billla

Team, bear with me as I give a little background and history here as this seems to be a tough issue. I've highlighted the key points to try to do a tl;dr path.

I bought my Z about 3 years ago with 86K on the clock. It's now at 125K and has been relatively trouble-free with the usual exceptions (EBCM/SWPS, etc.) About a year ago it threw an AC belt...and immediately started to overheat as well.

Now...I don't want to get into the "these things run hot" discussion, please. I keep the radiator cleared out of gunk and the air dam is in place. Typical road temps or long-term idle temps were between 200 and 220 or so. But now I had multiple situations in traffic where the temps hit 255-260 and triggered a CEL. That is by definition...overheating.

A good local Corvette shop replaced the A/C belt and the water pump but this didn't make any difference at all. I kind of lived with it through the winter, but now in the depths of summer I decided to get serious and find out what was going on.

First was using an ultraviolet dye kit, and I found coolant both at the overflow tank top fitting and at the shaft in the pump. I replaced both. Initially, this seemed to be better if not quite back to normal. At extended idle, the temp would get to around 230 and stabilize. But within a few weeks I was right back at 255+ in traffic.

Where do I go from here?

Sigforty

Have you got under the car and looked at the radiator? You may need to clean it out. Leaves, dirt, grass and other debris can be clogging up the fins making it not cool. You might need to clean it out with a water or air spray.

billla

Yep, it's clean.

3boystoys

Pull thermostat and put it in a pot of water with a thermometer and start heating it and see if it opens at temp stamped on it. Do a pressure test on the system too. You sure both fans kick on??

billla

I checked the thermostat before changing the water pump and it was correct. I'll do a pressure test tonight.

I have verified that both fans are on when temps are > 234 degrees.

Fed Up

These are relatively self-bleeding systems (not like some BMWs I've had where the coolant system will grenade on your from a little air), but it almost sounds like you have air in the system. My thoughts drift towards...

1. Air in the system
2. What is your coolant/water ratio. And is the coolant new or recycled?
3. Did they transplant the thermostat from the old water pump to the new one?
4. If not, replace it.
5. And last, and most fearfully, do you think your radiator has sediment in it to the point of being clogged?

All questions to think on. Good luck!

billla

1. It is hard for me to imagine where this would occur, but I'm interested.

2. 50/50. New.

3. On the pump I replaced it was a new thermostat and housing.

4. See 3

5. I really don't think so - the coolant that came out when I changed the 2nd pump looked pretty good.

Open for any and all ideas

Fed Up

I would lean toward air in the system then, if you're comfortable with all your hardware. When the system was opened up (big time opened up, like a WP replacement or other engine-side coolant loss/replacement), air can get into the block if it's not bled properly after the install. The air will travel around and spike your temperatures. Bleeding is pretty basic- Start from cold, and give it a shot. Directions are abundant on the forum.

neutron82

try blowing out all the debris from the ac condenser and radiator... it may look clean but you would be surprised how much crap can be stuck in there... you might also have a tuner check your fan on/off settings, that alone can get your car running much cooler

Ray 2000 C5 FRC

One of your catalytic converters could be going bad. If one of those converters started to melt (prolong engine misfire, ignoring or delaying the change of an O2 sensor, or the converter it at its end of life) , it would increases enough back pressure causing the engine (a mechanical pump) to work harder to push out the exhaust gasses, this will cause the engine to work harder and thus run hotter. Cars with this problem notice their engines overheat at highway speeds, and with a little loss of power (probably due to too much back pressure). Something to check is your certain all other issues are normal.
Ray

danieloneil01

^Wow, I might need to pull my aftermarket cats off and check them out then.

donald4972

with all you've checked, and from what you've written; i think it needs to be 'burped'.

Air trapped somewhere within the system.

Don

Texas1ao

Had air block after changing tstat. Check out the post to bleed air from heads. Do a search for post title overheating??! Worked like a charm on mine.

Racinjace22

yes, bleed air from lines on head. if not it will catch air and run hot everytime

PLRX

Check the radiator after your stat is new or working properly. The radiator it's 11 years old; they do accumulate dirt along the fins. It's time to get it cleaned up with compressed air or a power sprayer.

billla

Necroing my own thread, as I just got time to get back to this problem

Still hitting 250+ on more than about 20 miles on the freeway or driving around town. I can just barely keep it below that.

It's been "burped" multiple times, including getting a full stream from the crossover at the front of the heads.

Last night I used phosphorescent dye to check for leaks - none.

Today I pressure checked the system and the cap. The system held 14 PSI for 5 minutes with no change, the cap held 14 and burped at a little over 15.

Given this, the system seems tight and there appears to be only one place left to look. I'm pulling the radiator tomorrow AM and testing/replacing the thermostat.

If I'm off base, now's the time

gimp

Damn dude, it's still overheating like two years later? I assume it'll overheat when you idle it, right?

At this point, I'd be looking into wholesale replacement of cooling system - new radiator etc. Because screw it.

Have you hooked up an OBD2 scanner to check all the values the ECU reports? For example, engine load % - maybe something is totally screwed and is causing your engine load to be very high in normal conditions. I would try that, go through your various parameters and see if any are out of spec.

billla

It's not a daily driver, and in winter it's not really a problem.

If it sits long enough, yes.

Why would I do that? The system is tight - it's just not cooling. Working through the problem will get it fixed; the radiator is coming out today, and in the end I suspect it's just going to be full of crap. It ain't broke, and the system works for millions of C5s.

I appreciate the thought, but the tune is stock (verified, as I SS autocross now and again) there are no codes and the engine runs great. I could see an overheat if the timing was well advanced, but any issues with the tune would show up in other ways.

I think we'll have this solved today with cleaning the radiator - I'll post pics in an hour or so and we'll see what was in it I have cleaned it out a bit, but this is the first time it's been out. Doesn't look like too bad of a job.

ZigZag

I sent you a pm, did you get it ?

billla

Here's the new (top) and old (bottom) thermostats @190 degrees F - these both look fine to me, although I'd kind of expect them to be further open...