I have a 1992 full size blazer and the heater is no longer working. I've read the forums on this site regarding heater problems.I had a hot heater inlet hose and a cold outlet hose, which sounded like a blocked heater core. I triedflushing the heater core and radiator and I alsoreplaced the thermostat and radiator cap while I was at it. The heater core did seem to flow a bit slowly at first, but seemed to flow just fine after a few reverse/forward flushes.

However, now that everything's back together again, I haven't gotten any improvement in the heat situation and still have a warm inlet hose and cool outlet hose.The thermostat now also seems to be cycling open/closed since the temp rises to about 200-210 and then drops down to 160, repeating this cycle over and over -which didn't happen before.I also don't see any increase in the coolant level in the reservoir tank when the engine heats up. I know that this sounds like there's probably still air in the system, but I've topped up the radiator many times and it now appears to be full.

I was wondering if I might have a problem with the water pump. I bought the Prestone flush kit, which I used to flush the engine/radiator. (It comes with a tee that goes on the inlet side of the heater and attaches to the garden hose during flushing. You hook the garden hose to this tee, open the radiator drain, remove the radiator cap and turn on the garden hose, and run the engine until the water coming out of the radiator outlets runs clear.) Anyway, while doing this, the flow out ofthe top of the radiator was pretty weak- which makes me suspect that the water pump may be bad. I also noticed that the radiator cap doesn't get hot after driving the car and letting it warm up.

My thinking is that if the water pump is not working,it might explain the temperature cycling since no coolant it being circulated and the only flow is due to the temperature difference between the engine and radiator which causes the thermostat to open when the heat builds up high enough. Also, although I can't say for sure that the heater core was completely clear, I did see significant flow through it during flushing and would expect to see some waterflowing through it if the water pump were working properly. There are no leaks anywhere and no noise, which I thought was the usual sign of a bad water pump. Can thewater pumpactually just stop working with no other signs of problems?

Anyway, I'm certainly no expert and would appreciate any advice that you guys could give me regarding the next steps that I should take. Thanks.

 

It is possible for a water pump to quit working if the impeller shaft breaks. I've only seen this a couple of times in my life, so it's rare.
If you have the rad cap off, the engine is at operating temp with the engine running you should be able to see coolant flowing through the radiator. If you're not seeing the flow and you have already flushed the system, then I suggest changing the water pump.
You say your rad cap doesn't get hot, but does the lower rad hose?
If it was me, I'd replace the termostat again or remove the thermostat (only to test) and re-check for coolant flow in the rad and even temperatures in the heater and rad hoses.

 

Thanks for your help. The lower rediator hose does get warm, but not what I'd call hot, although the upper hose is definitely hot. I also noticed that the radiator cap was a bit warm after a short drive this morning.The temperature cycling does sound like a bad thermostat, but since it was new... Anyway, I'll try removing it to see if things flow better. I may also try putting the old thermostat back in to see it thngs return to the way they were before I started.

I suspect that I have a bad (new) thermostat and a (still) plugged up heater core. I'll also try some CLR or similar gunk dissolver in the heater core to make sure it's flushed out as good as possible this time.

Thanks again.

 

I think its your water pump, ive blown a few up on modified engines and your story sounds very similar to what Ive had. If you want to check your thermostat, put it into a pot/pan of hot water and use a thermometer from the kitchen drawer to see what temp it opens at. I do not think its your thermo or heater core but more towards the pump. Have you flushed your entire cooling system yet?

Test the thermo, and re-backflush the heater core and replace the pump.

Does your pump make any noise now? no grindings or anything?

 

Thanks DM. No noise or leaks from the water pump that I can see. The truck has close to 160k miles on it at this point, so it's probably time for a new water pump anyway. The thermostat definitely opens up, but then closes again and continues to cycle-open-closed-open...

I did flush the entire system but as I said in my previous posting, I'm going to have to get medieval ontheheater core's@$$ !

Thanks again.

 

Today I finally got to the root cause of my problem.

But prior to that, I aggressively re-flushed the heater core and filled it with a 2:1 mixture of hot CLR and let it sit while I replaced the water pump. The water pump replacement went very smoothly, although there was no sign of a problem with the pump.The pump bearing was a bit loose compared to the new one, so it probably would need to be replaced soon at any rate. After the water pump was finished, I drained and re-flushed the heater core and put everything back together. Drove it for quite a while - but still no heat.

At that point, by process of elimination, there was only one possibility left - the inlet line from the manifold to the heater core. The inlet line is a one inch ID hose that transistions to a 1/2 inch ID metal line, which then goes into the manifold throught a snap-in fitting attached to a 1/4" ID threaded inset in the manifold. As soon as I saw the new parts with the 1" - 1/2" - 1/4" transitions, I knew that this was going to be the source of the problem - not a very good design IMHO. When I pulled out the old line and the treaded insert, sure enough, I found that the 1/4" ID insert was fully clogged. Once the new parts were in, a test drive revealed that things are now HOT HOT HOT baby!

My advice to anyone who has a heat problem and this kind of heater inlet hose is to flush the heater core first, and then check the inlet hose if the flush doesn't work. I also recommend replacing the hose AND the threaded insert since the snap-in connection on the old line will be pretty much shot.

Thanks again to everyone for their help and advice. I hope this documentation of my experience will help others down the road.

 

We appreciate the follow-up, members like you help make this forum what it is. Thanks!

 

Excellent! I never even thought about those silly manifold "quick" connectors.