Most People Check the Wrong Things
The standard used car inspection goes like this: check the oil dipstick, kick the tyres, start the engine, listen for obvious rattles. Then either buy it or not.
This process misses the most expensive problems a used engine can have — and catches mostly cosmetic issues that don't matter. Here are the eight most common mistakes buyers make when inspecting a used car engine, and what to do instead.
Mistake 1: Checking Oil Level Instead of Oil Condition
The dipstick tells you how much oil is present. It doesn't tell you anything useful about the engine's history. What matters is the condition of the oil.
Pull the dipstick and look carefully at the oil. Healthy oil is amber to brown. Very dark black oil means it's overdue for a change — not a serious problem, but tells you the owner wasn't diligent. Milky, creamy, or grey oil is a serious warning sign: it indicates coolant contamination, usually from a blown head gasket or cracked head. Walk away.
Also smell the oil. A sharp, acrid smell can indicate the engine has been overheated.
Mistake 2: Not Checking the Coolant Reservoir
Open the coolant reservoir cap — only when the engine is completely cold — and look inside. The coolant should be clean: green, blue, pink, or orange depending on the type. What you don't want to see: oil residue or an oily film on the inside of the cap, brown or rusty-looking coolant, or any foam or bubbles.
Oil in the coolant system is another indicator of head gasket failure. Rusty coolant means the cooling system hasn't been maintained and may have internal corrosion.
Mistake 3: Not Starting the Engine from Cold
Always insist on a cold start. This is non-negotiable.
Cold starts reveal problems that disappear once an engine warms up. Specifically: diesel timing chain rattle on cold start that settles after 30 seconds (BMW N47, early Vauxhall diesels) means the chain is worn and replacement is imminent. Blue smoke on cold start that disappears when warm means valve stem seals are worn and oil is burning off. White smoke that persists after warm-up indicates coolant burning, suggesting a potential head gasket issue.
A seller who insists the engine "needs to warm up first" before you hear it is almost certainly hiding a cold-start rattle.
Mistake 4: Trusting the Service History Without Verifying
Service stamps in a book are not self-verifying. A surprising number of used cars have falsified service histories — stamps can be faked, and entries for "oil change" cost virtually nothing to fabricate.
What to do instead: cross-reference the mileage progression on stamps with MoT or inspection records (in the UK, free via the DVLA; in other EU countries, via national vehicle inspection records). Look for oil change stickers inside the door jamb or under the bonnet — these are harder to fake. If there's a specific major service that should have happened (timing belt at 120,000km, timing chain replacement for an N47), ask for the invoice, not just a stamp.
Mistake 5: Skipping the OBD2 Scan
An OBD2 scanner costs €15-30 and plugs into a port under the dashboard of any car built after 2001. It reads stored fault codes from the engine management system — including codes that have been cleared but recur.
Many sellers clear codes before a test drive. But if the underlying problem is still present, the code will often return after a short drive. Ask to scan the car before AND after the test drive.
Fault codes don't automatically mean a serious problem — a single P0420 catalytic converter efficiency code might just be an old sensor. But multiple codes, or codes related to timing, fuel trim, or coolant temperature, warrant further investigation.
Mistake 6: Not Asking About Oil Consumption
Ask the seller directly: "Do you add oil between services?" Most won't lie to your face. If they say yes, ask how much and how often. Adding 0.5L over 10,000km is borderline acceptable; adding 1L every 3,000km is a significant problem.
If you can't verify from the seller, check known oil consumption issues for the specific engine on enginecreep before you buy. The VW EA888 Gen 1 and Gen 2, BMW N20, and Alfa 1.4 Multiair are all engines with documented consumption histories — not necessarily deal-breakers, but things to factor into your offer price.
Mistake 7: Only Listening at Idle
Idle is the easiest state for a struggling engine to hide problems. The real test is during hard acceleration (knock, ping, or rattling under load can indicate worn bearings or detonation), during deceleration (popping or misfires under engine braking), and at operating temperature (any ticking that remains after full warm-up often indicates valve train wear or low oil pressure).
A car that sounds clean at idle and terrible under load is more common than you'd think.
Mistake 8: Not Doing a Quick Visual Scan for Leaks
With the engine running and at temperature, crouch down and look under the car. Any active drip is worth noting. Then open the bonnet and look at the engine surface — dried oil residue around gaskets, valve covers, or the sump indicates past or current leaks.
Minor seepage from an old valve cover gasket costs €100-200 to fix. A rear main seal leak on some engines can cost €600-1,000. Neither necessarily means don't buy — but both should come off the asking price.
The 10-Minute Engine Check: What to Actually Do
1. Arrive before the seller starts the car
2. Check oil condition, not just level
3. Check coolant with cap off and look for contamination
4. Listen to cold start — first 60 seconds are critical
5. OBD2 scan before the test drive
6. Hard acceleration test during drive
7. OBD2 scan after the test drive
8. Check for leaks with engine at temperature
9. Ask directly about oil consumption history
10. Cross-check service history mileage against official records
None of this requires specialist knowledge. All of it will save you money — often significantly more than the cost of the car inspection itself.