Sorry for using my first post here to babble on about my issues, but I need some help with a problem that I'm baffled with. This is kind of long, so bear with me. I do apologize.

I have an '05 Monte Carlo LS 3.4L with about 165,000 miles.

Last Saturday, I was sitting in a drive-through when my car got real hot and the temperature ended up red lining. I let it sit in the parking lot overnight and a friend and I went out to look at it. Coolant was blown out through the overflow tank all over the inside. So we put some new coolant in and it was fine and didn't overheat. I drove it out about 60-70 miles that day and it was fine. That evening, I went out and when I was driving down the road going real slow behind someone, the temperature got REALLY hot. It didn't red line, but came close. When I turned down another road and sped up, it cooled down.

After doing some reading on this forum, actually, I determined I had air pockets in my coolant system, which was causing it to bubble in the overflow tank. It actually sounded like popcorn popping. When it was cool, I'd put coolant in the radiator as it began refilling everything back up to where it was supposed to be. Monday, I noticed there was a chunk missing from the threads of the radiator cap. I purchased a new OEM cap and it put it on, but there was still bubbling. Took it to a mechanic yesterday who said he drained the air out.

So last night, I'm driving and I thought I heard the bubbling sound coming from the backflow tank, but I've been hearing so much and listening for it, that I passed it off as hearing things. Stupid, I know. Today, I get up early and hit the road for a couple job assignments. I drive to a town about 30 miles away and pull into a drive through to get something to eat. Within about two or three minutes sitting there, it started warming up really bad again. So I pulled out and was going to get down to a gas station about two blocks away and as soon as I hit the road, it cooled off back down to normal temperature, just below the halfway mark on the temperature gauge.

Continued onto my next town about another 30 miles away and got stuck behind someone doing about 15 mph. The temperature started climbing again. So I turned down a road and as soon as I sped up, it went back down. When I pulled into where I was going, I could hear the water bubbling inside the backflow tank. As I sat in the parking lot doing some stuff, I watched as the temperature steadily rose. As soon as it hit the red line, I shut it off immediately.

About an hour later, I come back out and turn it on and it's normal temperature. I drove straight back, with the gauge fluctuating slightly going down the road, but never going past the halfway marker, staying below in the range where it's normally at.

I let it sit for about 2 1/2 to three hours in the parking lot at the office. I went back out and checked the radiator and put some more coolant in there. It belched a couple times, but then held steady. Put the cap back on secure and went on another 35 mile drive. Gauge didn't move, even when I sat in one spot for about 10 minutes. On my way back, I made two other stops, sitting for about 10 minutes in idle with the air conditioner running and the gauge steadied slightly above the mid point and didn't fluctuate until I started moving again.

So I get home and I pull the cap off the overflow tank. It's not bubbling. But I can hear a faint, what sounds like a single drop and then I see a ripple in the overflow tank. That quickly stopped and it hasn't done anything since.

My mechanic swears he got all of the air out and it shouldn't be doing anything else. He said he pulled some coolant out of the system yesterday after he got done. He also says he saw some white smoke coming out of the tail pipe when he fired it up yesterday after I dropped it off. It was pretty humid and there was a lot of condensation on the windows and such. I've been watching for smoke and haven't seen any. I also haven't seen any condensation on the tail pipe. I keep checking my oil to see if there's any coolant leaking in there and I haven't seen any yet. The car runs smooth with no missing or any other issues. I thought maybe it was the fans on the radiator, but they kick on when it hits a certain point.

Honestly, I'd be "relieved" if it was a head gasket, because my warranty covers this completely. But I'm worried it's something else entirely. What I can't figure out is the randomness of it. So I was hoping someone here would be able to help.

 

Hello. I am not familiar with the 3.4 and it's nuances, So this is just from my experience with cars and cooling systems.
If you hear bubbling, it is more than likely air in the system. With the 3.8 it has to be bled 2 or 3 times before all the air is out. Even when you think it's all out there could still be air left. Question, did the mechanic bleed it through the bleeder. Albert or from the radiator cap?
If you have a bleeder valve then That's the best place to bleed it from.
Now, as far as the cause of the air in the system, if you haven't done a flush and fill lately, then you may have a blown head gasket. But as you know, that would NORMALLY cause white smoke from the exhaust. But just because you don't see it doesn't mean that the gasket is good.
I'm hoping 2 things for you; that this will already give you a direction to hunt and that a member with a little more experience with your engine will chime in. Good Luck!!!!!!

 

I haven't had anything done to the radiator since I purchased the vehicle about two years ago. My mechanic bled it through the bleeder valve, but there was still air in there. I've just spent the last hour bleeding the air and driving it around. Before I started, I saw some condensation in the tail pipe, but no white smoke. After bleeding it, I drove around for quite a bit and the temperature gauge didn't budge from where it's supposed to be. When I pulled it back home, there were still no bubbles and I checked the pipe and no condensation.

