⚡ When a car rolls into the workshop feeling gutless, misfiring, or just plain unhappy, one of the fastest ways to find the culprit is a relative compression test. Recently we had a vehicle where cylinder 2 was clearly slacking off, and thanks to some clever scope work we pinpointed the fault without cracking a single bolt. Here’s how it works and why it matters. 🔧
What a Relative Compression Test Actually Shows
A relative compression test measures the current draw of the starter motor as the engine cranks. Every time a cylinder reaches the top of its compression stroke, the starter has to work harder, which shows up as a peak on the oscilloscope. When one cylinder has lower compression than the rest, that peak dips noticeably compared to its neighbours. In our recent job, cylinder 2 was doing exactly that, its waveform sitting well below the others, a textbook sign of low compression.
The beauty of this test is that it’s completely non-invasive. No spark plugs pulled, no compression gauge threaded in, no injectors removed. Just a current clamp on the battery cable, a Pico scope, and a trained eye reading the waveform.
Why Accurate Diagnostics Save You Time and Money
Low compression can point to several underlying issues, and knowing which one you’re chasing matters:
- Worn piston rings allowing combustion pressure to escape into the crankcase
- Burnt or leaking valves failing to seal the combustion chamber
- Head gasket failure between cylinders or into a coolant passage
- Camshaft or timing issues affecting valve lift and timing
From Waveform to Repair Plan
Once we identify the weak cylinder, we can move on to targeted follow-up tests, like a cylinder leak-down test or an in-cylinder pressure transducer reading, to work out exactly where the pressure is escaping. That means no unnecessary parts thrown at the problem, no guesswork, and a clear repair path from the very first visit. It’s the difference between fixing a car properly the first time and chasing symptoms for weeks.
Book Your Diagnostic With Romano’s Automotive
If your car is misfiring, running rough, lacking power, or throwing cylinder-related fault codes, don’t leave it to chance. At Romano’s Automotive in Perth, we use advanced scope diagnostics and proper testing procedures to find the real fault, not just the easy one. Book your diagnostic online at https://shopcommand.com.au/book/romanos-automotive or give us a call on 0401 747 320 or 0483 932 306, and let’s get your engine breathing evenly again.
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