Why does the engine lose power under prolonged load?

When a customer rolls into my shop complaining their engine feels like it’s “running out of breath”—especially going uphill or trying to merge onto the highway—my first thought usually isn’t a cracked engine block. Most of the time, that kind of power loss under load, the hesitation and sluggish acceleration, comes down to simpler stuff: a clogged fuel filter, a weak fuel pump, a dirty MAF sensor, or even a restricted catalytic converter. These are all common issues, and they can definitely make your car feel gutless.

But here’s the thing: sometimes, it’s something far more serious. I’m talking about a compromised engine block. Unlike a blown head gasket or a failing sensor that might give you some warning, a crack in the engine block itself creates internal leaks that mess with combustion, contaminate your fluids, and accelerate wear at an alarming rate. These aren’t problems that just “come and go.” They get worse, and fast. If you ignore them, you’re looking at total engine failure. The trick is knowing when this rare, but absolutely critical, condition is the real reason your engine’s struggling.

A cracked engine block means the very core of your engine is leaking, and it’s usually leaking internally. We’re talking about the sealed systems—your coolant circuit and your oil galleries—getting breached. When they cross-contaminate or lose pressure, the symptoms are pretty distinct from your typical drivability problems. And believe me, you don’t want to miss these.

Unexplained Coolant Loss and Sweet-Smelling Exhaust

The first big red flag I look for is coolant disappearing without a trace. You’re topping it off every week, but there’s no puddle under the car, no obvious hose leak. If that’s happening, and you’re also seeing thick, white exhaust smoke that smells distinctly sweet—like maple syrup—and it keeps coming even after the engine’s warmed up, then you’re almost certainly burning coolant. Now, a cracked cylinder head or a blown head gasket can cause this too, but if those check out, the problem is deeper, right in the block.

Milky, Foamy Oil on the Dipstick

Another strong indicator, and one that sends shivers down my spine, is when the engine oil looks like chocolate milk or a milky foam on the dipstick or under the oil filler cap. That’s coolant mixing with your engine oil, and it’s a death sentence for your bearings. I’ve seen engines seize up fast when this is ignored. Sure, a failed oil cooler or a head gasket can cause this too, but if those have been ruled out, the block becomes the prime suspect.

Combustion Gases in the Cooling System

For me, the most definitive test is checking for combustion gases in the cooling system. We use a “block tester” or “combustion leak tester” for this. It’s a simple chemical test: you put a special fluid in a tool over the radiator opening, draw air through it, and if the fluid changes from blue to green or yellow, you’ve got exhaust gases in your coolant. That means combustion is escaping into the cooling system. But here’s my critical caveat: before you condemn the block, you absolutely need to confirm the head gasket is intact using a cylinder leak-down test. If the gasket holds pressure but you’re still getting gases in the coolant, then that leak path is through the block casting itself, or sometimes a very subtle head crack.

This is where experience really comes into play because, honestly, the symptoms for a cracked block, a cracked cylinder head, and a blown head gasket can overlap like crazy. Your job as a tech, or even as a savvy DIYer, is to methodically isolate exactly where that breach is happening. You don’t want to pull a cylinder head only to find the problem was deeper in the block.

Engine blocks are the foundation of your engine, designed to outlast the rest of the vehicle. So when they fail, it’s typically due to some pretty extreme conditions or, occasionally, a manufacturing flaw. In my 25+ years, I’ve seen a few common culprits.

1. Severe Overheating

This is probably the biggest cause. A single, really bad overheating event—say, a stuck thermostat, a collapsed radiator hose, a failed water pump, or just running critically low on coolant—can create massive thermal stress. The rapid expansion and contraction, especially in the narrow webbing between cylinders, can cause the cast iron or aluminum to crack. Aluminum blocks are particularly susceptible to this.

2. Casting Defects

Sometimes, it’s just bad luck. Manufacturing defects like porosity, sand inclusions, or thin spots from improper core alignment during the casting process can create weak points. I’ve seen certain aluminum V6 and turbocharged engines, especially some older European models, where thin cylinder liners or voids eventually develop into pinhole leaks. These are tough to diagnose because they can be slow to show up.

3. Incorrect Coolant or Neglect

Using the wrong type of coolant, especially in aluminum or magnesium-alloy blocks, is a slow killer. It leads to galvanic corrosion, where dissimilar metals react and literally eat away at the casting from the inside out. I’ve seen this more than once in European imports where non-spec coolant was used for years. Eventually, you get pinhole leaks in the block that are almost impossible to fix.

4. Catastrophic Internal Failure

This is the most dramatic. A spun bearing that locks up the crankshaft, or a connecting rod that breaks and “throws” itself through the side of the block—these are instant, violent failures that physically fracture the block from the inside out. There’s no mistaking these; you’ll hear a loud bang and see a big hole.

Let’s be clear: a cracked engine block is almost always bad news. Unlike a simple gasket, you’re dealing with the core structural component of your engine. Your repair options are usually limited, and often, they’re not cheap or permanent.

You can’t just cross your fingers and hope a repair held. Validation is absolutely critical, especially with something as serious as a cracked block. You need to confirm the fix actually worked.

• For an external epoxy patch: After the epoxy has fully cured (give it at least 24 hours), the cooling system must hold pressure at the radiator cap’s rated level (typically 13–18 psi) for at least 30 minutes with zero seepage. Then, drive the vehicle through at least three full heat cycles—cold start to operating temperature and back—and recheck the area with UV dye and a black light. Any tiny weep will show up.

• For a freeze plug replacement: Perform the same cooling system pressure test. Then, drive at least 50 miles under normal conditions before re-inspecting the area thoroughly. Any sign of moisture means the bore seal failed, or the bore itself is compromised.

• For a full block or engine replacement: This requires the most thorough validation. After installation, I run the engine under varying loads for at least an hour. Then, perform a combustion gas test on the coolant. A negative result—confirmed with hydrocarbon test strips showing less than 0.1% contamination—is the gold standard. You also need to closely monitor oil condition and coolant levels for the first few hundred miles. I’ll often do an early oil change just to be safe.

Alright, let’s talk real money. Repairing or replacing a cracked engine block is almost always a major expense. The decision often boils down to the vehicle’s overall value and your attachment to it.

Most block cracks are preventable. It really comes down to basic, consistent maintenance. The foundation of protecting your engine block is a healthy, well-maintained cooling system.

• Use the Right Coolant: Always, always use the coolant type specified by the manufacturer. This is especially critical for modern aluminum blocks, where incorrect coolants can trigger galvanic corrosion and gradually eat through the casting. Mix it at the correct ratio (usually 50/50 with distilled water) to ensure proper boil-over and freeze protection. And replace it at the recommended interval—it loses its protective properties over time.

• Address Overheating Immediately: Never ignore cooling system issues. A thermostat that sticks closed, a water pump with a weeping seal, a clogged radiator, or even a tiny leak can all lead to overheating. And remember, one serious overheat event is often all it takes to crack a block or warp a cylinder head. Pull over, shut it down, and get it towed if the temperature gauge spikes. You can learn more about the risks of overheating here: Can Engine Overheating Warp a Cylinder Head?.

• Regular Visual Inspections: Monthly, when you’re checking your oil, grab a flashlight and scan the block surface. Pay close attention to areas around freeze plugs, engine mounts, and where the cylinder head meets the block. Look for any fresh stains, rust trails, or crusty, colored deposits. These are early signs of a leak.

• Monitor Fluid Levels and Condition: Never ignore unexplained fluid loss. Whether it’s coolant disappearing or oil looking milky, early detection is absolutely key. Use coolant test strips every 6–12 months to check pH and additive strength.