Camarochriss

Car:
2000 Z28 M6 88,000 miles.

Problem:
Replaced the thermostat and flushed the coolant about 2,000 miles (4 months) ago. Now that it's cold I noticed the heat coming through the vents intermittently is coming out hot and cold, and mostly only hot while in motion. Figured the heater core needs to be cleaned, but since I don't drive it that often I hadn't gotten around to it yet. Today the temp gauge flew up to the red, and as I was pulling over dropped into normal. Parked for a while and when I got going I noticed the temp gauge would fluctuate, then sit at normal for a while, fluctuate again, then shoot up. Had to let it sit 3 times over a (usually) 20 minute drive.

Question: Is the heater core causing the car to over heat? If so, should I try just cleaning it, or do I have to replace it? Or does this sound like more of a water pump issue? I don't have a garage anymore, and have very limited access to one, so if I need to replace the water pump and heater core, I'd rather just do it all in one sweep.

:::UPDATE:::
The overheating went from intermittent, to a constant problem. The car will get to optimal temp, sit there for a few minutes and then spike. The coolant is down a litttle bit, but I don't see any leaks. I'm going to try to get a pressure tester and see if I'm missing something, but in the meantime I checked the oil cap - no milky residue, and no smoke in the exhaust, so it doesn't seem like a head gasket leak. I checked the temp on the exhaust manifold with an IR temp gauge and the driver side bank is about 50F degrees hotter when warmed up then the passenger side, not sure if that means anything or not, but was told to check each and see if there was much of a difference. Is there anything else I'm missing? I'm hoping it's not a head gasket, but the coolant has to be going somewhere, right? It's not just showing the signs of it.

ALSO - almost forgot. Took the belt off and spun the water pump, seems fine BUT didn't feel any pressure in the upper or lower radiator hoses when the car is running. Seems like a bad pump, but it feels fine, and doesn't explain the missing coolant (would see a drip for that, right?) Fans kick on just fine.

Thanks!

G Atsma

First try a new temp. sender. I had that problem once on a high mileage pickup. Also check the thermostat. Even new ones can be defective

Camarochriss

Thanks, will do! Appreciate the help.

Project GatTagO

iTrader: (7)

Also make sure that all air is bled from the system.

Andrew

JimsNovaLS

The heater core is a symptom not the problem. Air in the system/low coolant will cause you to loose heat from the heater and cause your intermittent temp rises.

Camarochriss

Oooh ok, thanks Jim! So question for you guys - I had read that the LS1 doesn't have a way to bleed the coolant, and you just need to fill it, run it, let the thermostat open up and top it off. Am I missing anything else? I've done that, but if there is still air in there, how do I get it out?

Thanks for the help!

Camarochriss

Just updated the post, but the TL : DR version:

*Now overheats consistently
*Coolant level down a little bit
*No sign of usual head gasket warnings (milky oil cap, smokey exhaust)
*10-15% increase in temp on driver side exhaust manifold vs passenger side.
*No pressure in upper or lower radiator hoses
*Water pump doesn't have any sort of grinding or bad noises, even when spun by hand.

Project GatTagO

iTrader: (7)

I've had good luck topping off the block through the coolant crossover tube. Get a long hose and a little funnel. It's a little tedious and time consuming, but it has always worked for me.

Andrew

eb110americana

iTrader: (4)

When my water pump died, I also had mysteriously vanishing coolant. It has a weep hole at the bottom, but it was only trickling out on the freeway. It was bone dry every time I checked. After a long road trip, I pulled off the freeway, into a gas station, and the WP finally puked up a puddle and I knew what was wrong. I never had the temp issues you have though.

For the bleeding, I just park the car on the driveway with the nose up and let it run with the rad cap open. Keep topping things up as the system will allow. When the thermostat opens and you start to see coolant freely circulating through the hoses, it will very quickly get hot enough to boil over, so be ready to pop the cap on quick, and be careful not to get burnt! Drive the car around to shake out any loose bubbles and get to running temp. Later, top up the overflow tank if necessary after it completely cools off (like overnight).

Camarochriss

Thanks! Hopefully it is just a bad pump and that's where it's leaking. I'll burp it today and then I'll do a pressure test to see if I can find anything under the pump.

That is, if I can burp it with a bad pump...

Camarochriss

It was the water pump - was only leaking under a decent amount of revs, so I didn't see it until i had someone rev it up while under the. Replaced that and everything is working beautifully. No leaks, heat comes in perfectly.

My only experience working on cars is when something goes wrong with this one, so still learning a lot on how to diagnose - thanks for the suggestions everyone!

eb110americana

iTrader: (4)

Great! Glad to hear it all worked out.

KW Baraka

iTrader: (7)

Cool! Now change the engine oil sooner versus later.

You don't want to take the chance that the oil hasn't gotten badly degraded from being heat cycled multiple times into the 250+ degree range.

KW

Camarochriss

Good point! i'll do that this weekend.