2000 Jimmy SLE V6 4x4 heating problems

The vehicle has 102,000 miles on it. At times there is heat and then other times there is cool air blowing out. When there is heat, it is not as hot as it was last winter.I checked the antifreez level in the reservoir and it was way below the ADD line almost empty (engine had been off an hour). That was last week. Yesterday, same problem, so I checked the anitfreeze level again. Very low again. I have been checking the driveway under the Jimmy but there is no sign of any antifreeze leakage. I have read the other threads and they helped some what. I do have a Haynes book and have read through that. The book said to check the inlet and outlet hoses. I need help locating them. I know where the resevoir is and the radiator. One thread also said that if the resevoir has been empty there may be an airbubble. Is flushing or reverse flushing the only way to get rid of it? Last winter the heater worked fine and this past summer the A/C worked fine. Any other information would help. I am trying not to have to take it to a mechanic (no $). Thanks in advance!

The vehicle has 102,000 miles on it. At times there is heat and then other times there is cool air blowing out. When there is heat, it is not as hot as it was last winter.I checked the antifreez level in the reservoir and it was way below the ADD line almost empty (engine had been off an hour). That was last week. Yesterday, same problem, so I checked the anitfreeze level again. Very low again. I have been checking the driveway under the Jimmy but there is no sign of any antifreeze leakage. I have read the other threads and they helped some what. I do have a Haynes book and have read through that. The book said to check the inlet and outlet hoses. I need help locating them. I know where the resevoir is and the radiator. One thread also said that if the resevoir has been empty there may be an airbubble. Is flushing or reverse flushing the only way to get rid of it? Last winter the heater worked fine and this past summer the A/C worked fine. Any other information would help. I am trying not to have to take it to a mechanic (no $). Thanks in advance!

Take the rad cap off and make sure the rad is full of coolant.
If rad is full first thing first start with a new thermostat.
Post back how that does.

I went to check the radiator this morning (engine was very cold). I could not get the radiator cap off. It had gummed up anti-freeze under the lip of the cap. I didn't force the cap in either direction (I did tighten the cap before I started the engine). The resevoir still had a good amount of anti-freeze in it, not at any of the measurement lines but more than the other times I checked it. Now, what?

I went to check the radiator this morning (engine was very cold). I could not get the radiator cap off. It had gummed up anti-freeze under the lip of the cap. I didn't force the cap in either direction (I did tighten the cap before I started the engine). The resevoir still had a good amount of anti-freeze in it, not at any of the measurement lines but more than the other times I checked it. Now, what?

YOU NEED TO GET THAT CAP OFF AND SEE IF RAD HAS ANY COOLANT IN IT.
That is always a first check or one thing to check. A bad cap,will not keep the rad full even if the overflow tank has coolant in it.
It does not sound like you have looked in the rad for a long time.:nono:

I checked the radiator. It does have anti-freeze in it. It was full up to the cap. The cap was not as gummed up as I thought it was and because it was so cold, I couldn't get a good enough grip on the cap. Strange thing, if I set the dial on A/C and the temperature dial on the red section, warm air does flow from the vents. The engine never over heats. Does that mean the thermostat is stuck?

I checked the radiator. It does have anti-freeze in it. It was full up to the cap. The cap was not as gummed up as I thought it was and because it was so cold, I couldn't get a good enough grip on the cap. Strange thing, if I set the dial on A/C and the temperature dial on the red section, warm air does flow from the vents. The engine never over heats. Does that mean the thermostat is stuck?

Go for a thermostat and see what it does.

Here are some tips ... I hope they are helpfull.
1. Check the temp when the blazer is warmed up. the guage should be almost right in the centre. You mentioned it does not run hot so I'm betting maybe the opposite ... that it runs a little cool. This is a symptom of a thermostat that will not close ... replace thermostat. If it runs hot, then its likely a leak and a system which will not hold pressure.

2. Is there any sludge like material under the rad cap? If there is this may be a symptom that the car has been operated for a period of time without a completely full radiator. Even down the slightest little bit, the orange coolant will sludge up.

3. Make sure your resevoir is always at the recommended fill level. It is imparative that no air gets into the coolant system. Orange coolant will turn to sludge. If this has happened a complete flush is recommended. Please note that this may not correct the problem in the long term. It is very difficult to get all the sludge out of the system. Anything left over tends to lodge itself into the heater core ... hence no heat. This has happened to me ... I just periodically back flush just the heater core to remove debris.

