ChrisLSx

Vehicle
2004 Manual Base C5, basically stock other than 180 T-stat and programming

Back story:
I went out for a New Year's Eve cruise tonight (~30 min romp on out of town roads and highway) and to my surprise, my coupe started running hot (~240). This shocked me as it was 50 degrees outside and even in the summer my C5 never goes above 205 (I have a 180 T-stat and my fans kick on at 195 low/205 high) and can actually run as cool as 185 on a cool day/night.

When the coolant temps were in the 235-240 degree range (displayed on the digital gauge), I was cruising low load at less than 2500 RPM. On a hunch, I decided to increase the RPM but maintain low load so downshifted and brought the RPM to 4000. IMMEDIATELY the temps plummeted. Bingo, low coolant. I pulled into a grocery store parking lot to top off the radiator and to my dismay she took a full gallon of distilled water!

On the way home, the temps never budged above 195 no matter what I did. But now I'm stumped. I did a coolant flush in mid October and confirmed the coolant was full. Since that time the car has always been parked in my garage and has never shown a drop of coolant on the floor. I looked high and low this evening and couldn't find a single drip. I even checked the oil (pulled the dip stick) and there's no indication (no white residue) of water getting into the pan either.


Question:
Car shows no signs of leaking/never a drop on the floor/no water in the oil yet I'm low a full gallon after 2 months of intermittent driving. What could it be?

C6_Racer_X

The first thing I check when coolant is disappearing without a trace is the pressure cap. They wear out and should be replaced when the coolant is replaced (every 5 years/100,000 miles).

If you have a pressure tester, test it. If not, toss a new cap on it. A good cap isn't expensive. Even at a $tealership parts counter, you'd have a hard time spending over $10 on a cap.

ChrisLSx

I'll grab one Tuesday morning and give it a shot. Thanks.

k24556

So, if you are running 240 deg (hot!), then a small leak somewhere the fluid will immediately flash to steam when it passes to the atmosphere, you may not see a leak. Look for dried residue, and there are lots of places where a small leak can happen, water pump, steam vents, reservoir tank plastic weld, cap, hoses, etc.

The more common, the cap has been mentioned.

That is also why you are running hot. a small leak will reduce system pressure just a bit and you are getting boiling of the fluid, which lowers the heat transfer capability of the fluid.

Also, boiling ruins the corrosion inhibitor capability of your fluid, and boiling fluid will etch the water passages, and lead to corrosion of places where air can enter and provide more oxygen. You will see pitting at the hose connection locations, for example.

jdmvette

^water pump

ChrisLSx

I'll check the pump when I do my cam swap (next month or so). I checked the surge tank last night as soon as I got home (pulled the 3 nuts and rolled it over) while the system was hot. Zero indication of leaks there. I'm planning to replace all of the hoses and the surge tank when I do my cam.

I did find a small puddle (about the size of a dollar bill) on the passenger's side garage floor (just inside of the passenger's side front tire) this morning, but everywhere I looked on the car was bone dry and I couldn't find any residue or discoloration.

ChrisLSx

As I mentioned, the high temps were entirely from being low on water. Being a gallon down, the water pump wasn't moving enough water at low RPM to keep the heads cool. When I increased RPM (to 4K), the pump began moving more water and the temps rapidly fell to normal. Once I refilled the system, it never went hot again.

C6_Racer_X

If it was under the overflow hose from the overflow catch tank, that's definitely a symptom of a failing pressure cap.

scott5

I had this same problem but what it ended up for me was I had air in the system. So I took the overflow cap off and started up and let it run and then added coolant as it flowed and my temps don't even get to 190-195 now and no more "missing coolant"

Purple92

First off - since you said that New Years Eve was 50 degrees - I guess you live where you don't regularly worry about your cooling system freezing. BUT - Antifreeze also has other stuff in it - like corrosion inhibitors - so I would recommend getting some more antifreeze into the system in the not too too distant future.

As far as the leak goes - fill the system up - then buy or borrow a cooling system pressure tester, pump the system up to about 13 - 15 psi. If there is a leak - you'll probably find it pretty quickly !!!

ChrisLSx

farmington

why has nobody mentioned a leaking head gasket?

