On a healthy engine at idle, a vacuum gauge on the intake manifold holds steady between 17 and 21 in-Hg (MOTOR). The number is only half the reading: how the needle moves tells you which test to run next. The A1 task list words each of these tests the same way: perform it, interpret the results, determine needed action. This page follows that order.
17–21 in-Hgsteady idle vacuum (MOTOR)
5–10%leak-down, engine in great shape (ALLDATA)
18A1 questions in general diagnosis
7questions on this page
Vacuum test: read the needle
Gauge on a manifold vacuum port, engine warm, at idle first. Readings and patterns from MOTOR's vacuum gauge guide:
Steady, 17 to 21 in-Hg: normal. Rings and valves that seal well pull more vacuum.
Steady but low: a vacuum leak, an EGR leak, or ignition or valve timing. Smoke-test the intake before you suspect timing.
Drops regularly and predictably: one or more leaking valves. Find the cylinder with a power balance test, then confirm with compression and leak-down.
Drops erratically at idle: a sticking valve.
Snap throttle: vacuum should rise above idle, to about 20 to 25 in-Hg, as the throttle closes, then fall slowly and settle at the idle reading.
Exhaust restriction: every reading under load is lower than normal, and a little throttle drops the needle to zero.
Altitude: air pressure at sea level is about 29.9 in-Hg and lower higher up, so the same engine reads less vacuum in the mountains.
Power balance
A power balance test disables one cylinder at a time and watches the drop in rpm. A healthy cylinder pulls rpm down when it is disabled; a cylinder that barely changes the rpm was doing little work already. On a late-model engine the scan tool runs the test, and its misfire counters do much the same job. The result only says which cylinder is weak. Spark, fuel, compression or a vacuum leak at that runner is the next question.
Compression: cranking, wet, running and relative
Compare each cylinder with the specification and with the others. The pattern carries more information than any single number:
One cylinder low: add a little oil and test it again (a wet test). If the reading rises, the rings are leaking; if it doesn't, look at the valves or the head gasket.
Two neighbors low: a head gasket leaking between them, or a crack. Confirm with leak-down.
All low by the same amount: check the cranking speed first, since a weak battery lowers every reading. With good cranking, suspect cam timing, such as a stretched chain or a belt that jumped a tooth; leak-down then shows the cylinders sealing well.
Running compression is taken with the engine idling and at snap throttle, and shows how well a cylinder breathes, which cranking compression can't.
Relative compression reads starter current, or crank speed, for each compression stroke. A short hump in the pattern marks the weak cylinder without pulling plugs.
Leak-down: where the air goes
The leak-down test pressurizes a cylinder at top dead center on its compression stroke and reads the percentage lost. ALLDATA's guide puts 5 to 10 percent loss on a naturally aspirated engine in great condition, 15 to 20 percent as no reason on its own to condemn it, and 30 percent as major trouble. Cylinders within 4 to 5 percent of each other are consistent. Where the air escapes matters more than the percentage:
Oil filler, dipstick tube or PCV: past the rings.
Throttle body or intake: an intake valve.
Tailpipe: an exhaust valve.
Bubbles at the radiator filler or coolant reservoir: a head gasket or a cracked head or block.
Out of the next cylinder's spark plug hole: a head gasket between the two cylinders.
Oil pressure test
Fit a mechanical gauge in place of the oil pressure sender, warm the engine, and compare the readings at idle and at the specified rpm with the manufacturer's figures.
Normal cold, low hot: worn bearings (excess clearance) or oil too thin for the engine, since warm oil leaks through clearances faster.
Low on the dash, normal on the mechanical gauge: the sender or its circuit, not the engine.
Very high, filter gaskets blowing out: a pressure relief valve stuck closed.
Drops in hard cornering or braking: low oil level or a pickup that uncovers. Check the level before anything else.
Cooling system pressure test
Pressurize the cold system to the rating on the cap and watch the gauge. A slow drop with a wet hose, radiator or water pump is an external leak. A drop with no external leak points somewhere you can't see: the heater core, a head gasket or a crack. Test the cap on its own adapter, since a weak cap lets coolant boil early.
A system that holds pressure can still take in combustion gas. A chemical block tester at the filler neck, or an exhaust gas analyzer sniffing above it, finds hydrocarbons in the coolant. A tester gauge that climbs while the engine cranks or runs does the same job.
