high temps idling but cools while revving

[chomper149]

Car has had issues with high temps before, previous owner theEZV had issues. He replaced the thermo, coolant temp sensor, fan relay, and replaced the fan unit with a lower mileage.
He concluded it was a bad fan since no problems occured after doing that work.
Ive had the car for a few months now, only had one issue with high temps while first getting it, threw the heater on and the temps dropped quickly.
Recently it has been getting a lot hotter. Ive looked at the coolant level and there is no visable change at this point. havnt checked the oil yet though.
Normally i drive with the heater on/ have had this issue till recently.
I will drive around fine, once i idle for a bit the temps will rise, even with the heater on full, with hot air coming out, it still overs as high. But once i start to rev the car, the temp starts to drop. come to think of it i should have tried turning the heater off and revving to see if the temps drop. I havnt looked to see if the fans come on yet since it just started happening.
but has anyone else seen this problem?
I was thinking its a circulation problem rather than fan problem at this point, Possible bad water pump? but wouldnt that cause overheating at all conditons?
Also i noticed this earlier, i parked the car and came back to move it and the temps said high, not sure if this is a recurring theme or just a one time thing though.
experiences, thoughts? internet searching has yielded an array of possibles

 

[Spiced MSP]

Air pockets?

 

[chomper149]

could be pockets of air, but why would the symptoms show now? form what i an tell there hasnt been any loss in coolant, and EZV also had a shop(i believe it was a shop, not himself) flush all the coolat and all when he changed the thermo and everything. So im not sure why its occuring more now than before.
But if i take off the radiator cap, while cold, turn it on with the heater on high, will that properly bleed the system? or will i need to do something else on these cars. Thanks!

 

[BRIAN_MP5T]

Head Gasket

 

[chomper149]

what about the symptoms say head gasket? thought it would normally get worse if you drove with a bad gasket/ not loosing coolant either

 

[BRIAN_MP5T]

No performance issues with a bad headgasket

Several things can occur, the one that sounds like your issue is compression passes the gasket and enters the cooling system. that would displace the coolant into the bottle and eventually, you will have an air pocket then overheating.

To diagnose, run up the engine to temp with the system full and the cap off. Keep topping it up if it drops. If you notice small bubbles constantly (Steady Stream or predictable occurrence) coming out of the filler cap, see if they smell like gas.

If not, then forget all that I just said and keep looking.

 

[Installshield 2]

^^you can also take a small sample to certain auto shops that can do a quick test...which can easily tell you if there is something in the coolant that shouldn't be...but it'll take some searching, as i'm not sure of any in the philly area off the top of my head.

its tricky...the system will push some coolant into the overflow bottle nearly any time its fully warmed anyway...its how pressurized systems regulate themselves...then when its cool again, it'll vacuum more out of the bottle...and level in there will drop again...its the bubbles that you'd be looking for...

First...does your temp gauge do anything funny? Does it get to the normal 'almost half way' point under normal warm up? If you have access to a scanner of some sort, the coolant temp reading would be great...don't worry if the coolant temp is staying low, thats what i'm hoping the case is...if it won't heat up, the temp sensor isn't reading raw coolant...its reading air pockets as well, which won't heat the sensor properly...making you think the coolant is cooler than it is...

If you're unsure of the last time the coolant was changed..i would first simply replace the coolant...with the engine cool, raise the rear of the car, pull the rad cap and the screw at the bottom of the radiator...collect everything that comes out in a big bucket...this will only drain the radiator and the lines running to the engine as the thermostat will be closed...drop the rear, raise the front, and Top the radiator back off with either clean coolant (if you don't mind wasting some), or distilled water...you can use a hose, but there is some debate about using 'unpure' water...idle the engine up to operating temp, and keep the heater core on high with the fan on...you should do this without the radiator cap on, but just be ready to shut it off as soon as coolant starts to bubble out of the radiator fill neck...its much like bleeding a brake system...its ok if the cap is off, as long as the radiator has coolant in it...if the level drops out of the radiator fully, it'll pull air into the pump and cavitate...and then you've definitely made the problem worse than it was...so work slowly, and keep a close eye on the coolant level in the radiator...if you can't see coolant/water just under the neck, add more slowly until you can...

repeat that as many times as needed until your sure its clean stuff coming out...Try to only idle up the engine temp with the front raised, cap off, as that will promote air leaving the system (the fill location needs to be the highest point in the system)...Also, pay attention to what you're doing with this...its not hard, but its tedious and isn't an exact science...if you've never done this kind of engine service, find someone to help that has...i'm not kidding, you could boil your arm off if you try to pull a radiator cap off of a fully heated engine...when i say 'operating temp'...i'm saying just past where the thermostat opens and lets coolant flow through the entire system...not operating temp as in 'i just raced my car for 2 hours'...keep an eye on how much coolant has come out of the system relative to what you have added...its a 7.9 liter system iirc...so roughly 2 gallons...you need to be idling up with a good bit in there, or you're just going to make the problem worse...

if you're not getting heat properly after you're sure its been properly filled and bled...the problem isn't just air in the system...and is something else...we can address that if the time comes...but even small air pockets will interfere with the heat exchanger at the heater core...making it blow not so hot air into the cabin...this problem WILL be relative to engine rpm, as we have a crank driven water pump...the faster the coolant moves through the heater core with air in it, the less it'll heat up the air passing through...i.e. won't work nearly as well at raised rpm, but may be fine at idle...i'm fairly confident this is your only problem...

 

[chomper149]

My problem isnt with my heater core at all, at least i dont think so, im still fairly new to this car. But it always blows hot. My cars engine temp actually decreases while i rev the engine. It cools obviously faster if im moving, but even standing still, if i rev the engine it helps cool the engine according to the stock engine tmep gauge. But i will look into bleeding it first then flushing and so on.
There isnt a tensioner or anything on the water pump that would cause this? maybe not allow it to spin at idle but it will while revving?
Thanks!

 

[chomper149]

so i diagnosed the problem a little further now and im 90% sure its my fans not coming on during idle.
So i started using the AC/ once i did the fans kicked on right away. now while sitting at stop lights i can actually feel the fans kick on sometimes and turn off. something i never really noticed before while running just heat. I have def seen the car turn on the fans while only on heat. but right now it seems to not be working.
Yesterday when i drove the car it was fine the first half, drove only heat on the way home and when i was closer to home started to over heat, threw on AC, upped fans and revved it a bit and it dropped pretty quick.
Went out today with AC on the entire time, did a lot of stop and go and i heard the fans come on a lot, and not a single overheat.
So why would the fans work with AC on but not with heat/ nothing on?
im about to just wire in a fan switch so i can just keep it running all the time/ turn it on after it heats up. next mod was supposed to be a water temp gauge too.

 

[chomper149]

no one else has had a problem with it working on AC but not regularly and apparently now it doesnt work with the heat on too?