$10 cap or $1,000 repair

[KliffsAurora]

Since I've had my newest Aurora, I've been battling all sorts of issues from bad calipers to bent suspension, loose subframe bolts, fuel delivery problems, and the most recent, cooling system issues.

I have replaced every hose, the radiator, thermostat, reservoir and "aftermarket cap", 2 complete flushes with new Dex, and at WOT, the cooling system is building so much pressure that its spewing coolant out the cap.

I have bought 3 caps since a got the car, 2 from Autozone, one from a Cadillac dealer that I later found out THEY BOUGHT FROM AUTOZONE because they didn't have one in stock. Long story there...

So after doing the radiator and thermostat on Friday night, it was still spewing out the cap at WOT. It has come down to 2 things it could be. Help me out if I'm wrong.

1. It really is a cap problem. The cap is bleeding off pressure to early. A dealer cap might fix it.

2. Headgasket...

I have tested the coolant with test strips right before the radiator pooped out and it tested fine. Could it really be a cap problem?

I'm at my wits end with this car. I have never had so many problems with one car in such a small amount of time.

[oldsmobile99]

have you made sure all the air is out of the system? is it getting hot? i would also try a cap from rockauto just to be sure before you spend more money. Thats the way mine happened, it just started building pressure. but mine would get hot, it also took the shop 3 times testing the coolant to actually find it. with mine the more pressure it built less distance you could drive before it got hot. hope its not head gasket for you.

[Aurora40]

I don't see how a bad cap could cause surging like that in the overflow tank. Even a zero pressure cap that just sealed the opening shouldn't do that.

[Superdemon]

Jan 18, 2010 8:01:58 GMT -6 Aurora40 said:

I don't see how a bad cap could cause surging like that in the overflow tank. Even a zero pressure cap that just sealed the opening shouldn't do that.

Keep in mind the Auroras use a pressurized overflow tank. It operates at the same pressure as the rest of the coolant system. If the tank isn't holding pressure the coolant will begin to boil and start overflowing from the cap as it is the only opening in the system.

Double check the air dam under the car to make sure it is in place and directing airflow to the radiator. If that is good you may have a partial blockage in the radiator that is preventing it from cooling as efficiently as it could.

[Marc]

Jan 18, 2010 7:46:39 GMT -6 oldsmobile99 said:

have you made sure all the air is out of the system?

Agreed.

[bobsblue95]

Jan 18, 2010 6:59:25 GMT -6 KliffsAurora said:

Since I've had my newest Aurora, I've been battling all sorts of issues from bad calipers to bent suspension, loose subframe bolts...

yikes! did you buy the car from the Dukes of Hazard?

It sounds like a head gasket to me, but make sure the cooling system is free-flowing before anything else.
- How hot does the heater get?
- What's the temp when fluid first spews out the cap?
- Does the coolant smell like exhaust when spewing?

If you do have a HG/bolt problem, you're looking at an easy 1000 as you say. If you plan to repair the engine, be careful with it. If you plan to replace it (you can find a used 4.6 for 1000), I'd suggest trying the 3-step HG repair kit to get as much life as you can from this one in the meantime.

Good luck!
Bob

[Deleted]

It does not sound like a headgasket issue to me. I went through two pressure caps from autozone and they were both the 18 psi. Went to GMC and bought an actual OEM replacement rated at 15 psi and never had an issue up to the day I sold her. If you are having this kind of problem I would suggest looking at the reservoir tank closely to see if you can find any damage to the tank where the cap goes on. My grand am currenty suffers from a stripped reservoir tank but the GA's overflow tank is hardly pressurized.

[Aurora40]

Jan 18, 2010 8:20:41 GMT -6 Superdemon said:

If the tank isn't holding pressure the coolant will begin to boil and start overflowing from the cap as it is the only opening in the system.

Hmm, that's a good point. I figured the loss of pressure would lower the boiling point into maybe the 240's, but it appears it would actually be as low as 225F for a 50/50 mixture. So you are right, it certainly could reach boiling temps with just ambient pressure.

