How to Use the AutoLine Pro Proton 4-in-1 Digital Cylinder Diagnostic System
The AutoLine Pro Proton runs four engine-cylinder tests from one digital unit — compression, cylinder leak-down, top dead center (TDC) location, and pressure/vacuum. Pull the spark plugs and coils, thread the hose into a spark-plug hole with the adapter that matches your engine, pick the test on the touchscreen, and the gauge logs each cylinder automatically — including doing the leak-down percentage math for you — then charts every cylinder side by side so you can compare them at a glance.
An analog compression gauge tells you one number at a time and leaves you to write it down and do the math. The Proton keeps the readings, calculates leak percentage from your input pressure, and helps you find top dead center by watching pressure and vacuum instead of guessing with a screwdriver. Below is the full walkthrough for all four tests, in the order you'd actually run them on a car.
Watch: the Proton 4-in-1, unboxed and run on a real engine
Sam from AutoLine Pro unboxes the Proton cylinder gauge and then runs all four of its tests on a running six-cylinder vehicle. The unboxing shows what's included: the digital cylinder gauge itself, the hose that threads into the spark-plug hole, an extension hose for hard-to-reach cylinders, the engine-cylinder adapters for different thread sizes, and the rest of the accessories. He starts with a vacuum test on the running engine, showing the display reading roughly −16 inHg, how to switch units between inHg, kPa and PSI, the high/low pressure modes, and the graph-over-time mode that shows whether vacuum is holding steady or fluctuating. Next is a compression test: he notes that the fuel-pump fuse has to come out before cranking, shows the spark plugs and ignition coils already removed, installs the red one-way valve on the side of the unit, cranks each cylinder in turn, and points out that every reading is logged automatically to the cylinder it was taken on — releasing pressure between cylinders with the plunger valve and stepping from cylinder one through six. Finally he demonstrates finding top dead center by turning the crankshaft with a ratchet and watching the pressure/vacuum screen (pressure builds on the compression stroke; the moment vacuum appears you've just gone past TDC, so you back the ratchet up and fine-tune until neither builds), then runs the leak-down test on each cylinder by connecting shop air, opening the plunger valve, dialing the regulator to 100 PSI on the input gauge, and reading the leak percentage the unit calculates — 3%, 5%, 4%, 3%, 2% across the cylinders in the demo. He closes by pressing the chart button, which puts every cylinder's numbers on one screen for comparison, and notes that AutoLine Pro is an American family-owned business that developed the product from the ground up.
What's in the box
Everything needed for all four tests ships in the case:
- The Proton digital cylinder gauge — the touchscreen unit in the center of the case, with the input and cylinder pressure readouts, the red one-way valve on the side, and the plunger (bleed) valve for releasing pressure between cylinders.
- Cylinder hose — threads into the spark-plug hole and connects to the gauge.
- Extension hose — adds reach for cylinders buried under intake plumbing or set back against the firewall.
- Engine-cylinder adapters — different spark-plug thread sizes and reaches, so the kit fits a range of gasoline engines.
- Additional accessories — the remaining fittings and hardware for connecting to the engine and to shop air.
You'll also need two things that aren't in the case: a ratchet and socket to turn the crankshaft (for TDC and leak-down), and an air compressor capable of holding 100 PSI (for leak-down only). Compression and vacuum tests need neither.
Before you start: prep and safety
Pull the fuel-pump fuse before any test that involves cranking the engine. A compression test means cranking with the plugs out — you don't want the injectors firing raw fuel into open cylinders and down the exhaust.
- Warm the engine, then shut it off (for compression testing). Compression numbers taken stone cold read low and inconsistent. Let it cool enough to be safe on the plug threads before you start pulling parts.
- Remove the ignition coils and all spark plugs. Every plug comes out, not just the cylinder you're testing — leaving plugs in makes the engine hard to crank and skews the readings.
- Pull the fuel-pump fuse or relay so the engine can't deliver fuel while you crank.
- Pick the right adapter for your engine's spark-plug thread and reach, and add the extension hose if the cylinder is hard to get to.
- Thread the adapter in by hand first. Aluminum heads are easy to cross-thread. It should spin in freely before you snug it.
The vacuum test is the exception — it runs on a running engine and doesn't require pulling anything.
Test 1: How to run a vacuum test with the Proton
A vacuum test reads intake manifold vacuum on a running engine. It's the fastest overall health check you can do — a steady, strong reading says the engine is breathing and sealing normally; a low, drifting, or rhythmically dropping needle points at vacuum leaks, valve problems, timing, or exhaust restriction.
- Connect to a manifold vacuum source with the engine off — a vacuum port on the intake manifold, or a line you've capped off at the other end.
