Been meaning to post this for a while...I flushed my cooling system this summer with one of those Peak (i think) brand kits that attaches to the valve near the firewall that feeds the heater core. I drive my truck maybe once a week or 2, I've noticed now that it's cold, i rarely have any heat when the engine is at normal temperature. Occasionally, i'll get "warm" air but it's hit or miss and never full temperature. Usually the engine temp is normal regardless of the outside temperature. Today i noticed the temperature gauge running hot, i'd turn the "heat" full blast and it would drop...again, no hot air; not even close... i mean, cold. I'm thinking i screwed something up when i flushed it. I was describing this to my Dad, he thinks i may have the heater core air locked. With my limited mechanic knowledge, i would argue that with the engine "usually" operating at normal temp, this would rule out the thermostat but with today's behavior, i don't know...Any ideas?

 

Simple test see if the coolant in the overflow bottle rises as the engine warms up & drops as it cools down. If that is'nt happening theres a leak in the cooling system

If you're not afraid to do a couple of very simple tests go to Autozone & borrow a cooling system pressure gauge.

First thing to test with the gauge is the radiator cap. Atatch the cap to the tester & pump it up to 15 PSI, if it does'nt hold the pressure theres your problem. Just replace the cap.

If the cap tests good connect the gauge to the radiator, pump it up to 15 PSI and see if the pressure holds. The gauge will have instructions with it. Only remove the radiator cap when the engine is cold

 

I'll give that a shot, thanks. I haven't noticed any leaks on the ground...

 

The pressure cap is used to prevent the coolant from boiling should it reach temperatures above the boiling point of the water:antifreeze mix. It should not affect coolant flow through the heater core.
If you check the temperature of both heater hoses with the engine at temperature, you should not be able to keep your hands on either without getting cooked fingers. If one is hot, the other warm, you have limited coolant flow through the heater core.
You could also have an air bubble in the cooling system. You can park on a slope, radiator on the 'high' end, and some bubbles will migrate to the radiator tank or degas bottle. You may have bubbles in the heater core lines. You can loosen one or both clamps, and manipulate the hoses to allow the air to get to the 'high spot', and then release it by removing the hose from the tube for a second.
tom

 

if it's leakiing internaly you would'nt know it.
Just curious, how many miles have you driven since the flush? 500 miles "should " be enough to purge any air out of it.
Bottom line with the engine overheating the sooner you find the problem the better.

 

A better way to put it is a dozen warm up/cool down cycles "should" have purged the air out of it.

 

I'd be getting close to that amount, like i said, i don't drive it very often

 

My '96 with a 4.0 has a vacuum operated valve that controls coolant flow to the heater. Does yours have it and could the vacuum line be pinched or off?

 

Can't say i rightly know...

 

Have you checked the coolant level in the radiator? With my 4.0L, I have to top off the system for several days after I do any repair that requires draining some coolant. That means waiting for the engine to cool down completely and then adding coolant until the radiator is full. Repeat as often as necessary. This is what 87 XLT is referring to when he talks about air being purged out of the system after many warming/cooling cycles. The air being expelled needs to be replaced by coolant until the system is full.

Thermostats can -and do- get stuck closed, including partially closed (I have personal experience). So I wouldn't automatically rule it out as a possible culprit.

The vacuum-operated "heater control valve" (a/k/a hot water valve) on my '98 is located under the hood, near where the heater hoses go through the fire wall. It looks like this

 

All very good information, thank you. Today is finally going to be warm enough to take a look at things... no garage . I think i'll start with getting the front end in the air and trying to work out any air and checking coolant levels. Then i'll get it up to temp and see what happens.

 

Ok, i just checked things out. Before starting the engine, i checked the radiator cap and reservoir... i gots problems at the top of the radiator, i couldn't see bright green coolant, i saw thick brown sludge. The fluid in the reservoir was the same color brown. I bet i can't have 500 miles on the thing since i flushed it . i started it up and let it get to normal temperature, before it did, i opened the cap and had good fluid flow. Once at normal temp, i started feeling the hoses. The top radiator hose was hot but neither of the heater core hoses were anywhere near warm. The one with the vacuum valve, which i would assume to be the feed wasn't warm on either side of the valve. Speaking of which... stupid question here, that valve has an arrow on the side of it, should it be pointing to or away from the firewall? Another question, would the 3-4 times the engine got hot be enough to "cook" the new coolant, turning it brown? I've got the truck parked on an incline, when it cools down, i'll crack open the cap again and look for bubbles.

 

The coolant likely turned brown because it managed to loosen more sludge that was stuck in the water jacket or bottom of the radiator, or the heater core. When you flushed, did you connect the Tee to the correct heater hose? I do not remember, but the idea was for the pressure from the garden hose water to overcome the normal output of the water pump and flow the water backwards through the system, and have the 'stuff' coming out where the radiator cap snaps on.
The vacuum valve on the heater core hose needed to be open for the flow to occur. I think that means with the engine idling, the heater should have been set to the HOT position, HEAT mode(not A/C or vent only).
The heater hose that connects near the water pump should be the hose that flows into the heater core, the other being the outlet hose, dumping its flow into the system where it can flow back to the radiator with the rest of the coolant. When the thermostat is closed, coolant pushed by the pump should be able to flow to the heater core, so it is the pump-attached(or near) that is under pump pressure, and will flow through the heater core, if the valve is open.
Your valve may be stuck. You can check by applying vacuum or manually moving the lever to see if it affects flow(temp) through the heater hoses.
tom

 

Pretty sure i did, i had to take the valve out and attach a hose fitting, i could only do this to one heater core hose. I'm doing a little bit of research online, the hose with the valve has to be the heater core feed, it's also feeding directly from the water pump so that makes the most sense. It would appear that i may have installed it backwards as the arrow on the side was pointing away from the firewall or what i would consider, against the direction of water flow? I also noticed, when i tested it in the sink, it appears the valve is normally open, moving the arm on the side actually closes the flow. Does that sound right?

 

I actually think i'm going to contradict my previous reply. I did some more testing on this, i removed the valve to make sure i had good fluid flow... i did not. Then i noticed that what i would take to be the "return" line was the hotter of the 2. In my case, the hose that has the valve in it (feed) attached to the engine below the thermostat, at the water pump. The hose that i would deem the "return" line attaches right at the thermostat. I had the truck at the mechanic earlier in the year to fix the 2 hoses that feed hot water to the intake manifold, i'm not wondering if they got my heater hoses reversed.