Overview

Testing for a bad engine in your vehicle is a crucial diagnostic process that can help identify whether your car's powerplant is suffering from catastrophic failure or more subtle issues leading to inefficiency and potential breakdowns. This article provides a step-by-step guide to performing visual inspections, electronic tests, compression checks, and interpreting results, with a focus on common problems in engines like those found in popular models driven on Australian roads. By following these methods, you can determine if repairs or replacements are needed, potentially saving time and preventing further damage under harsh Australian conditions such as extreme heat and dusty outback drives.

There are few things more gut-sinking than that moment when your car starts acting up and you think, "is the engine actually cooked?"

Whether you're daily-driving through Sydney traffic, hauling gear across the Nullarbor, or just trying to keep your old ute on the road for another year or two, a healthy engine is what everything else depends on. Without it, you're not going anywhere.

The tricky part is that engines don't always fail in obvious, dramatic ways. Sometimes they go out with a bang, literally. Other times they just quietly deteriorate over months or years until you're burning through oil like it's going out of fashion and wondering why your fuel bills keep creeping up.

Knowing how to actually assess what's going on under the bonnet, rather than just guessing or throwing money at a mechanic hoping for the best, puts you in a much better position to make smart decisions about your vehicle. This guide walks you through the whole diagnostic process from start to finish. You don't need to be a trained mechanic to follow most of it, though some steps do require a bit of mechanical confidence. Even if you end up handing the job off to a professional, understanding what they're checking and why means you can have an informed conversation about your options rather than just nodding along.

Understanding How Engines Actually Fail

Before you start poking around, it helps to understand the two broad categories of engine failure. They behave very differently and knowing which one you're dealing with shapes everything that follows.

Catastrophic Failures

These are the ones nobody wants to experience, but at least they leave no room for doubt. A catastrophic engine failure is usually sudden, loud, and terminal. You might hear a deep knock or a sharp bang, and then the engine either seizes up completely or continues running so badly that you know something is seriously wrong. In the worst cases, you'll find metal fragments in the oil, large external leaks, or even parts that have punched through the engine block.

Common causes include running the engine dry of oil, which is more common in Australia's hot climate than people realise. A quick stop at a servo that takes longer than expected, a slow leak you hadn't noticed, or simply falling behind on servicing in the middle of a long road trip can all lead to oil starvation. Bearings fail first, then rods, and if things get really bad you end up with what's sometimes called a "thrown rod," where a connecting rod breaks loose and damages everything in its path.

Overheating is another big one. Blown head gaskets can allow coolant to leak out without any external sign, and if the temperature gauge isn't watched carefully, you might not realise until the engine has already cooked itself. On long drives through inland Queensland or the Northern Territory, where ambient temperatures are brutal and the engine is working hard, this is a genuine risk.

The good news, if there is any, is that catastrophic failures are usually obvious. The bad news is that by the time they happen, major engine work is almost always required.

Soft Failures

Soft failures are sneakier and arguably more frustrating, because the car keeps running, just not well. The engine isn't dead, but it's not healthy either. Performance drops gradually, fuel economy gets worse, oil consumption creeps up, and you might notice the car struggling in situations where it used to feel fine.

These failures often stem from worn piston rings, damaged valves, or degraded seals. The compression inside the cylinders drops, which means the air-fuel mixture isn't being squeezed tightly enough for efficient combustion. You might see blue-grey smoke from the exhaust on startup or under acceleration, which is a classic sign of oil burning. White smoke when the engine is warm can point to coolant getting into the combustion chamber, which is a different problem but equally serious.

Some modern engines, particularly certain V6 units found in Australian-market SUVs and people movers, use variable cylinder management systems that deactivate cylinders under light load to save fuel. When these systems malfunction, they can cause misfires, uneven power delivery, and accelerated wear on the affected cylinders. The car might feel lumpy or hesitant at cruising speeds, which owners sometimes write off as a fuelling issue when the real problem is mechanical.

Soft failures are worth catching early. There's often a range of repair options available, from minor seal replacements to more involved head or ring work, that can extend the engine's life without needing a full rebuild or replacement. Leave them too long and they become something much more serious.

Step 1: Start With Your Eyes and Nose: The Visual Inspection

The best diagnostic tool you have at the start is careful observation. A thorough visual inspection takes maybe 20 to 30 minutes and can tell you a lot before you touch a single tool.

Engine Oil

Do this after the car has been sitting overnight, or at least for several hours. This gives any burnt oil time to drain back into the sump and gives you an accurate reading. Pull the dipstick, wipe it on a clean rag, reinsert it fully, then pull it again and check the level and condition.

