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Brown sludge in cooling system, 1999 GM 3.4L engine

While driving my recently purchased minivan (a 1999 Pontiac Montana) for about 40 miles, I noticed the temperature gauge was reading high when I wasn’t moving very fast, though short of what appears to be the overheating zone. Opening up the radiator revealed no coolant that I could see, but plenty of brown sludge. Looking under the oil cap revealed more brown pudding-type stuff (that wasn’t there when I did an oil change when I first got it).

The lack of coolant makes me nervous that the engine might have overheating without the temperature gauge reading it properly. Is that possible? The heater was working, which searches seem to suggest means there must have been some in there.

I figure that either the intake or head gasket must be bad, so I’m nervously setting out to replace both using a gasket set like this:

What I’m more worried about though, is clearing the cooling system out of all the gunk. It sounds like oxalic acid and washing soda and backflushing might work? But I’m anticipating a large quantity of sludge, oily, rusty coolant water. How am I going to dispose of it all?

Also, would it make sense to preemptively replace the water pump, and install it after flushing the system?

I wouldn’t use anything, just drain the radiator and block, then refill with 50/50. Plan on repeating as soon as the new coolant becomes cloudy. Chemical flushes will damage the rubber components of the cooling system.

On the second drain and fill, if the hoses look original, then I would pre-emptively replace all of them. A new thermostat and radiator cap would also be in order.

GM used a,coolant that I think was called Dexcool or something like that. If contaminated, this coolant was known to turn to sludge. My 2006 Chevrolet Uplander came with Dexcool. When it was time to change coolant, I had the system completely flushed and went with regular antifreeze.

I run DexCool in all my GM cars that specify it. I have never had a problem in hundreds of thousands of miles and many years.

As with any coolant, it does need to be changed every few years.

How is the engine oil? Is the level good? Does it look like pure oil and not like a milkshake?
How is the transmission fluid? Is the level good? Does it look like clean, red/pink fluid and not like a milkshake?

Leaks in head gaskets, oil coolers, internal radiator, etcetera, can cause cross-contamination and other problems.

It could just be that the coolant has not been touched since the vehicle left the factory. Any coolant would go bad in that time.

Dex-Cool, or as us in the business call it, Death-Cool or Dex-Kill, is an Organic Acid Technology antifreeze. This means that if air enters the cooling system, the antifreeze reacts with the air and turns acidic. This acidic antifreeze then corrodes the inside of the engine and you end up with this brown sludge of rust and gelled antifreeze.

It’s possible for the engine to overheat and not show it on the temperature gauge because this sludge coats the tip of the temperature sensor, and that gives a false reading of the actual coolant temperature.

From what you’re describing, I’d stick a fork in it.


The oil level is fine. It doesn’t look discolored that I can tell, but there is pudding-type stuff under the cap. The radiator fluid is a bit dark, but doesn’t look contaminated.

Tester, are you saying it’s not worth trying to change out the gaskets and clean the cooling system?

Let me ask you this simple question.

How are you going to clean the sludge from out of the engine block?

Because that’s were the rust originated from.


My plan was to get as much out as I can manually when I have the engine taken apart, and then use the oxalic acid and sodium carbonate (which is what GM recommended for the sludge at some point) to hopefully get what’s left.

Good luck!


I bought a handicapped conversion van, 92 Chevy 20 a few years ago for my father in law. The coolant looked like brown sludge. I did just what I described above and didn’t have any more trouble. I used a universal long life coolant.

Mine was a little tricky though, it had a rear heater core and that was not easy to drain.

BTW, I would not change any gaskets, that is asking for trouble if you are not doing a complete overhaul or rebuild.

Keith, what sort of trouble?

Once you start opening something up, you often find things that you don’t expect. Also things don’t always came apart as expected and break so that has to be addressed.

But go ahead and find out for yourself.

I would take a moderate gamble

Do NOT touch the heads

Replace the intake gaskets, cap and thermostat. Felpro makes pretty decent gaskets, for example. If you buy the kit, it’ll probably come with the gaskets and the intake bolts, which I would also recommend replacing

Drain the radiator AND the block, if you can access any block drains

Flush the radiator with a simple hose

Refill with 50/50 mix of dexcool and water

You heard me right . . . we have tons of GM vehicles in our fleet. The only coolant problems I see on ANY vehicles are those where the coolant system has been neglected for 10 - 15 years or more

I don’t have a problem with dexcool

BTW . . . I would advise you to do all this work YOURSELF

if you pay somebody to do it, then it’s not worth it, and you may as well shop for another vehicle

it’s only worth it if you do it yourself

I’m always inclined to do mechanical work myself anyways, but it’s especially necessary for me since I’m extremely tight on money. That’s also the reason I don’t want to give up on it, I’ve already invested too much money in buying it.

From what I understand, the intake gaskets are more likely the culprit in this particular engine. But I also heard that you might as well replace the head gaskets if you get that far anyways?

It has aluminum heads, you really don’t want to disturb the original head gaskets if you don’t have to, but go ahead, you seem determined. BTW, the brown sludge under the oil cap is from condensation.

I’m determined to try to fix the engine, but not to replace the head gaskets if it’ll make it worse. Since I’ve changed the oil, I’ve been on one short trip to get it inspected, and then one 40 mile trip, which is why I thought it’s probably not condensation.

What happens with disturbing aluminum heads? I guess I figured it would also be a good time to inspect them for warpage or cracks.


leave the heads alone

Tester: That definitely describes the condition of the engine. I specifically went looking for a non-2eh oat coolant for my bike, but now I’m wondering if I shouldn’t switch it to conventional. The different OAT could be just as bad for all I know.

db4690: But… what might go wrong with taking off the heads?