ChrisLSx

ChrisLSx

I should mention my wife and I took the car on a day trip to Prescott today. 2 hours each way of maintained minimum 80 MPH (spurts of 90 and 100) mountain running. Water temps never bumped over 196 the entire time and I sure wasn't focused on gas mileage .

C6_Racer_X

Check your coolant level both in the overflow bottle and on the radiator. Did you replace the cap yet?
That's also possible. Bad pressure cap is far more likely, but head gasket or intake manifold gasket can both cause coolant loss.

ChrisLSx

Yup, went to the dealership and purchased a new cap before we went on the drive. How do you check the coolant level in the radiator (it's sealed/no cap on the radiator)?

I haven't noticed any loss, but then again it was a very slow leak to take almost 2 months. When you say "intake manifold gasket", are you talking about the crossover/vent tubes (LSx's don't have fluid going through the intake manifold)?

k24556

The manifold has no coolant flowing through it. the "steam lines" as they are called are a high point vent for the engine back to the reservoir. They have a gasket between them and the cylinder heads. Since the engine tilts to the rear slightly, the front is the normal high point. That's why GM got away with removing and blocking off the rear "steam vents" in the LS6 as the front is the true high point vent. GM calls this the "engine coolant air balance line", so steam line or steam vent is is a good term to use for it.

If it just did it once, well, maybe not a cause for concern, but watch it close. After all, the car is now on the order of 14 years old, and there are lots of plastic parts and damage to other stuff that you will have to prosecute until you find the true root cause. There are two things you don't want it to be, a head gasket or a heater core.

The system DEPENDS on there being a 50/50 mixture of DEXCOOL (or equivalent that has the corrosion inhibitors for aluminum engines). The reason for the 50/50 mixture is that the fluid boiling temperature increases as a function of system pressure, AND concentration of [DEXCOOL]. If you have the wrong mixture, you may have a functioning coolant SYSTEM, but you might be boiling the fluid, therefore ruining the heat transfer capability of the fluid, and driving up indicated temperature. See if the attached document explains it better.

If you have the right coolant concentration, no air in the system, a system with a capability to maintain 15 psi at temperature, and a clean radiator so it functions efficiently, then unless the fan settings have been changed, your system should operate as the second attachment describes. If not, then you start looking for a small leak that upsets system pressure at temperature, which is usually hard as all to find when cold.

k24556

The coolant level is basically at the seam on the reservoir. I think there may be a min/max set of lines embossed in the reservoir tank on the side. Not sure, and I sold my C5 so I can't go out and look. The owners manual may show it, but if you ar e like me the only time you really look at is is when the car is new.

C6_Racer_X

Yes, I meant the crossover tubes/pipes, steam tubes or what they are called. The front one has the coolant temperature sensor in it, doesn't it? And possibly a vent/bleed plug. If there's a plug, that's to let air out when filing it. If there's no plug, pulling the coolant temperature sensor is usually a good bleed point to let the air out. I'm not looking at an lsx engine right now, so I'm not 100% sure.
As for checking, the coolant reservoir is one item I look at every time I fill up. I open the hood for a quick visual check of coolant, brake fluid, washer fluid, maybe check the oil if I'm stopped long enough. If I find anything a little low, a post it note is stuck to the steering wheel tof remind me to top up when I get home. If it's dangerously low, I find what's needed close by and address the issue quickly.
Generally, anywhere between 60 (water)/40 (coolant) and 40 (water)/60 (coolant) will work fine. In southern climate, more water (60/40) works better in the summer. Water transfers heat better than dexcool. In northern climates, more coolant (40/60) may be needed to prevent freezing in the winter (like this week).

ChrisLSx

Coolant temp sensor is in the driver's side cylinder head. The front vent tube (rears are blocked off in 01+) just has a single barb (on driver's side) which goes to the TB and on to the low pressure/cool side of radiator. There's nothing to open (at least not a plug or valve) to vent, though I suppose one could just undo the crossover tube.

I tend to check those every fill up as well, though my coolant reservoir is pretty clouded (plastic is fogged) due to age. I fully intend to replace it when I do the head/cam swap.