rageface. I just wrote a whole long-ass detailed post about this but the forum logged me out while I was writing and I lost the whole damned thing. ARGH!!! Not what I needed.

I don't feel like writing all that up again, no time right now in fact. Take my word for it that the radiator, heater core, all hoses, the thermostat and water pump are good. I've checked the new and old thermostats 3x in hot water, good.

Here's the problem in a nutshell: I was driving in normal/cool temps last summer and the car started overheating. Using the heater got the temp down and I drove it home but from then on the car started overheating every time it ran without the heater on, sitting or driving didn't matter. The only things I know for sure are that the thermostat is GOOD and that it's never opening in the car. The hose coming off the thermostat never fills and never gets hot, the radiator doesn't get hot.

My theory as of now is that some chunk of something broke off and is blocking or highly restricting a coolant passage in the head or at the head gasket. I'm about ready to pull it and see unless someone has a better idea or a way to test something else. I've worked on a lot of cars for myself and friends but I've never really had a complicated cooling issue....this one I just can't figure out.

This car is in great shape and has always run like a top until this overheating problem started. I haven't had time to work on it in months but a friend hit a deer last weekend and totaled out his car so I want to hook him up with this for the winter but I'm running out of time I can be working on it outdoors this year so any help is appreciated.

Thanks,
Mac

Here's the only text I had copied from the lost post. (&$*&@#$&(@!!)

About air trapped in the system, possible but the symptoms are the same now as they were the day the car started overheating while driving in clear traffic. I drained the coolant once and replaced the (good) thermostat, bled it well and still had the exact same issues. Now I've drained it all again, new thermostat (same good one), new water pump and careful bleeding with new coolant...same issues. I believe I got almost all of the air out before starting because I got 10 quarts of coolant in there and the spec is 10 and change.

More detail from the lost post: I'm pretty sure the head and head gasket are okay as far as cracks go. Plugs are fine (no coolant deposits), no "steam cleaned" piston crowns, coolant I removed looked fine, a little sediment dropped out in the pan but not much and no signs of oil or grunge from gasses, oil is fine, no mousse on the dipstick or under the cap.

What year is your car and motor size?

Nicolas Torres wrote:What year is your car and motor size?

lol... sorry, I was really pissed when I had to rewrite things earlier...

The car is a 2000 2.2 5spd Sunfire.

When the car is overheating is the back part of the thermostat cold/warm? The side that is facing the block.

if your overflow cap is old it could bleed pressure and cause overhearing. worth a try. you can get them at o-reillys.

I'm thinking your heater core is blocked and the hose coming out from it back to the motor is not heating up the thermostat causing it not to open and causing the car to overheat

Thanks, Alex, I'll stop by a local garage and have them pressure test the cap just to rule it out. It may bleed pressure too soon but it definitely holds some pressure at least.

Nicolas, I'm positive it isn't a blocked heater core. 100% Even if the heater core is bad I don't think that would make the engine overheat because it isn't the only path. The thermostat housing (engine side) gets warm but not hot-hot.

I've tested all the flow paths outside the block/head unit and the radiator, heater core and hoses all flow without restriction. I mean obviously it isn't like the rad or core are empty containers but if you pour a gallon of water in it comes back out just about as fast as it goes in.

I'm going to give it until tomorrow morning to see if someone here or I come up with a better idea and then I'm going to pull the head. The fact that (so far) I haven't found anything outside the block/head that could be causing the problem combined with the way the problem first started more and more leads me to think that this is a blocked passage somewhere not letting much coolant get back to the pump.

The way this all started was when I was driving last summer on a fairly cool day (60's) in open traffic, so moving and with plenty of airflow, when the car suddenly starts to build towards overheating. Even when moving along at 55 or so I had to use the heater to keep the temp down. Of course my origional thought was that the thermostat had failed closed but that turned out to be fine (replaced anyway) but then it seemed like it wasn't pushing coolant very well so I thought maybe the pump. I checked all the external stuff then replaced the pump and nothing is different.

Thanks for the ideas so far!

Any progress man? I say bypass the heater core and see what happens?

No progress so far but I'm about to get on it today. It's been like 14-15 degrees here so I had to make room in the garage. Fortunately the garage is heated, unfortunately I've got three bikes under tarps outside until this gets fixed.

I've got to trace out the cooling system for myself but I don't think it can be the heater core. Isn't the core just a side loop of the main system? This is based on other vehicles but I can't picture all the coolant passing through the core all the time... That said I did find a thread on another site that sounded like a similar issue and apparently bypassing the core solved it for them so I'm going to give that a shot I guess and then pull the head tonight unless it turns out to be some sort of magical fix.

One way or the other I aught to be able to come back with something tonight or tomorrow...

Sounds good man keep us posted.

Nicolas Torres wrote:Sounds good man keep us posted.

Just like I said, it was the heater core.

.. okay, I'm still surprised by this. I looked at the plumbing of everything and decided to try bypassing the heater core and....it worked! For the first time in six months the thermostat opened. I'm still not sure how this is happening though... I guess the heater core is flowing enough to pump out good heat inside the car but still flowing too little volume melt the wax or whatever in the thermostat before the heat gets sucked away. Maybe the pump was cavitating and flowing some bubbly BS past the thermostat.

The job isn't done yet, I've got to try and flush out the core and pressure test it and/or replace but I should be able to have it on the road tomorrow but I'll post again when it's all wrapped up.

Are there screws right at the firewall on the lower heater core cover? I think I've got 'em all out but I can't get it out, it feels like it's still attached at the firewall. I can feel at least one screw up there but I'm not positive it goes into the lower cover and I've got no idea how I'm going to get it out. I've tried ever combination of wobble sockets and flexible extensions but I can't get on it.....yet.

Just flush the heater core, do the outlet side first and then the inlet side. I used the water hose to do it last night and hooked everything back to normal and it is working fine.

Ha! Too late for that... I'm getting ready to put the new core in. I decided just to replace it so take it out of the equation if the problem comes back, I won't have to wonder if it wasn't flushed well enough.

FYI, on the 2000 Sunfire at least there is one little MOFO screw in the middle of the heater core lower cover that you can't get at in place, at least I couldn't with universals, wobbles, extensions and everything else I had in the shop. I read a few places that said that screw didn't actually go into the lower cover and you just at to pull it away from the firewall... well on the 2000 Sunfire that screw does have to come out. That or you can do what I did which is break it off while you're trying to "just pull it out of there".

A little epoxy on the lower cover and it'll be okay and F that screw, it's never going back.

Thanks, Nicolas.

The new core is in and the car is sold and back on the road. In hindsight I should have tried just back-flushing the heater core. Even if it had to be done again in a few months it would have been worth it to get back to warmer weather... Looking at the old core I think it would have been just fine if I'd flushed it.

Also, SUPER IMPORTANT for anyone who searches and finds this thread later. (Sorry if this was mentioned before in this thread..not sure):

Make sure the new core goes in the right way! Because the pipes can twist you can put it in the wrong way and nothing will line up right and you'll never get the bottom cover back on. Be careful with the metal bands that hold the core in place too, I only have one now... the end where it hooks in place is super fragile. The tab it goes into, not the strap itself.

...now to dig up a new headlight harness because those went out three days after my friend got the car. All sorts of corrosion at the headlight harness to body harness plug under the battery tray.