A healthy engine should have oil sitting between the minimum and maximum marks. But what matters almost as much as the level is what the oil looks like and smells like:

• Fresh oil ranges from amber to light brown
• Old but serviceable oil goes darker
• Concerning oil is very dark, has a burnt smell, or has a sludgy, gritty texture

The other thing to check is whether the oil looks milky or frothy. This is a sign that coolant is mixing with the oil, which usually means a head gasket problem. Even a small amount of coolant in the oil can cause serious damage quickly, because coolant doesn't lubricate the way oil does. If you see this, treat it as urgent.

Also unscrew the oil filler cap and look underneath it. A little condensation is normal in some conditions, but a thick, creamy residue under the cap is another strong indicator of coolant contamination.

Spark Plugs

Spark plugs are brilliant little diagnostic windows into the combustion chamber. Removing and inspecting them takes a bit of time but gives you cylinder-by-cylinder information that's hard to get any other way.

Before you pull them, grab a piece of cardboard, punch some holes in it, and number each hole by cylinder. As you remove each plug, place it in its corresponding hole so you know exactly where it came from. This matters, because if one or two cylinders are showing problems, you need to know which ones.

A healthy spark plug should be dry, with a light tan or greyish colour at the electrode. Here's what the abnormal ones look like:

On a V6 where rear cylinders run hotter and are harder to service, oily plugs often appear on specific cylinders first, which helps pinpoint where the wear is worst.

Coil Packs and Ignition Leads

While you're in there, take a look at the coil packs and, on older vehicles, the ignition leads. Coil packs should be clean and free of visible cracks. Check the connectors for corrosion or heat damage. A cracked coil pack can arc to earth and cause an intermittent misfire that's maddeningly hard to pin down under normal driving conditions.

Ignition leads on older vehicles should feel firm, not brittle or cracked. Even a visual inspection of the outside condition tells you a lot about what's happening internally.

External Leaks

With the bonnet up and the engine cold, spend a few minutes looking for signs of oil or coolant leaks. Fresh oil looks wet and dark. Older leaks leave staining and dried residue. Check around:

• Valve covers
• The sump gasket area
• The timing cover
• Any obvious sealing surfaces

Coolant leaks can be less obvious because coolant evaporates, leaving a powdery white or pinkish residue rather than a wet puddle. Look around the radiator hoses, the water pump, heater hoses, and the area where the head meets the block.

Small external leaks don't necessarily mean the engine is in terrible shape, but they do mean it's losing fluids faster than it should and that components are starting to fail. They also tend to get worse if left alone.

Step 2: Electronic Diagnostics: Using a Scan Tool

If you have access to a scan tool, or can borrow one from a mate or a parts store that loans them out, this is the next step up in diagnostic capability. The OBD-II port on Australian-market vehicles is usually located under the dash near the steering column.

Reading Fault Codes

The first thing to do is pull any stored fault codes. The engine management system continuously monitors sensor outputs and flags anything outside normal parameters, including misfires, oxygen sensor faults, and fuel trim issues.

Don't just clear the codes and hope for the best. Write them down, look them up, and understand what each one means. A P0301 code, for instance, means cylinder one misfire detected. Seeing individual cylinder misfires is very useful information to cross-reference with your spark plug findings. Some codes are red herrings or secondary symptoms of something else, so look for patterns rather than treating each code as an isolated problem.

Cranking Compression Test via Scan Tool

Some scan tools, particularly the more advanced ones used by workshops, can perform a cranking compression test. This doesn't measure actual cylinder pressure, but it monitors crankshaft speed during startup. When all cylinders are working properly, the crank accelerates smoothly and evenly as each one fires. A weak cylinder creates a small hesitation or dip in crank speed during its cycle.

The software translates these speed variations into a percentage contribution for each cylinder, giving you a relative picture of how evenly the engine is performing. It's not a replacement for a physical compression test, but it's a quick, non-invasive way to flag a problem cylinder before you pull plugs and hook up gauges. Not all vehicles support this function, particularly older or pre-OBD-II models.

Live Data Monitoring

A good scan tool also lets you watch live sensor data while the engine runs. Key things to watch:

• Fuel trims — large positive numbers mean the system is adding fuel to compensate for a lean mixture (possible vacuum leak). Large negative numbers mean it's pulling fuel back because things are running rich.
• Oxygen sensor data — shows how exhaust composition behaves at various load points
• Coolant and intake air temperature — flags cooling or heat management issues
• Throttle position — helps confirm the engine is seeing the inputs it should be

Step 3: Power Balance Testing: The Hands-On Cylinder Check

A power balance test is a practical way to evaluate individual cylinder contribution without needing anything more than your hands and a bit of patience. The idea is simple: if a cylinder is contributing normally, switching it off temporarily causes a noticeable drop in engine speed. If a cylinder is already weak and barely contributing, switching it off makes almost no difference.

Do this test outdoors. You'll be disconnecting ignition or fuel components briefly, and you don't want to fill a garage with unburnt fuel vapour.

