94 Z28 Automatic...176k miles...total stock...

Within the last couple of weeks I've had my water pump, radiator, t-stat and spark plugs replaced. At the time all of this was being done, the dealer I took it too had also done a pressure test as well as a chemical test of the coolant to check for a head gasket leak, and it passed the pressure test and the chemical test.

But they still said that while looking into the radiator, every now and then they would see a bubble or two, leading them to think there might be the beginning of a head gasket problem.

Anyway, despite the low coolant sensor also being replaced along with the radiator, I was still getting the low coolant lamp coming on almost as soon as I drove off with all of new (and expensive) replacements.

After about a week of driving (and mind you, the coolant temperature was perfect...as good as new), I popped the hood one day after about 25 miles of highway driving (and the low coolant light coming on again), and as I was gazing around the area of the radiator cap, I heard a faint POPPING noise and saw a bit of coolant suddenly appear around that small pipe that leads to the neck of the radiator.

I thought a pipe had burst or something, but then I realized it must have been the radiator cap relieving pressure or something, which of course isn't normal.

I drove it home without incident. After sitting overnight, I was only about one mile into my trip to the shop (I was going to have them investigate this radiator cap popping thing), and while I'm watching the coolant temp gauge, I am realizing it is rising a lot more quickly than normal, AND IT'S NOT STOPPING!!!

So, before it red-lines, I pull over and turn the car off and have it towed the rest of the way....the funny thing is, about 5 or 10 minutes after turning the car off and waiting for the tow, I turned the key just to see what the temp gauge was reading, and lo and behold, it was almost back to normal!!!

So after about a day at the shop, they finally call me and tell me I blew a head gasket...but the reason I'm not seeing any smoke or anything is because it's an external head gasket leak...and that the reason the car almost overheated so soon into my last trip was because the leak had formed some sort of air pocket...which somehow "equalized" while I was waiting for the tow, thus the reason why the temperature was almost back to normal 5-10 minutes after pulling over.

Does this sound like an external head gasket leak to you guys out there?

Wouldn't an external head gasket leak send coolant running over the outside of the hot engine and produce some noticeable smoking under the hood?

 

I`m guessing what I had on a previous ride would be called an internal head
gasket leak.There was a breach between the cylinder and a water jacket.It
would suck in a small amount of coolant on intake stroke and pump air into
the cooling system on compression.It would run fine and have no external leaks,
but would boil over every time I stopped and would need coolant added after every trip.
I would think with an external leak you would see steam,and have a puddle under it
after you parked it for awhile.

 

Have you checked the coolant level in the radiator?

 

So the shop said there were bubbles in the radiator after they had replaced parts of the cooling system? Could have just been the system naturally bleeding itself of air.

Check the oil to make sure it's not milky, but it sounds like you might just still have air in the cooling system. Like above, did you check the radiator level? And which hose was leaking, the small one from the steam tube to the radiator neck? Fix that first.

 

The hose wasn't leaking...what I thought was a leaking hose was actually a little coolant that had been "spritzed" out from under the radiator cap...which apparently popped up for a split second (to relieve pressure???)

As you can probably tell from my posts, I'm not much of an "under the hood" kind of guy...just trying to get a bearing on what is going on from those better informed...

 

I called the Vette Doctors, and they said it sounds like it's probably a head gasket leak...said they could replace it for around $700 but that doing so in this engine (176k miles and some iffy oil pressure problems...remember, I have already put a thicker grade of oil in there due to one instance of a low oil pressure reading at idle during the summer) might open up a whole other can of worms...

So, I am regrettably thinking of junking the car...which sucks, because of the money I've already put into it in the last month or so...plus the body is in excellent condition...this sucks....

 

I would use this as a good learning experience. First, if the radiator cap is leaking, replace it. That might be your problem right there. Did you check your oil yet? That will be a dead give-away for a leaking head gasket. If it's milky, the HG is leaking internally. And if it's leaking externally it should be easy to tell.

Don't be so quick to let mechanics tell you what's wrong, most could care less about helping you. They want your money. And if it does end up being the head gasket, do it yourself.

 

It's a brand new cap (not to mention a brand new radiator and brand new low coolant sensor).

I wish I could do it myself, believe me...but I just don't have the time or the patience at the moment.

This whole thing started with the low coolant light coming on...and coolant in the recovery tank dropping slowly over the course of the week...but no smoke, pressure tested fine and they ran a chemical test on the anti-freeze for exhaust gases...it passed that test too...

But I kept bringing it back because it kept slowly losing coolant...finally, after three weeks they said my water pump was leaking out the weep hole...so i replace water pump and thermostat...then a week later I notice a little white smoke coming out of the hood, and a little puddle near the front passenger side...they tell me my radiator has a crack in it...new radiator, new low coolant sensor......drive it for a week with no problem, but STILL low coolant light keeps coming on....then I look under the hood after driving to work and hear a POP noise and notice a bit of coolant magically appear on that small pipe leading to the neck of the radiator...radiator cap must have popped up for a split second because of pressure and spritzed some coolant???? Are they designed to do that if the pressure gets too high? And if I have an external head gasket leak, wouldn't pressure be relieved at the source of the head gasket leak?

So, I drive it to the shop and about a mile into the trip (mind you, this is one mile into a trip after sitting all night long), the temp gauge is fast heading towards the red zone....so I shut her down before it gets to red, tow her to the shop, and they tell me it's a head gasket, and it's been sitting there at the stop untouched for three weeks now...while I lean towards junking it...like I said, this sucks, as I love that car (15 years, 176k miles).