I'm still leaning toward head gasket, and I have a mechanic scheduled to work on it Monday with my warranty. I just want to make sure there's not some other major issue that could cause this that could jump up and bite me for a lot of money that wouldn't be covered under my warranty.

 

Cool. Hope it all works out for you!!!!!

 

I've dealt with a 3100 (which is almost identical to your 3400) with very similar symptoms. If I was to bet, I support the head gasket issue.

True, air pockets do cause the temp sensor to through crazy readings, but you also have that "bubbling" issue that at least once, forced coolant all over the place, it's been bled at least once and finally the white smoke from the tail pipe.

I'm not sure about all engines, but in the 3100/3400, when the head gasket fails, it can force compression gases into the cooling system. This causes the cooling system to become over pressurized, the end result can be the coolant explosion all over the place and the bubbling (it's the pressure in the system forcing itself out). In my case, it took so long to narrow down the problem, it blew the gasket on the side tank to my radiator (so I changed the radiator out when I did the head gaskets).

The white smoke is a result that sometimes when the car sits, that pressure might force coolant reverse the way the compression gases got in, start the car and it may idle rough for a few moments, white smoke and then run fine.

I believe you said this is under warranty, that is AWESOME! I will make a recommendation, while the motor is apart, even if you have to pay for the parts, have the Fel-Pro aluminum lower intake gaskets put in (the original plastic/nylon is junk).

Best of luck and let us know how this shakes out.

 

Well, the mechanic sat on it for a week and did some tests. After six days, they said it was the thermostat causing the issues. They replaced it and everything was "fine" for a while. The car didn't overheat, but was running a bit warmer than before. Before all this started, the temperature gauge stayed at about between the 3/4 and the 1/2 hash marks. After I got the car back, it would run just barely below the 1/2 mark or barely above. Some times it would raise a bit higher, but then I'd hear the fans kick on and it'd drop back to the new normal temperature. Didn't overheat in idling, didn't have any bubbling in the overflow tank, no signs of coolant in the oil, no white smoke and no rough idling -- even at start up. So I thought it was fixed.

Tuesday, I was going down the road in traffic, doing about 40 and it spiked. I pulled over, let it cool down and it kept running pretty hot -- but not red lining -- as I drove it back to the mechanic. So after sitting on it most of the day yesterday, they say the coolant isn't pushing through the system. They said it's either a bad water pump or a radiator. There's no external dripping and I haven't noticed any leaks and I haven't heard any noise from the water pump, so it can't be that, can it?

 

Just on first thought, if you are asking us, I assume you question your mechanic's diagnosis. If you don't trust the mechanic, find one you can trust.

This is from my experiences (and I'm NOT a mechanic, just a guy that has works on a bunch of 3100/3400 engines):
Usually if a pump goes bad on these cars, they leak coolant. If they seize or start to seize there would be other issues and noises (like a pulley scraping or belt rubbing).
You could have gunk built up in the system. Before chucking money at radiators or other parts, I would grab a bottle of coolant flush additive and follow the directions and then do a coolant flush and re-fill with good coolant if I felt my issue was a blockage in the system.

Again, what you are describing, I went through with my old '94 Grand Am. No leaks, passes a radiator pressure tester, I could drive it a couple of days and no problems, but when it began and gave me the first signs of an issue, my temp would climb out of no where. Everything else initially was fine, no issues.

I actually carried a ratchet with the socket to open the bleed screw on that car (I'd pull over, leave the car run). And I was in dis-belief at how many times I kept "bleeding" air off, it made sense later when I found it had a bad head gasket).

This stuff can be obnoxious to troubleshoot when it's not consistent.

 

Run a compression check on the heads!

Check to see if it has plastic coolant elbows. They crack off inside the tensioner on the 3.8 and in the manifold. Check the heater core hookups to the block they are plastic on the 3.8. You may be into the dreaded intake manifold gasket failure on the lim and the uim.

My uim gaskets were failing but the lim gaskets were really bad, coolant was being ported into the cylinders and burned. A sweet plasticy burned smell, as my wife put it. There were also stings of stuff in the uim where the coolant and oil coagulated going into the cylinder heads. The inside intake bolts had a sealant failure so oil was being sucked into the uim along with the coolant. Oil does not make long black fine hairy things, got to be the mixture of the two. Surprise, surprise, no coolant in the oil.

D&S Engine Specialists

A site fyi buddy, good luck, my wife's 3.8 is still torn down in the garage. Had to get a junkyard intake where the coolant and oil mix ate holes in the aluminum.

 

So not to confuse the subject, the original poster informs they have a 3.4/3400 engine. This engine does not have coolant elbows. That is exclusive to the 3.8.

 

Thanks for the info, learn something new every day here.