Seized Engine?

170,000 MILES • 3.8L • AUTOMATIC
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STAN FREE
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I just got a 96 V6 Mustang pretty much got it as a project. The guy got it from said the motor was locked up, but it was extremely old and didn't specify why it was locked up. The hood was left up when I first seen the car so I'm guessing it was like that for a while. My question is how do I remove all the rust that is down in the motor? Without pulling the motor if it's possible. I've already took out the spark plugs drain the oil. I got the belt off and it won't turn my hand. It's solid. I don't have the money right now to do the obvious fixes. Like replacing it or taking it to a shop for a rebuild. I just want to know if there's any kind of something that I could do to save it myself thanks.
Oct 20, 2026 at 1:14 PM
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CARADIODOC
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Actually, this is more common than you'd think. The first thing to try is pouring a little diesel fuel in through the spark plug holes and let that sit for a few days. Unlike gasoline, diesel fuel is an oil with some lubrication properties, and it can free up rusted parts like piston rings. I've seen people smarter than me use a finger on the end of a drinking straw to get the oil in the holes. A couple of inches up the straw should be enough in each cylinder. Don't get carried away thinking "more is better". Too much liquid in a cylinder will cause hydro-lock. That's when there's too much liquid and no air that can be compressed. Hydro-lock when trying to start an engine will prevent the starter from being able to spin the engine. When catastrophic hydro-lock occurs in a running engine, such as with a sudden major leak in a cylinder head gasket, can cause a bent connecting rod.

The high mileage can actually work in your favor. With the extra wear on the cylinder walls and piston rings, rust has to work harder to make those parts tight, and there's a better chance of the diesel fuel being an effective solution.

Once the engine does free up, the diesel fuel will lubricate the cylinder walls. That aids in sealing the rings for higher compression to build up early on. Left dry, if you do get the engine to rotate, it can take some time to build enough oil pressure to get cold oil to squirt onto the cylinder walls. We'd like to avoid those dry cylinders for the first minute or two. By that time, warmed oil will be happy to spray up there to lubricate the pistons and rings.

It's also surprising, if you do have to pull the engine apart, that the rust on the cylinder walls where the rings were stuck actually cleans up pretty well. It's not uncommon to find new pistons, new rings, and machining cylinders are not necessary. Just clean everything up, lightly hone the cylinders, then stuff everything back together with normal Engine Assembly Lube.

There's no problem allowing diesel fuel to seep past the piston rings and into the oil, but it would be a good idea to change the oil soon after the engine gets running, especially if you don't know the previous maintenance history. Additives in the oil break down, typically after roughly 3,000 miles. We want to get those additives replenished. That includes detergents that dissolve gunk, dispersants that carry that dissolved gunk to the filter, including iron oxide, (rust), seal conditioners, and anti-foaming agents that reduce the development of compressible air bubbles in the oil at higher engine speeds.

Before you try cranking the engine, check all over under the hood for acorns or other signs of a mouse nest. Check around the air filter too, on both sides of it. Years ago, with engines with carburetors, it was common to walk away in frustration and leave the air filter housing laying off to the side in a smoldering pile of tears. Somehow that seemed to advertise to mice that this was their new vacation home. You don't want those shells going through the engine, or plugging the muffler or catalytic converter.

Please keep me updated on your progress.
Oct 20, 2026 at 6:06 PM
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STAN FREE
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thanks for the reply. After rereading my message, I realize my audio picked up my words wrong, so it may have been a little confusing, but I think you got the gist. I'll definitely try the diesel. I have the top of the motor pulled off so I can see where the push rods connect to the engine, there is significant rust. sorry very new to working on cars so I'm not sure what everything is called. i'm gonna see if I can get that rust out of there as much as I can. Any tips at all I would gladly take them. Oh, and for sure mice had gotten into the car before I got it. Seemed like they were quite comfy too. Must've been there a while. I've got most of the wiring fixed from there a little snacks I'm guessing.
Oct 27, 2026 at 10:46 AM
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CARADIODOC
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I'm told the insulation on wire is being made with some kind of a soy product, so mice love to chew on it. Guess the engineers never lived in the real world where we have mice.

When you see scale and rust that you can wipe off easily, in chunks, with your fingers, that stuff will dissolve away if you try to collect it. That makes it more aggravating to clean up. I'd start with a wet / dry vacuum and fashion an extension hose with a really small diameter, like a piece of vacuum or heater hose, so you can stick it into tight places. Once the bigger pieces are gathered up, use Brake Parts Cleaner or Carburetor Cleaner in a spray can to wash the rest down. The goal is to get it to run down the oil return passages, into the oil pan. From there, it can be drained out when changing the oil.

Brake Parts Cleaner takes longer to evaporate, but both products should not be left in the engine as they not only don't lubricate; they remove the oil that has already left a lubricating film behind. Don't crank the engine, then no cleaner will go up the oil pickup tube and into the oil pump. With the drain plug out and the valve covers removed, the cleaner can evaporate out overnight. What little that might be left inside will get pulled out through the PCV valve and burned. Once the engine is running, any remaining rust deposits will gradually be removed by the detergents in the oil, then carried to the filter by the dispersants. Those are two of the additives in engine oil.
Oct 27, 2026 at 6:40 PM
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