Sludge in Radiator NOT from Crankcase? [1996 Buick Century]

I recently noticed this terrible flaky-burnt sludge around my radiator cap (see photo) and the fluid dark and oily. This, after running my radiator a few weeks (one-mile daily trips) with about 25% Vinegar to Water (no Coolant). The car purrs like a kitten and NO water in the crankcase or discolored oil or even any apparent oil loss. I am second owner and have only put 2,000 miles on it since buying it. I am suspicious perhaps first owner put oil in the radiator and its only now surfacing after the vinegar-treatment. What is most suspicious however is the burnt-flakes, like that which you’d find inside a cyllinder head that is burning a lot of oil. Yet the engine doesnt idle roughly so no apparent cyllinder damage. So if it was caused by previous owner, they would have to pour dark sludgy oil in the radiator (not new oil). I dont know what to make of this.

A. Clean Crankcase Oil,
B. No Measurable Oil Loss,
C. Smooth Engine,
D. Recently Purchased,
E. Seldom Driven,
F. Burnt-Flaky Sludge in Radiator,
G. Vinegar Treatment for Two Weeks with car mostly sitting off.

Seen this before? What is your best guess as to the cause?

Best guess? There’s no telling what the previous owner dumped into the radiator. Possibly some stop-leak product, either commercial or home-made. You are probably seeing the results of his quirky effort.

Rather than ponder the source of the contaminant, your only thought should be to get the cooling system cleaned out and filled with the correct antifreeze mixture.

Go to the shop and request a power flush. Show them the problem first so they don’t assume they can get by with a simple drain-and-refill.

Do this ASAP. Then monitor all fluids, including transmission fluid, for anything that seems abnormal.

Do a google search for “dexcool, V-6, intake manifold gasket” and you
’ll have some good reading to do. You probably need a lower intake gasket, coolant flush, and you’ll be all set.

BTW, the replacement intake gaskets are steel and will not crack or break like the original plastic ones.

Vinegar?? What were you hoping to accomplish? Vinegar is very acidic and will react with the metals it comes in contact with…It will not remove grease and or oil…So flush it all out, fill with the recommended coolant and drive on…

I have never heard of anyone using vinegar in their cooling system, but it seems like a bad idea to me. Vinegar is acidic and engine coolant needs to be alkaline or it will eat the engine, radiator, and heater core apart from the inside out. Your best bet now would be to consider replacing the lower intake gaskets if you have the 3.1L V6 (a possible, and likely, source of this oil contamination), buy a flush and fill kit (like this: ), and hope for the best.

I do some zinc plating of old parts at home and my wife gets all over me about the smell from a measly 1/2 gallon of vinegar based electrolyte so I can imagine what several gallons of solution can do the sense of smell.

Anti-freeze also has a certain amount of lubricity to it so you might seriously be considering a new water pump and thermostat along with flushing all of that garbage out of the cooling system.
You might check the transmission fluid level while you’re at it too…

I have had a lot of success using Cascade dish washing liquid to clean out radiators when oil or transmission fluid had contaminated the coolant. And the crud most certainly appears to be some combination of stop leak products and rust.

It looks like this radiator has had zero maintenance over its life. The crud, as other point out, is likely stop leak residue.

A thorough flush is indicated, together with a check for leaks, since all the loosened crud will reval any leaks in the radiator. A new radiator may very well be needed.

Make sure you go to a good specialty cooling system shop.

“one-mile daily trips”

These short trips are very hard on a vehicle.
You should be changing the oil at least twice a year regardless of mileage.
It would also be a good idea to get a small automatic battery charger and charge the battery overnight every month or two.

Back in the day ‘a friend’ would clean out cooling systems with lye, which is a base, not an acid like vinegar.