sure.
I still can't help thinking it's a bit weird that the problem has occured so sudddenly - can air really stop the coolant from circulating to the extent that the heater goes totally cold?
I really must say this is a most excellent forum - it has gotten me out of trouble on more than one occasion. Yourself & the other administrators do a first class job here - well done! The information on here is invaluable - especially seeing the Blazer is such a rare vehicle in the UK & consequently technical information is rare too!

 

I jacked up the front today & got it as high as the jack would allow. Vehicle was already facing uphill as I have a steep driveway. I ran engine until the thermostat opened with the cap off keeping it at high revs. Whenever I let the revs drop the radiator overflowed for a few seconds, level dropped while it was revving. I got a friend to rev the engine while I topped up the coolant & replaced the cap.
Went for a 12 mile drive & heater is slightly better, although nothing like it used to be. Funny thing is, the heat gets a lot better if i'm at higher revs (such as when I kickdown or hold in a lower gear) which seems strange.
I will try to bleed again tomorrow & maybe again the day after to see if things improve. seems weird this problem started so suddenly - can't imagine the heater matrix could become blocked so quickly - especially seeing I drained, flushed & refilled system only a few months ago?

 

Well this heater problem is really bugging me now.

I have tried to bleed the system 4 times now. I have parked on my sloping driveway (facing uphill) & jacked up the passenger side as high as I can get it, ran the engine with the radiator cap off, revved it, topped it up & allowed it to cool and still the heater is virtually cold. The pipe that goes from the manifold to the matrix is hot, the other that goes to the water pump is cold.
This afternoon (in desperation) I disconnected both hoses at the pump & manifold & ran my garden hose through the matrix via the hoses. Had some brown rusty bits for a few seconds to begin with but water quickly ran clear & there didn't appear to be any restriction at all. I let the hose run for about 5 minutes at a resonable pressure. Reconnected everything & bled through like i'd been doing & still hardly any heat. I went out for a drive & found that if I put the shifter in 1st or 2nd & allowed the engine to almost redline it gets a lot warmer - although nowhere near as warm as it used to be. As soon as revs drop it goes almost cold again. Have double checked blend door & all is OK there.

To me it is acting as if it is partially blocked but I can't believe it could happen so suddenly? A week ago I had plenty of heat. Surely a blockage would occur over a long period of time accompanied by a gradual deterioration of heat? I always used to get a slight gurgling behind the dashboard on high revs before this problem started but used to live with it as the heat was always good so figured things were best left alone. I no longer get the gurgling yet the heat has gone. I can only believe the air has gone somewhere else - but where?

Any help at all would be appreciated. If it wasn't such a bitch of a job i'd change the matrix for the sheer hell of it but I find it so hard to believe that the matrix is at fault? That plus the fact that the matrix for the right hand drive models are different & mega expensive here.

 

I would go ahead and take the 2 heater core hoses off and take a water hose and flush out the heater core, do it from both ways and flush it until the water comes out clear, my guess is that some of the old crap from before you changed the coolant is blocking the heater core, Hope this helps and good luck.

 

I don't know if you read my "Failure to Burp" thread but did you try connecting a garden hose to the outlet of your heater core (the smaller hose) and let it run back into your radiator? You can leave the return line sticking up in the air. I got super heat even at idle that way but when I connect the line back up I get almost nothing. Something inside the water pump is preventing the return flow...but only when the engine is running.

 

Many thanks for the advice guys.
Dobyken - that's an interesting idea about connecting the garden hose up & letting it run into my radiator, I will give that a go in the morning when I finish work (working nightshift).
Nvidia78 - I flushed the core in both directions & it has no restrictions at all.
One thing that I was thinking of trying was to disconnect the heater hose from the water pump, plug the water pump inlet & fill the heater hose while holding it high in the air. I was then planning to quickly reconnect it to the pump so that i don't let much air back in. Getting desperate now so I won't ignore any suggestions anyone has to get this damn thing sorted - however radical they might sound!
Will let you know if I have any luck.

 

might wanna try one of those system cleaners. you drain and mix with water. i know prestone sells one. my heat sucked in my truck and i flushed it both ways from the heater core.

 

Well I spent another couple of hours today trying things to improve my heater - am just about at my wits end now.

I have concluded that the problem could be one of 2 things.
Either I have a partially blocked heater core, although it seems to have happened very quickly, 12 days ago things were fine - or my water pump has maybe got corroded vanes that don't circulate the water as powerfully as it should? However these 2 theories do slightly contradict each other as i'll explain below.

Although water seems to flow freely through the core when using a hosepipe, is it possible that the water is just taking a "shortcut" through the core header tank without actually flowing through the thinner tubes? Have never actually seen a Blazer core but I googled some pictures of it & it looks as though this may be possible? Or, has the header tank got some kind of partition in it to ensure that coolant has to go through the core tubes before it returns to the water pump?
I am now 100% positive that there is no air in the system - I actually disconnected the heater return hose, held it up in the air & filled it with coolant, I then held a finger over the end & quickly reconnected it to the pump connection, which was overflowing water. If there's any air in there at all it is minimal.

The reason I think the waterpump could possibly be at fault is the fact that when I rev the engine the heat gets hotter the faster the engine goes. When I went for a drive earlier I kicked down on a couple quiet roads & almost went up to the redline before the transmission shifted up - heat was much better but dropped back within a few seconds of the revs dropping again. However, just to contradict the water pump theory - the engine never overheats, gauge reaches halfway withing 2-3 miles & stays there so the pump has gotta be doing something?

In desperation I am considering trying a flushing solution as suggested by blueblazer1982. maybe something stubborn has been sticking in the system somewhere & suddenly dislodged & blocked the core? We don't have Prestone products here in the UK but I am sure my local Auto parts store will have something equivilant.

Incidentally, out of curiosity I phoned & enquired about the price of a new core today. Nearly had a heart attack - £277 including tax, that's about $387 at todays rate! A core for the left hand drive model is $79 at Autozone. When I asked why the right hand drive ones are so expensive I was told that it was due to the relatively low production volumes. I though that was bull****, it is only a mirror image of the LHD one, can't see how it is more expensive to produce?

So - i've still got little or no heat. Any more suggestions are welcome. Will let you know if I have any more luck.