Older Vehicles: Disconnecting Plug Leads

On vehicles with distributor-based ignition and separate plug leads, pull one lead at a time off the spark plug, note the RPM drop, and move to the next.

• Healthy cylinder — significant and immediate drop in revs, often 100 to 200 RPM or more
• Weak or dead cylinder — almost no change in RPM

Be quick. Don't leave a lead disconnected for more than a few seconds per cylinder, because unburnt fuel can wash down the cylinder walls and dilute the oil.

Modern Vehicles: Disconnecting Coil Packs or Injectors

On modern coil-on-plug systems, unplug the connector from each coil pack in turn and observe the RPM change. Alternatively, if your scan tool supports it, some systems allow you to cut injectors individually through the software interface, which is cleaner and more controlled.

Be aware that many modern vehicles will trigger fault codes and may enter a limp mode when you do this. That's normal. Just clear the codes after you're done.

What the Results Tell You

If one or two cylinders cause almost no RPM drop when disconnected, note their positions and cross-reference with your spark plug findings. Consistent results across multiple tests, for example a rear cylinder showing oily plugs, minimal RPM drop, and low compression, gives you a confident diagnosis rather than a guess.

On engines with variable cylinder management, the deactivation cylinders are particularly worth scrutinising. If the solenoids controlling deactivation are sticking, those cylinders may be deactivating at the wrong times, causing rough running that the ECU struggles to fully compensate for.

Step 4: Compression Testing: Getting Actual Numbers

If you want certainty about how much usable compression each cylinder is producing, a compression test gives you real numbers to work with. Compression testers are widely available from automotive tool suppliers and the test itself is within reach of anyone with basic mechanical skills.

Preparation

Getting the prep right matters as much as the test itself.

Start with a warm engine, warmed to operating temperature then allowed to cool just enough to safely work around it. A warm engine gives more consistent readings because the metal has expanded to its operating clearances.

• Disable the ignition so the engine can crank without firing, and pull the fuel pump fuse or relay to prevent fuel spraying out of open plug holes.
• Remove all spark plugs, not just the one you're testing. This reduces the load on the starter motor and lets the engine spin at a consistent speed across every cylinder.
• Check the battery is fully charged. A weak battery will cause the engine to crank slower as the test progresses, skewing your results.

If the vehicle has a cable-operated throttle, prop it wide open to allow maximum air in during cranking. On drive-by-wire systems, skip this step.

Running the Test

Thread the compression tester into the first spark plug hole by hand only. Using a ratchet risks cross-threading the hole, which is a painful and expensive mistake on an aluminium head. Once hand-tight, connect the gauge and crank the engine for about five seconds, roughly five to eight compression strokes. Note the peak reading, then repeat for every cylinder.

For most Australian petrol engines, you're looking for readings above around 1000 kPa (150 PSI), though exact specifications depend on the engine's compression ratio. Your workshop manual will have the precise figures.

What matters just as much as absolute numbers is the variation between cylinders. Any cylinder reading more than 20 per cent below the highest reading is a concern. If your best cylinder reads 1100 kPa and another is down at 800 kPa, that's a significant problem regardless of whether 800 would be acceptable on its own.

The Wet Compression Test

If you find a low cylinder, the next step is figuring out why. Squirt about a teaspoon of engine oil into the spark plug hole of the affected cylinder, then repeat the compression test.

• Reading jumps significantly — rings are the likely culprit. The oil temporarily seals the ring gap, boosting compression.
• Reading stays low — valve problem is more likely. A burnt or poorly seating valve can't be sealed with oil the way rings can.

This simple extra step points the diagnosis in the right direction before any teardown happens.

Step 5: Leak-Down Testing: Tracking Down the Source

A compression test tells you how much pressure a cylinder holds. A leak-down test tells you exactly where it's going. This requires a leak-down tester, a two-gauge device that introduces regulated compressed air into each cylinder and measures how much escapes. You'll also need a compressor capable of sustained output at around 700 to 800 kPa.

Setting Up

The cylinder must be at top dead centre (TDC) on the compression stroke, meaning both valves are closed and the piston is at the very top of its travel. At this point, the cylinder is fully sealed, so any air escaping is genuine leakage.

To find TDC, bump the starter briefly with the plugs out, or rotate the engine by hand using a breaker bar on the crank pulley bolt, while watching the piston through the plug hole with a torch. When it reaches the top and both valves are confirmed closed, you're in position.

Thread the tester in, introduce compressed air, and read the leak rate as a percentage:

Where Is the Air Going?

