OK, we'll I've had various issues with the cooling system in general on this car over the years. I think I solved my mystery leak, was a small leak where the water pump casting meets the pipe that enters the casting. That is no longer leaking that I can see.

On the ride home from an autocross last Sunday when it was very cool about 40*, on the highway just cruising in 6th gear at 75mph I saw 205 and 207* That isn't right!

So I investigated today and using a pyrometer I could check the external temps of the lines. As the car warmed up the top hose running over to the driver side radiator was consistently warming, while the hose right off the thermostat neck was always way cooler, as it should until it opens. Once the car was around 185* the hose off the thermostat was rising quickly like the other hose. This check leads me to believe at least the thermostat is opening, no idea if it is opening all the way is another question???

Just sitting in the driveway with the temps outside at about 66* the car ran up to 205* Took it for a ride and at first with getting air over the car it came down to 194 but then it rose back to 201* and then at a light sitting 205* still to high but not going up into the 220-230* full overheat area!

I'm now thinking about 3 things.

1. Partial open themostat
2. Worn serpentine belt
3. Bad water pump

I'm thinking if the thermostat won't open all the way it wouldn't get full cooling? Thoughts?

The thought on the sepentine belt is I see it drives the water pump with the outside of the belt compared to the other accessories, maybe it is slipping? I'm not too convinced about this idea? Thoughts?

Then of course the water pump could just not be pushing the coolant? I'm used to the pumps leaking before they fail to circulate? Thoughts?

 

Have you checked to see if there is a bunch crap on the front of the a/c condenser/radiator? That would be the first place i would check.

 

I am having the same thing with the fluctuating temps. I smell coolant, but can't find the leak. No drips, did dye test still can't find it.

 

1st most STOCK C5's run hotter than that !!!!!
205 is not that hot
But if it concerns you try the following:
1st a stock thermo opens at 195*
I like installing a 160* thermo--it never runs that cool and settles around 180-185* driving normally
2. clean in intake shourd from all bebris-leaves paer etc.
3. Check to see if the #1 fan comes on--Stock settings on a C5 is #1 on at 226* off at 219* #2 fan on at 235* off at 227*---If your fans are coming on sooner or at colder temps then you must of had it tuned already to come on earlier
4. make sure you don't have any air bubbles in your coolong sysyem-

IF you have a colder thermo already--unless you had the fan settings changed lower- it WON"T run any cooler

IF you have a STOCK thermo and set the fans to come on sooner it won't make it run any cooler either as the thermo and the coolant isn't opening and circulating until 195* anyway-----

 

Well, I had checked the radiator and it had a few leaves but really not much, so that doesn't look like it.

I do no this i regards to temp, this car running down the highway never used to run at 205* that is one of the things that really has me looking, especially given it was about 40* air temp at the time!

I've got to do this burp sequence to see if I have air in the system someplace. I had to pull the top hose off last week to drain a bit of coolant and fix the joint of the water pump casting to that top metal pipe, and sense then it seem to be high. Hope it is air in the system.

The burp sequence I've seen online seems odd, but I'll give it a go.

 

I also did the burp procedure, and the temps went down imediately, in the 180's but soon went back up, fluctuating between 198 and 208. I assume that means a small leak and it's sucking in air. No luck finding the leak.

 

To the OP, I would certainly check for debris in the condensor vanes and plan on cleaning it out regardless. Depending on how much you drive it, minimum would be to clean it out once a year, twice a year if you drive a lot. You can fabricate a water-wand or buy a air-wand to do this with. Remove the radiator cover and wash out forward from between.

For Guido, If you smell coolant, then you have a leak. Always check the weep hole first for evidence of coolant therefore indicating a new water pump is in order. If not, two other areas come to mind right away...the coolant tank nipple will crack at the edge of the tank and start dripping along the edge of the tank. Another place that just happened to mine was a coolant leak I could smell but not readily find but got tired of it and started really digging. Turned out to be a leak on the top left of the radiator itself in the form of a hairline crack. New radiator in there now and all's good.

 

Can't find coolant anywhere.

 

Like I said, if you can smell it, its leaking

 

That's what makes it so frustrating!

 

Here was mine...I could smell it but nothing dripping to the ground.

 

I did the "burping" last weekend, it was one round of more "vomiting" as it got coolant everywhere at one point! But then got everything done.

Today in 65* weather I drove it to work, 21 miles, 16 highway. On the highway it is running pretty good at 201*, a couple times 203* at 70mph, that seems OK.

But then I get off the highway and it begins to rise, on surface streets to my office get to about 210 between all the lights, then driving slow through he parking deck and then backing the car in and letting it run for 15-20 seconds it rose to 230* when the fan kicked on, This behavior is what concerns me!!!!

I just wonder if the thermostat is not really opening all the way and so with 70mph of air it cools but when slow the coolant isn't getting around at the right rate. Also concerned the water pump is just not pushing as needed.

Anyone have any thoughts on a test to narrow down the likely issue?

 

It's hard to believe it wasn't leaking from that big of a hole!

 

OK, another data point, today on the route home from the office with temps of about 80* the temp was pretty good at about 201* on occasion 205*, but then as I hit a big jam it started to rise with no movement of the car.

I decided to switch on the A/C since it was hot and I knew this would also force on one of the fans. When I did this teh temp was leveled out at about 201-203* and never went up. Even when I got home and was just pulling into the drive and garage slow it still was in that range. Much different than in the morning in the parking deck with no A/C on!

What I'm getting from this is a low speed it just isn't getting any flow through the cooling radiator, so I'm going to check again on the lift for debris up in that area.

It's the only thing that makes sense to me, if it were the water pump or thermostat I don't think the A/C and that other fan on would make such a difference?????????

 

If you pull your radiator shroud off you will flip out when you the crap in the rad and condenser coil. You can't see it with the shroud on. This needs to be done a least every couple of years. You can buy an aftermarket water pump for a reasonable amount, buy one and change it out. These cooling problems will just lead to other problems which are more costly.

 

So far you haven't mentioned any terrible water temps. On hot days I used to see water temps peak in the 240s and then drop to 219 while running an autocross. That was where the fans would kick on and off. If I ran the A/C I could get it down a little more. Never worried about it and the car is still running fine 5 years and many thousands of miles later.

Bill

 

That's not a hole..just a crack and the wetness from it. The crack was just barely even there...but it was enough.

 

those temps are about right........id clean the condenser and radiator fins and check coolant level. usually thermostats are good or bad not in between.. (partially open) notice i said usually.........

 

Ran the car today in a SCCA autoX and no big issue with the temps, but it was overcast and never about 62 today.

I agree that temp range things aren't out of range, I guess the last thing I'd say is that even though my coolant level stays constant my car seems to have a heavy coolant smell from the engine bay, after runs a few other around agreed that is smells of coolant even if you don't see any.

Several other 04 Z06's at these events and they don't have the issue, I'd like to see if I could track it down once and for all.

 

Alot of this sounds so familar. I could smell coolant for along time on my 97 but couldn't find a leak anywhere. Pressure tested nothing, it was maddening. I also suffered from mild temp swings. Till one day my a/c compressor belt popped. Thought that was odd, the belt wasn't that old. While changing it I saw one drop of coolant fall from the weep hole in the water pump. Replaced the pump, she has been golden ever since.