- Set your units. The Proton opens in inches of Mercury (inHg), the common unit for manifold vacuum in the U.S. You can switch it to kPa or PSI on the display if you prefer.
- Choose high or low mode depending on whether you're reading engine vacuum or testing something at higher pressure.
- Start the engine and let it idle. In the video the demo vehicle settles at about −16 inHg, which is in the normal range for a healthy engine at idle at low elevation.
- Switch to graph-over-time mode to watch the reading live. This is the part an analog gauge does poorly: on the graph you can see instantly whether vacuum is holding constant or oscillating. A flat line means steady; a regular dip is the classic signature of a burnt or leaking valve, and a slow drop as you hold RPM points toward a plugged catalytic converter or restricted exhaust.
Most healthy gasoline engines idle around 17–22 inHg at sea level, and you lose roughly 1 inHg per 1,000 feet of elevation. Steady but low usually means late timing or a general vacuum leak; unsteady means something is happening cylinder-by-cylinder.
Test 2: How to run a compression test with the Proton
A compression test measures the peak pressure each cylinder can build while cranking. It's the quickest way to find a mechanically weak cylinder — but on its own it only tells you that a cylinder is down, not why. That's what the leak-down test is for.
- Confirm the prep is done — plugs and coils out of every cylinder, fuel-pump fuse pulled.
- Install the red one-way valve on the side of the gauge. This is what lets the unit hold the peak reading instead of bleeding back down between compression strokes.
- Thread the hose into cylinder one using the correct adapter, adding the extension hose if you need the reach, and connect it to the gauge.
- Select cylinder 1 on the screen, then crank the engine. Hold the key until the reading stops climbing — usually four to six compression strokes.
- Let the unit log it. The reading is captured and stored against that cylinder automatically; there's no notepad step.
- Release the pressure with the plunger valve on the unit, disconnect the air, and move the hose to the next cylinder.
- Select the next cylinder on the screen and repeat. Work through every cylinder in order — one through six on the demo engine in the video.
- Press the chart button when you're done. Every cylinder number appears across the top with its compression figure beneath it, on one page, so you can compare and see how far each cylinder deviates from the others.
The absolute figure matters less than the spread between cylinders. A common shop standard is that no cylinder should be more than about 10% below the highest one (some manufacturers allow 15%, and some publish a hard minimum — check the service data for the engine). Two adjacent cylinders both low together is the classic head-gasket pattern. One cylinder low on its own points at that cylinder's valves, rings, or piston — which is exactly where the leak-down test takes over.
Test 3: How to find top dead center with the Proton
Top dead center is the point where the piston is at the very top of its travel with both the intake and exhaust valves fully closed. You have to be at TDC on the compression stroke before a leak-down test means anything — if a valve is even slightly open, air escapes past it and the tool reports a huge leak on a perfectly good cylinder.
Most people hunt for TDC with a screwdriver down the plug hole or by lining up timing marks. The Proton lets you find it by watching pressure and vacuum instead, which is both more precise and easier to see.
- Remove the spark plug from the cylinder you're testing and connect the Proton's hose to that cylinder.
- Open the pressure/vacuum screen. Pressure displays on the right, vacuum on the left.
- Turn the crankshaft by hand with a ratchet on the crank pulley bolt, in the engine's normal direction of rotation.
- Watch for pressure to start building. Nothing happening means a valve is still open and you're not on the compression stroke yet. Once pressure climbs, you're on the compression stroke and the piston is coming up.
- Keep turning until pressure stops building and a small notch of vacuum appears. That vacuum is the signal that you have just passed TDC.
- Back the ratchet up slightly. Reverse direction and come back a little — when vacuum builds going backward, you've passed TDC the other way.
- Fine-tune. Nudge back and forth in small increments until you find the spot where neither pressure nor vacuum changes. That dead zone is top dead center, and you're ready to run leak-down on that cylinder.
The unit also has a TDC shortcut on the display so you can jump straight to that screen for each new cylinder, and a leak-down shortcut in the bottom-left with on-screen instructions for switching between the two.
Test 4: How to run a cylinder leak-down test with the Proton
A leak-down test pressurizes a sealed cylinder with shop air and measures what percentage of that air escapes. Where you hear it escaping tells you which component failed. This is the test that turns "cylinder three is low" into "cylinder three has a burnt exhaust valve."
- Get the cylinder to TDC on the compression stroke using the method above. This step is mandatory — skip it and your numbers are meaningless.
- Switch to leak-down mode. Use the leak-down shortcut on the bottom left of the screen; the unit displays simple on-screen instructions, including pulling the plunger valve up.