4. You mention that the coolant level was low. This bothers me as 2 possible things can cause this. First if the rad cap does not seal properly no pressure will build up in the system. this will lead to a lower boiling temp of the coolant. Orange antifreeze needs to ne under pressure to raise the boiling temp and work properly. If the cap or system has a leak then the coolant will slowly boil off ... hense lower levels.
Secondly coolant can leak out if under pressure and there is a leak.

5. Sometimes you can get a really inconspicuous leak in the back of the water pump near one of the seals. this can lead to a slow drip type leak that will lose a lot of fluid over time while driving and the pressure builds up. It may not leave a big enough leak that it would pour on the driveway that you would even notice.

6. Dealership may recommend an acid flush if it comes to flushing the system. GM actually has a tech note that recommends this for the sludging problem. This is fine but the acid flush will reak havoc on all seals in the engine to some degree. nn particular the water pump seals and rear intake manifold coolant seals. these are also notorious for springing leaks after 200K miles.

7. Check the oil regularly to see if any coolant is getting inside the engine. Again intake manifold coolant seals can wear out and leak coolant internally. this may lead to a white sludge in the oil or other visible contaminants. If this happens. Do not delay this needs to be fixed right away. The result of driving in this condition will likely lead to a complete engine replacement. The engine failure happens quite quickly and dramatically as opposed to a long period of maybe I'll get this fixed soon. Main bearing failure, severe knocking and nothing that cannot be fixed without a major overhaul.

8. Another possibility is that there is a small leak in the heater core. This will not leave any puddles if it is small. Do all the heat settings work. Does the transfer from defrost to vent work well. If not (big delay) this may be a sign that the door actuators are gummed up because of coolant in the heater cavity. Do the windows mist up? Is there any odour in the car? Again symptoms.

9. A symptom of air in the coolant system and really low coolant and possibly a big leak is intermittant hot and cold air from the heat vents with the temp on full "red". Coolant boils and sporatically gets pushed to the heater in between vacant periods when there is no heat. Watch the engine temp. A symptom here is the temp going to super hot and back to just above normal temp.

Bottom Line: Fill the coolant to a level in the resevoir and mark with masking tape when cool/cold. Monitor this and determine if coolant is disappearing. If it is, it has to be going somewhere. Keep things topped up and find out where.

Oh ya one last thing ...
When the thermostat is open or closed, coolant still cycles slowly through the heater core. There is a small bypass around the thermostat. A really bad assumption is trying to diagnose a bad thermostat based upon heat coming from the car heater. You need to look at all the symptoms to pinpoint a thermostat.

The tendancy is jump to the cheapest solution for a problem. If the thermostat was locked closed, the engine would overheat with major drama. This is not happening so the alternative would be to be blown wide open. Some thermostats are fused and do this if the engine overheats. Even in this state after running the engine for a while would lead to very hot almost untouchable air from the heater.

1.) replace the rad cap.
2.) Replace the thermostat
3.) get a cooling system flush.

those are the things that need to be done and they are regular maintanence items anyways so no harm done doing it.

Can't believe I'm reading this. I have the same exact problem. The wife reminded me earlier this fall the heat wasn't working good, so I changed the thermostat and while it was out, and the orange goo was four years old, I decided to flush the system (with the thermostat out, I stuck a garden hose in the upper radiator hose which was still connected to the block and ran it through the block and had it come out the lower radiator hose which I disconnected from the radiator. It took a while but eventually all the orange goo ran clear, so I then refilled it with green Prestone. Thought that would be the end to my problem , but doing a check when everything was up to temp still had less than desirable results, especially when the fan was turned up to four - what little heat I had was warm and it went to cold. I then went under the hood again and I could hear the AC unit engage for a bit then disengage and it continued that cycle. Thinking the selector switch oin the dash was "going", I disconnected the AC unit under the hood by unplugging it, and I got heat. Come next spring, I'll just plug the AC unit in again. Becasue this isn't the vehicle I run, I never checked it again to see if it was really heating up. Now then it is December, the wife is complaining again that it is cold. Checked it again and what little heat there is doesn't take long to get real cold , real fast. I have had problems with the 4WD drive switch twice and another electrical problem that was rectified by replacing the ignition switch. Im ready to replace those switches, but I decided to check out the forum first. Glad I did, not saying that my theory may be wrong, but I guess I should try flushing the heater core. By the way my level is right to the brim, but the resevoir ended up not maintaining what I put in after the flush refill. Much trouble flushing just the heater core whcih was a thread I just saw something about. I do have a Haynes manual.

Thanks for all the information. The thermostat has been changed and there is heat and the engine temp registers at a normal number. The mechanic checked all of the hoses and checked for leaks. The hoses were fine and no sign of leaking anti-freeze.