 

Head gaskets are not difficult to replace and can easily be done in a weekend with some basic tools. With the help of websites such as this and others created by Shoebox, you could probably get away with the job for $150 (gaskets, bolts, fluids, beer).

 

Let me ask a simple question...

What causes a radiator cap to "burp" like I discovered mine doing...and shortly thereafter, on my first trip of the day (after the car sat all night long), it almost overheated a mere mile into my trip (I think I let it idle for maybe a minute or two after first starting the car). Now, keep in mind, up until this point, I had been driving for about two weeks without incident after having the water pump and radiator replaced.

Is a burping radiator cap a sign of a bad cap or a sign of too much pressure in the radiator? Is it also a sign of a bad head gasket?

 

The radiator cap itself does not "pop up.... to relieve pressure" or "burp". The internal, spring loaded plunger lifts off the seat in the radiator neck, the coolant flows past the plunger, and out the neck nipple to the hose that goes to the reservoir. If you saw coolant at the point you described, you either have a faulty radiator cap, or the overflow hose is leaking. You should never see coolant at that location.

If the overflow hose is leaking, the coolant will correctly flow out of the radiator as the coolant heats up and expands, but when you shut the engine down, and the coolant cools down and contracts, the natural vacuum in the radiator will not be able to pull replacement coolant out of the reservoir, up the hose, and into the radiator.

Sounds to me like a defective cap, the radiator neck is damaged, or you have a leaking overflow hose. Is your "new" cap rated to hold 18psi?

Are you sure you are fully tightening the radiator cap? Its got a "detent" about 1/2-way closed, so you need to be sure you are tightening it fully.

 

I've decided to try and drive the car home from the shop and procrastinate some more as to what its ultimate fate will be.

I asked the shop to flush the system and get whatever air pocket caused it to almost overheat out of there so I could limp it home...

I also asked the service rep about the radiator cap and the two little burps that I saw after driving it for about 25 miles on the highway (this was before it almost overheated, not when it almost overheated), and he said the cap COULD burp like I saw if there were funky pressure problems going on due to the head gasket...said basically that it is designed to act that way if the pressure gets too high...something tells me he isn't really understanding what I am saying, and your explanation here makes more sense...

It's designed to pop up and allow coolant to flow back into the overflow tank, but it should never allow any coolant outside the system no matter the pressure buildup, correct?

 

Based on your last question, the over-flow catch tank, down near the headlight, could become full and over-flow if enough coolant is forced into it. Under normal operating conditions, though, no.

The radiator cap, itself, if working properly, shouldn't ever allow coolant to be forced out at the cap. If you are using the correct cap and it's properly tightened down, there should be no leak there, even under the 15 psi (or so) of pressure.

What psi cap are you using? 'F' body cars take a different pressure cap from 'Y' bodies.

During your original experience, pressure is like water, finds the path of least resistance. As pressure builds it'll burst through at the weakest link in the chain. Since you saw evidence of leakage at a hose, that was your weakest link. Tighten that up and yet another, different weak link would be created.

After replacing the low coolant sensor, all the air wasn't purged from the system causing the light come back on. As I recall from reading an article, the system is designed to self-purge air once the original bleeding is done.

I believe you have a tiny head gasket leak. Once all your connections are tight and leak-free, original burping done and over-flow tank with the proper level of coolant mix, absent anything else not mentioned (like water pump, sticking thermostat, trash in front of radiator, etc.) you should be good to go.

I have a long, detailed tech article on how the LT cooling system works (differs by body design "F", "Y", etc.) and if you'd like to read it I can email to you.

Jake

West Point ROCKS! Nation's TOP COLLEGE per Forbes Magazine!! Graduation Day Parade 20 May 2010!!!

 

Thanks for the reply Jake. Actually, what I thought was evidence of leakage at a hose was actually a tiny spritz of coolant that escaped from the radiator cap after I turned the car off.

So, to continue my saga, after having the car sit for four weeks at the dealer, I decided I don't know whether I want to junk it or not, thus I took the chance and drove it home the 9 miles to my house...it made it home and the temp gauge acted exactly as it should have...no signs of overheating...

I start the car up a few days later just to test it out, and of course the low coolant light immediately comes on (I had checked the overflow tank before I started the car, and it read a wee bit below normal, but not too bad).

I drove it around after letting it warm up a minute or two, and I noticed the temp gauge was once again rising faster than normal less than a mile into my trip (again, this is after sitting for several days)...once it hit about 185-190 degrees, the temp gauge plunged back to under the first tick mark....I assume the thermostat opened at that point as it should...I drove around for a little while longer and the temp remained within normal operating range.

I then started it up today and did the same test...only this time, the temp gauge took a lot longer to get to 185...not like the way it quickly rose the day before.

Unfortunately today, I quickly noticed that about one mile into my trip, the temp was continuing to rise, until it almost hit the red zone at which point I turned the car off...let it sit for about 15 minutes...then drove another .5 mile when it almost went into the red again....let it sit 15 minutes again...then started driving, but this time I cranked the heat...and then I noticed...NO HEAT!!! But then, after cranking the heat for about a minute, I notice the temp starting to COME DOWN back into the normal range, and the HEAT started coming out of the vents.

What do these recent symptoms sound like to you? Trapped air? Bad thermostat, bad water pump, head gasket?

To reiterate, about a month and a half ago I had the water pump replaced (along with t-stat), and it drove fine for a week until I had to have the radiator replaced...then it drove fine for another week until I started getting these overheating problems I've detailed in my last couple of replies.

Even though I know next to nothing about these sorts of things, I would think my latest round of symptoms would point to something rather definitive.

 

Check the coolant level in the radiator, not the overflow. Check after it's been sitting over night.