i thought i fixed the problem but i was wrong... problem i am having is my heat seems to fluctuate from luke warm to hot when in the same position and i dont change anything when its on vent....when i turn it on defrost it seems to get warmer and stay there...when i change it back to vent from defrost it seems to stay pretty warm for awhile then decreases back to luke warm...any suggestions i changed the heater control value seemed to not do anything

 

It could be a thermostat, but not likely.

The most likely cause is a clogged or blocked heater core.

Take off the heater hoses, aim the upper into a bucket. Blow compressed air or water from hose into the lower hose, which is the return.

Basically backflush the core.

Some suggest using CLR to break up the clog(s), but I've been told that doing so can eat through the heater core.

I had very good success with the backflush without CLR.

It was a night/day difference.

Try grabbing the upper hose when the engine is warm. Then grab the lower. If you feel any real difference, the core is clogged.

 

Yes the Heater Core is the first thing I thought about also, But quickly a second thing came to mind was a split water pump Impeller, because of the intermittant heat yuo described. If the Heater Core is plugged, then Heat wont come hardly at all. It will be steady COOL, instead intermittant heat.

 

I would suggest it could be inconsistent vacuum to the heater control valve.
(seems like I remember seeing on my 87)
So its pulling wide open sometimes and other times its flowing just a trickle.
When its blowing luke wash feel the heater hoses to see if the hose is the same temp from the pump to the core and past the valve.

 

I agree that a partially clogged heater core is a good possibility.

Worth trying to backflush it. Use garden hose pressure ONLY as high pressure will toast the innards of the core.

 

If that were the case other issues would be popping up, such as high running temp. and several other things.

It could be as simple as an air bubble. Have you opened the cooling system recently, or checked to see if everything (radiator, overflow bottle, etc) is completely full?

Could be a really small hole in a coolant hose letting really small amounts of air in. Over time it could create a large enough bubble to cause heat issues.

 

well i replaced both lines going to and from the heater core... i back flushed the heater core at first it was coming out really slow then got better i put it all back together took it for a drive...maybe it was just me but it seemed to be hotter when i was driving then siting at a light it seemed to cool back down again...but it for sure is better maybe i will have to try to flush it one more or a couple more times


and it is not showing any signs of over heating at all

 

You could potentially have an air bubble now, since you opened the system. Not super likely, but possible.

I used an air gun at first, and a HUGE chunk of crap came flying out. After that it was fairly clean.

Its still not as hot as I'd like mine to be, but its easily 20* hotter than it was.

You may have to try a little CLR to break it up.

I imagine its like a clogged drain. You may be able to push some out, but there may be crap still clinging to the insides of your core.

Just don't go too crazy if you do use CLR. I'd hate to hear your heater core started leaking. Then you'd have nothing.

 

Don't think I have an air bubble...someone had a flush kit in it so I cracked the top to that and ran it for awhile