The car is a 2003 Monte SS. Previously we had a problem with the cooling system being completely clogged up. Mechanic completely flushed out the engine and replaced the radiator. Did this just over a year ago. Just recently the car overheated. Once again the colling system is completely clogged. Likely caused by mixing dexcool with regular antifreeze. Mechanic says only way to fix is to replace everything, including the engine. Anyone else have this experience? We already brought a replacement car so this is either for sale as is or being parted out.

 

Did the engine overheat to the point of failure? If not, you might just need intake gaskets. I had my ENTIRE cooling system clogged at one point (never overheated tho) and it was pretty easy to flush. The dexcool eats the stock gaskets and introduces oil into the cooling system. Sometimes it will put antifreeze into the oil but thats less common.


EDIT: It wasnt clogged but rather completely full of sludge.

 

I agree, unless the engine actually broke as a result of overheating, I sure wouldn't replace it.

IMO it seems like the mechanic got you into this bad situation. It sounds like they didn't flush the engine well enough when doing the previous job. If they werent confident they got everything, they sure shouldn't have put dex back in. Id have used that universal coolant just to avoid any risk.

Id take it somewhere else, get it flushed / cleaned as best they can and refilled with the universal coolant. Maybe the radiator is too gummed up to save, but I sure wouldn't think it would be that bad if they did even a half hearted job at flushing the engine out last time. If its this gunked up again, they either didn't flush the block at all or didn't bother to clean the heater core loop.

 

i ran mine with distilled water and water wetter for months, with one or two flushes between my dexool to green antifreeze. No issues.

 

Like I always tell people, "Why pay some inept wrench turner to break something, then claim it was that way when I brought it in, when I can brake it myself for free and learn something along the way."

The composition for Dexcool orange is ethylene glycol with diethylene glycol as the antifreeze constituents with sodium 2-ethylhexanoate or potassium 2-ethylhexanoate added as a corrosion inhibitor. Conventional green Prestone is ethylene glycol with diethylene glycol as the antifreeze constituents with sodium nitrite as a corrosion inhibitor. The coolants are not chemically incompatible with each other so the monkey you took it to is wrong, but, I'm only a degreed chemist with more than thirty years in the chemical industry, so, what do I know?

However, I cannot dispute wht02monte's claim that the Dexcool, or rather the corrosion inhibitor used therein, causes issues with gaskets, that is, I do not know enough to say one way or the other. But I can tell you from personal experience that the times I've seen a cooling system clogged with what appears like brown gelatinous goop is when oil gets into the cooling system. The most common source of oil leakage is via the automatic transmission cooler inside the radiator. Have you looked at your transmission fluid? If, when checked at temperature while the engine's running, it's foamy, you've got coolant in your transmission and transmission fluid in your cooling system. If that's the case, you'll need to get the transmission redone and replace the radiator.

Assuming it's not the transmission and the car's broken, the worst case scenario is; after whatever repair you do, the car's still broken, right? So, may as well see this as an opportunity.

If this were my car (mind you, I have an uncommon fondness for, and attachment to, my 1974 Monte Carlo — by the way, this process outlined hereafter assumes a small block Chevy engine), I would take the radiator to a reputable radiator shop for a proper cleaning or, if damaged beyond repair, re-core job. If you need a re-core, go for broke and get a four-row high-flow core. Meanwhile, back at the ranch, to flush out the cooling system, remove the thermostat housing, remove thermostat, then replace the thermostat housing. Move and orient the car to where you won't flood your own garage when you flush the cooling system. This will get VERY messy. Get a 55-gal drum; plastic would be ideal because it's much lighter than steel, easier to drill, and much easier to see the fluid level. Make a fixture to fit the bung that will fit a radiator hose. This won't need to be elegant because this is the low pressure side of the water pump. You could just slap something together using epoxy and PVC. If the drum has a vent hole on the lid, open it, otherwise, you'll need to drill a vent hole. Get two really long large diameter hoses, the longer the better. They'll need to be large enough to fit the thermostat housing and water pump fittings. Orient the drum on it's side with the bung, where the hose will be attached, at the bottom. The drum should be elevated to where bung hole is about the same height as the radiator output. Attach the hose from the water pump to the 55-gal drum which you've filled with water. The other hose attaches to the thermostat housing and goes to wherever you want spent water to go. Turn the heater on to full-blast. Start the car and keep an eye on the output hose. If water starts flowing, up the RPMs to ~2000 and you should see all the gunk spewing out the hose. Continue to keep an eye on the hose output and the water level in the drum! Once the drum's empty, shut off the engine. Based on your observation of the appearance of the output water, make your own judgement on weather to repeat or not. Once you're satisfied the system's clean, put it all back together with your clean/re-done radiator and new thermostat.

If water doesn't start flowing out the hose attached to the thermostat housing within the first minute, shut off the car; don't want to cook the engine. You will need to tear appart the engine to manually clean as much gunk as you can from all the cooling passages. I never said this would be easy. After you put everything back together, do the method described above to flush out the rest of the gunk.

I'm compelled to reiterate that this is what I would do with my car. I've driven my '74 Monte Carlo as a daily driver since 1984 so, no matter what, I will put in whatever effort to keep her on the road. You are, of course, free to proceed however you see fit.

 

That is the one benefit the OP has here, theres a zillion good 3800s in every junkyard so if it really was to this point it would need torn down for that level of cleaning, it would only be a couple hundred bucks for a replacement to get out of that level of work.