Antifreeze leaking from muffler and knocking noise

The first thing you need to do is determine where the coolant is leaking from. Until we know that, it is very unlikely it was caused by changing the oil.

Different feller,

And you checked the oil dipstick, when you picked it up at the oil change place and again after this situation?

The Medic

It is very likely your car is going to develop a coolant leak at some point in its life, but you have no way of knowing when. Naturally when that occurs you want to look at the car's history to see if there is a cause that might be prevented from happening again. There are dozens of things that happened in the last four days, including changing the oil, but there is no way to lay blame on any of them until we know what is wrong.

What concerns me more is the knocking sound. That is indicative of serious engine problems and is very unlikely to be caused by low coolant unless you overheated the engine the night before. Then the legitimate question would be if the mechanic checked the coolant level as part of the oil change service. Typically they top off all fluids other than brake fluid, and they document anything they find wrong including a really low fluid.

There is another problem related to the cooling system that only affects GM vehicles. That is their use of the red Dex-Cool antifreeze. On the cars it says to replace it every three years, but they advertised it as "lifetime" antifreeze to make their cost of maintenance appear lower than that of their competitors. Even the Dex-Cool company says to replace it every two years because that is when the additives in it wear out. Those include water pump lubricant and corrosion inhibitors. Even if the antifreeze is replaced every two years, acids form in the coolant that do not get neutralized, and that can lead to corroded and leaking radiators and heater cores. As I said, this is real common on GM vehicles. You might notice an oily film on the inside of the windshield that gets slowly worse over many weeks, from a leaking heater core. You could have a sudden cracked radiator. You could have a corroded cylinder head gasket that is going to corrode all the way through in a week, but one brief burst of wide-open-throttle builds enough pressure to make that leak occur sooner. That type of leak was non-existent one minute, and serious the next minute, and is just the nature of the way this type of problem occurs. It can have nothing to do with anything that was done to the car. This is why you have to determine exactly what is leaking before you can figure out why, and potentially lay blame.

If you cannot find coolant leaking under the hood or onto the ground, a leaking head gasket is the best suspect. Looking for water dripping from the tail pipe is not a good observation because one of the byproducts of a properly-working catalytic converter is condensed water vapor, so that is normal. However, if you see red water dripping, it is a leaking head gasket. If that is bad enough it can cause knocking in the engine that is otherwise not harmful, and will go away once the repairs are done.

You can also add a small bottle of dark purple dye to the coolant, then search a day later with a black light to find the leakage. The dye will show up as a bright yellow stain that you can follow back to the source. If the head gasket is leaking, you will find the dye inside the tail pipe. Auto parts stores will have the proper dye for the fluid being checked, and those that rent or borrow tools should have a black light.

Just to add to this one if you have coolant pouring out of the exhaust then sounds like a blown head gasket or a cracked head most likely one of those two things. Also the knocking noise is probably coolant entering the combustion chamber causing a knock. Coolant does not burn/explode under heat and pressure like gasoline.

Just so I am not confused and make you confused, Dex-Cool antifreeze is red, not green like we are used to seeing. A lot of mechanics call it "Dex-Mud" and want to get rid of it. If someone did get it out of there, they most likely put some in that is green.

Remember, there is supposed to be a little water dripping from the tail pipe, and there is usually a tiny drain hole at the rear of the muffler to reduce its tendency to rust out from that water, but we are talking little drips, not pouring out. It does sound like a head gasket is leaking, and that correlates to my original story that it can happen anytime and it has nothing to do with the recent oil change, or any other service, or driving too much after dark, or too many left-hand turns. Wait, that was too many right-hand turns!

Probably not the best time for humor, but corroded / blown / leaking cylinder head gaskets are a common occurrence on every car brand and many engine sizes. At the mileage you listed, some cars would be getting repaired for a head gasket a second time.

Replacing head gaskets is a fairly expensive job, but look at it as two more car payments. It is a lot less expensive than buying a different car. This job should be left to an experienced engine specialist. I love working with do-it-yourself-ers and backyard mechanics, but this is one job you do not want to have to do over if you mess it up the first time. A conscientious mechanic who has your best interest at heart will recommend replacing the timing chain and tensioner at the same time to avoid problems in the future and another expensive repair.

Yep you have a blown head gasket, the head will need to be removed and the gasket replaced, it had nothing to do with the oil change, just bad luck, sorry.

Best, Ken

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