This is where the test gets really useful. With the cylinder pressurised, move around the engine and listen carefully:

• Hissing from the intake or throttle body — intake valve not seating properly
• Hissing from the exhaust pipe — exhaust valve leaking
• Bubbling in the coolant overflow tank — head gasket failure, air entering the cooling system
• Hissing from the oil filler or dipstick tube — piston ring blowby, air bypassing the rings into the crankcase
• Air escaping into an adjacent cylinder — head gasket breach between those two cylinders

Putting It All Together

By the time you've worked through all five steps, you should have a clear picture of what's going on. The real value of multiple tests is that they confirm each other. One abnormal result can be a fluke. When three or four tests all point to the same cylinder or the same type of problem, you can act on that with confidence.

Here are the most common patterns and what they mean:

Piston ring wear — oily spark plugs, low compression that improves on the wet test, blowby heard during leak-down, minimal RPM drop during power balance testing, all pointing at the same cylinders. Depending on severity, this might mean a ring job, a full rebuild, or an engine replacement.

Valve problems — low compression that doesn't improve with the wet test, leak-down air escaping through the intake or exhaust. Often addressed by pulling the head and having it reconditioned, which is less involved than a full rebuild.

Head gasket failure — bubbles in the coolant during leak-down, milky oil, white exhaust smoke when warm, coolant level dropping with no external leak. The engine might still run for a while, but leaving it causes progressively worse damage.

General wear throughout — widespread low compression across multiple cylinders in a high-mileage engine with a history of infrequent servicing. At this point the question is whether a rebuild or a replacement engine makes more sense given the vehicle's overall value and condition.

What Are Your Options?

Once you know what you're dealing with, the path forward generally falls into one of a few categories.

Targeted repair makes sense when the damage is localised, like a single head gasket failure on an otherwise healthy engine. Replace the gasket, have the head checked for flatness and cracks while it's off, and you're likely good for a long time.

Partial rebuild or top-end recondition is worth considering when ring or valve wear affects one or two cylinders but compression elsewhere is still reasonable. Less involved than pulling the whole engine, but not a beginner DIY job.

Full rebuild or replacement engine is usually the more sensible long-term option when compression is poor across most cylinders, wear is widespread, or the engine has suffered a catastrophic failure. A proper rebuild with new bearings, rings, and reconditioned wear surfaces should give you an engine that's essentially as good as new. A low-kilometre replacement from a wreck is another solid option depending on availability for your specific vehicle.

Cylinder deactivation eliminators are worth looking into for vehicles with recurring variable cylinder management problems. These aftermarket kits permanently disable the deactivation system and keep all cylinders firing all the time. There's a minor fuel economy trade-off, but it removes the source of the problem entirely.

Preventing This Whole Situation

The best engine diagnostic is the one you never have to do because you've stayed on top of maintenance. In Australia's climate, that means a few specific things.

Oil change intervals matter more here than in cooler parts of the world. Heat breaks down oil faster, and if you're regularly doing long highway runs in summer or working the engine hard off-road, shorter intervals are cheap insurance. Use a quality oil rated for your engine and the temperatures you're actually operating in.

Coolant condition is easy to neglect. Old, degraded coolant loses its corrosion inhibitors and its ability to transfer heat efficiently. A flush every couple of years using the correct type for your vehicle keeps the cooling system working as it should.

Air filter condition affects how clean the intake charge is and how hard the engine works to pull air in. In dusty outback conditions especially, check and replace it more often than the standard service interval suggests.

And pay attention to what your car tells you. A small change in engine noise, a gradual uptick in oil consumption, a new vibration at certain speeds, or a fuel economy figure that's quietly drifted upward, these are all worth investigating rather than ignoring.

Catching problems early, when a compression test shows one cylinder slightly below ideal rather than barely making pressure, is the difference between a manageable repair and an engine replacement. Engines are built to last, but they reward attention. Treat them well and they'll generally return the favour.

Frequently Asked Questions (FAQs)

What are the first signs of a bad engine? Early indicators include unusual noises like knocking, reduced power during acceleration, excessive oil consumption, or fouled spark plugs. In Australian conditions, heat can amplify these symptoms, so monitor closely during summer drives.

Can I drive with low compression in one cylinder? It's possible short-term, but it leads to inefficiency, higher fuel use, and potential damage to other components. Address it promptly to avoid a full breakdown.

How often should I check my engine oil? At least monthly, or before long trips. In dusty or hot areas like rural Australia, check more frequently to catch burning oil early.

What's the difference between a compression test and a leak-down test? A compression test measures pressure buildup during cranking, while a leak-down test identifies where the pressure is escaping, providing more precise fault location.

Do all engines have variable cylinder management? No, it's common in certain V6 and V8 models for fuel efficiency, but not universal. If your vehicle has it, failures can mimic general engine wear.

Is professional help always needed for these tests? Basic checks like oil and plugs can be DIY, but advanced tests benefit from experience. If unsure, consult a mechanic to ensure accuracy and safety.