[lanceslambos]

i was going to suggest the 18psi cap vs the 15psi one. but aura1 just shot that out the window. if it was a bad head gasket seems like it would be losing pressure and therefore not be coming out of the overflow tank at all. you say you replaced the thermostat so this is a bit of a mystery to me. possibly a coolant hose is kinked or in a bind? but Randy can concur that the autozone caps are junk and will appear to fit snugly but actually only strip out the threads. i say get an 18psi cap from GM and see if it cures the problem. they are only about 12 bucks. autozone cap = your problem

[KliffsAurora]

Ok, so I went to the local Buick dealer and bought my cap. Its an 18PSI GM cap and its seems to be doing well. I am not having any coolant loss that I can see but the smell is there. I probably used 4 gallons of water to clean off the area before putting the new cap and and so far so good. I did notice however that all of the other caps were 15PSI. Maybe that amount of difference really made all this happen?

bobsblue95, yeah your half way there when you asked if I got it from the Duke's. It was involved in a wreck not to long ago and I got her with everything in tact... so I thought.

[Superdemon]

I never had a problem with a 15 PSI cap.

[lanceslambos]

3 psi = roughly 9 - 12 degrees difference in boiling point.

[XJSman89]

But is the PSI rating on the cap simply the maximum PSI the cap can take before blowing, or does it actually control what pressure the coolant is at?

If it actually controls the pressure and/or boiling point of the coolant (making it run at 18psi constantly instead of the normal 15) then I guess it could make it cooler but I don't think this is the case. I believe that most people just use an 18psi cap in case the coolant pressure is a bit more than 15psi and the cap fails, thus saving the car from losing coolant and ultimately overheating. That's what I thought at least.

If it's just a failsafe in case there's too much pressure then that means even though you have an 18psi cap the coolant would still run at 15psi. If this is true then it wouldn't have any direct affect on the boiling point during normal operation.

I guess what I'm asking everyone is how does the 18psi cap provide greater protection for overheating? Does it use the extra 3 psi to lower the boiling point or can it simply just tolerate greater pressures so that you don't lose any coolant when its running over 15psi?

[hiphophead]

Spend the extra $30 or so and get the exhaust gases in coolant block test. Put your nerves at ease from wondering.

[bobsblue95]

xjs,
basically the cap regulates the cooling system pressure. Higher pressure means higher boiling point. It doesn't keep the engine any cooler *unless* you were having boil-over issues. If the coolant is boiling, cooling efficiency drops and the engine will overheat faster. A weak cap fails to maintain enough pressure, allowing coolant to boil over at lower temps.

hth,
Bob

[Aurora40]

Jan 19, 2010 13:42:20 GMT -6 XJSman89 said:

But is the PSI rating on the cap simply the maximum PSI the cap can take before blowing, or does it actually control what pressure the coolant is at?

If it actually controls the pressure and/or boiling point of the coolant (making it run at 18psi constantly instead of the normal 15) then I guess it could make it cooler but I don't think this is the case. I believe that most people just use an 18psi cap in case the coolant pressure is a bit more than 15psi and the cap fails, thus saving the car from losing coolant and ultimately overheating. That's what I thought at least.

They don't "blow". Rather they bleed off coolant into the overflow, lowering the pressure.

When cold, there will be no pressure. As the temperature rises, the pressure will slowly rise until it reaches the cap's pressure limit. The cap will then keep the pressure around that point by releasing and resealing.

Pressure also doesn't cool the car, it simply raises the boiling point further than anti-freeze alone can.

[elgato]

I have the same problem. 2002 3.5 98,000 miles.

Often the display will show coolant low even when it is at the proper level. The gauge appears to be in the normal range. After driving about 10 miles in the heat and stopping the expansion tank will be boiling over. The car does not appear to be overheated but the expansion tank is boiling like crazy.
No external leaks.
No water in the oil.
No white smoke at the exhaust.
Fans appear to be working fine.

I have not seen it happen at a long idle, just after a long drive in the 100 degree heat.

The cap was original GM and obviously the seals were failing. I replaced it with an Autozone 18lb and now instead of just spraying out around the edges it snaps open with a loud pop.

I borrowed the block test kit from Autozone and the fluid stayed blue. If it is a head gasket leak it only opens up after a long drive.

A mechanic suggested that a thermostat stuck half closed might cause these symptoms.

Thanks

[Randy T.]

You may want to start a new thread, but I will answer here for now. Have you ever heard a sloshing sound that sounds like it's coming from under your dash?

Throw the Autozone radiator cap in the trash, the don't fit on our coolant tanks right, you would be better off getting used GM caps from the salvage yard, I have a stack of them.

The "coolant low" message could be a bad or sticking sensor in your coolant tank, may need to replace the tank to fix that, some of the sensors are not able to be changed by itself.

I have my coolant sensor unplugged on purpose, when it says "coolant low" the fans kick on and stay on, I like it like that, only goes over 200 at drive through windows and when I shut if off.