- Connect your air compressor hose to the input side of the gauge.
- Open the plunger valve so both the input and cylinder gauges are reading.
- Turn the pressure regulator knob on top of the unit until the input reads 100 PSI. That's your reference pressure.
- Read the leak percentage. The Proton does the math for you the moment you dial in your input PSI — no dividing one analog gauge by another. In the video the cylinders read 3%, 5%, 4%, 3% and 2%.
- Log the data, then reduce the pressure back to zero before you move on.
- Select the next cylinder, find its TDC, and repeat through the whole engine.
- Press the chart button to see cylinder numbers across the top and the leak-down figures on the row beneath — the whole engine on one screen.
With a cylinder at TDC and 100 PSI behind the piston, the engine can suddenly rotate if the crank slips off exact TDC. Keep hands, tools and clothing clear of the belts and pulleys, make sure the vehicle is in park or neutral with the parking brake set, and bleed the pressure back to zero before moving anything.
What the leak-down numbers actually mean
Leak percentage is only half the diagnosis. The other half is listening for where the air comes out while the cylinder is pressurized:
| Where you hear or see the air escaping | What it means |
|---|---|
| Out the throttle body or intake manifold | Leaking or burnt intake valve |
| Out the tailpipe or exhaust | Leaking or burnt exhaust valve |
| Out the oil filler cap, dipstick tube, or PCV | Piston rings or a worn cylinder wall (blow-by into the crankcase) |
| Bubbles in the coolant / radiator, or coolant loss | Head gasket or a cracked head |
| Into an adjacent cylinder | Head gasket failure between the two |
As a general guide, most technicians treat under 10% as healthy on a well-maintained engine, 10–20% as worth watching (and normal on a high-mileage or performance engine), and over 20% as a real mechanical problem worth tearing into. As with compression, the consistency across cylinders matters as much as any single figure — the 2–5% spread in the video is a healthy engine.
Compression testing is fast and finds the weak cylinder. Leak-down is slower but tells you which part is leaking. Running compression first to identify suspect cylinders, then leak-down on those cylinders, is the efficient order — and it's why having both tests in a single unit with shared logging saves real time on a diagnosis.
Frequently asked questions
What four tests does the AutoLine Pro Proton do?
The Proton 4-in-1 Digital Cylinder Diagnostic System performs a compression test, a cylinder leak-down test, top dead center (TDC) location, and pressure/vacuum testing — all from one unit, with automatic data logging per cylinder and a chart view that shows every cylinder's results on one screen. It's designed for gasoline engines.
Do you have to remove all the spark plugs for a compression test?
Yes. Remove every spark plug and the ignition coils, not just the plug in the cylinder you're testing. With the other plugs still in, the engine is much harder to crank and the cylinder you're measuring won't reach its true peak pressure. You also need to pull the fuel-pump fuse or relay first, since you'll be cranking the engine with the cylinders open.
What's the difference between a compression test and a leak-down test?
A compression test measures how much pressure a cylinder builds while cranking — it tells you a cylinder is weak, but not why. A leak-down test holds the cylinder at top dead center, feeds it a fixed 100 PSI of shop air, and measures what percentage escapes — and where you hear the air escaping identifies the failed part: intake valve, exhaust valve, rings, or head gasket. Run compression first to find the suspect cylinder, then leak-down to diagnose it.
How do you find top dead center for a leak-down test?
With the Proton connected to the cylinder, open the pressure/vacuum screen and turn the crankshaft by hand with a ratchet. When pressure starts building you're on the compression stroke. Keep turning until pressure stops rising and a small amount of vacuum appears — that means you've just passed TDC. Back the ratchet up slightly, then fine-tune back and forth until neither pressure nor vacuum changes. That dead spot is top dead center with both valves closed.
What is a good cylinder leak-down percentage?
As a general rule, under 10% leakage is considered healthy on a well-maintained engine, 10–20% is worth monitoring (and is common on high-mileage or performance engines), and over 20% usually indicates a real mechanical problem. Consistency between cylinders matters as much as the absolute number — the demo engine in the video read 2–5% across all six cylinders.
Does the Proton calculate leak percentage automatically?
Yes. Once you dial the regulator so the input gauge reads 100 PSI, the unit calculates and displays the leak percentage for you. With a traditional analog dual-gauge leak-down tester you have to set 100 PSI and then work out the percentage yourself for every cylinder.
Can you use the Proton on a diesel engine?
The Proton 4-in-1 is built for gasoline engines. Diesel compression testing requires much higher-range gauges and injector- or glow-plug-port adapters. If you're diagnosing a diesel, use a dedicated diesel compression kit rated for those pressures.
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