Chrysler Repair: Coolant Fans not running on high when needed, coolant temp, coolant temperature

Question
Thanks Doug,

it is a 99 Pontiac Montana.  There are two fans. We recently flushed the radiator, mechanic said it had sludge in it, and a week later the temperature in the area went over 100.  That's when the AC started blowing hot air, and the coolant would boil when we stopped the car.  We then noticed the coolant temperature fluxuating while driving, and getting up to 210 until we would turn on the heater.  On the freeway or at night with cooler air the coolant temp stays low, but in stop and go in 90+ heat it begins to climb.  Turning on the heater usually works.  I stopped when temp was getting higher to check if fans were running, they weren't.  When I turn on the AC the fans are running in low speed, and the temp stays down, except when it is really hot outside.  I was charging the AC(so I had the AC running on high), and the coolant fans were running on low until the temp got over 210 when they kicked into high.  I think the thermastat is working.  The temperature will go up and level off, and I can feel the hose get hot.  We have owned the van for 3 years and we have not had to do any repairs.  The mechanic said the flush did not get all of the sludge out of the radiator, we have since had it flushed again.  I am starting to think the heat transfer in the radiator is not what it used to be, but also that the fans are not being kicked into high when they should be, or even started when they should be.  When I park the car with high temp, the fans do not run after turning the car off.  Seems like another indication they aren't being turned on when they should be.  But they can run,  because with the AC they run on low, and I have heard them kick into high.  Sorry about the novel, I hope that helps,

Thanks,

Matt
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Followup To

Question -
Hello Doug,

my vans temperature is fluxuating, especially in the 90+ heat.  At night it remains cool, and fairly constant temp, and AC is cooler, though not as cool as should be.  Then coolant fans don't seem to be kicking into high when they should be.  If the temp gets over 210 then they kick into high, but seems a little too late.  Is there an easy way to check if the computer is working correctly here.  I am guessing that because I have seen the fans on high that there isn't a problem with the relays or fans.  I originally thought it was the Engine Coolant Temperature Sensor and changed it, but it seems to be working fine.  If I unplug the ECT the fans kick on high but I can no longer see what the coolant temperature is, and would rather fix the problem.

Thanks for your help,

Matt

Answer -
I need a bit more information about the vehicle.  Year, Make, Model and engine size would be the bare minimum.  Also, I need to know if you have one or two cooling fans.  What's the history of the water pump, thermostat, radiator cap, and coolant?  
Thanks,
Doug

Answer
I would question whether the air was properly purged out of the system or not.  If there was no over-heating prior to this service then chances are the mechanic may not have purged out the air after service was done.  

Your engine has several high points of where coolant flows including the throttle body.  If there is any air in these passages then you may have a lot of fluctuation in the temp gauges.

Once your engine has cooled down you will need to look for bleeder valves anywhere the coolant runs.  This is typically near the throttle body, the intake manifold, upper radiator hose where it meets the engine or other various places.  I tried looking on one of my tech sites but they didn't mention where to find the bleeders.  Bottom line is that if you had no coolant problems before and you have them now then you more than likely have a simple air pocket in there.

Keep an eye on your coolant resevior.  With it overheating like it is make sure there is plenty of coolant in it (keep it to the full hot mark) so that in the event the engine wants to pull in more it can.  Otherwise it will just pull in more air.

If you have to take it to another mechanic then find one that has a vacuum operated coolant adding device.  This tool is made by snap-on as well as other major tool providers and is really good at adding coolant without air.

After draining out the antifreeze like anyone would, you install this tool and hook shop air to it.  The contraption actually applies a vacuum the coolant system and you can watch the hoses suck inward thus pulling out as much air as possible.  You flip a couple of knobs and put a hose in to the fresh coolant bottle and the coolant flows back in to the car without any air.  So long as the fresh coolant bottle has fluid in it and the hose is fully submerged in it then you cannot put air back in.

This is by far the best way I've seen a coolant system serviced by far and haven't seen a come-back from it yet.

Also, I would go ahead and replace your radiator cap.  This should be done anytime you replace the coolant and can also be another reason why you may get air in the system or overheat.
Doug