Head gasket: a decision path
A blown head gasket is the conclusion of several tests, not a guess from one symptom.
1
Symptom
Coolant loss with no visible leak, white exhaust smoke, overheating, or oil that looks milky.
2
Pressure test
Cold system to cap rating: a drop with no external leak means the coolant is going inside.
3
Combustion leak test
Block tester or gas analyzer at the filler neck: combustion gas in the coolant.
4
Compression
Two neighboring cylinders that read low point to the gasket between them.
5
Leak-down
Air into the coolant or the next cylinder confirms it; air at the tailpipe means an exhaust valve instead.
6
Before you replace it
Find out why it failed: check the head and deck for warpage and cracks, and fix the cause of any overheating.
On the A1 and the A8
Shared with A8. The four tests sit inside general engine diagnosis, the largest A1 area at 18 of 45 scored questions. The A8 task list includes them too, and its pages link here instead. Oil and cooling pressure tests belong to lubrication and cooling.
7 questions on test results
0 of 7 answered · 0 correct
Question 1 of 7 · General Engine Diagnosis · 40%Not answered
A customer reports a rough idle and a flashing malfunction indicator lamp. Scan tool data shows a P0300 (Random/Multiple Cylinder Misfire) code. Fuel pressure is within specification, but a mechanical compression test shows 60 psi on cylinder 2 and 65 psi on cylinder 3. What is the most likely cause?
Explanation
Incorrect: Plugged injectors lean out the cylinders but do not lower the compression gauge reading.
Correct: Two adjacent cylinders reading low together, while fuel pressure is fine, is the classic sign of a gasket failure between them.
Incorrect: Bad coil packs cause misfires but have no effect on a mechanical compression reading.
Incorrect: A crankshaft sensor wiring fault affects timing and misfire detection, not cylinder sealing.
Incorrect: A faulty MAF sensor skews fueling for every cylinder and cannot lower compression in two of them.
Question 2 of 7 · Lubrication and Cooling Systems Diagnosis and Repair · 18%Not answered
An engine frequently blows out its spin-on oil filter gasket during heavy acceleration. A mechanical gauge reveals that oil pressure climbs well beyond the manufacturer's maximum specification as engine RPM increases, though idle pressure is normal. What is the most likely cause of this over-pressurization?
Explanation
Incorrect: A restricted pickup screen starves the pump and lowers pressure, especially at higher rpm.
Incorrect: A filter bypass that has failed open only sends unfiltered oil past the element; it does not raise gallery pressure.
Incorrect: Worn main bearings bleed off oil and lower pressure instead of raising it.
Incorrect: Extra clearance between the pump gears lets oil slip back inside the pump and lowers output.
Correct: Pressure that is normal at idle but climbs past the maximum with rpm means the relief valve is not opening to cap the pump output, and the filter gasket blows out.
Question 3 of 7 · General Engine Diagnosis · 40%Not answered
An engine has uniformly low compression readings of 110 to 115 psi across all cylinders. Cylinder leak-down tests show less than 10% leakage on all cylinders. Which of the following is the most likely cause?
Explanation
Incorrect: Carbon holding the intake valves open would show air at the intake during the leak-down.
Incorrect: Worn rings would leak air into the crankcase on the leak-down, and leakage is under 10% here.
Incorrect: A worn oil control ring raises oil consumption but does not cause an even compression loss on every cylinder.
Correct: Even, low compression with good sealing on the leak-down means the valves are closing at the wrong time, which fits a stretched chain retarding the cam.
Incorrect: A blown gasket affects the cylinders next to the breach, not all of them evenly, and would show leakage.
Question 4 of 7 · General Engine Diagnosis · 40%Not answered
A technician performs a compression test on an engine. All cylinders read about 10% below specification. The battery is weak, causing slower-than-normal cranking speed. What should the technician do?
Explanation
Correct: Slow cranking lowers every reading, so the battery has to be brought back up before the compression numbers mean anything.
Incorrect: The starter is not the fault here; the battery is weak, and a new starter on a weak battery still cranks slowly.
Incorrect: Readings taken at low cranking speed are artificially low, so accepting them leads to a wrong diagnosis.
Incorrect: A wet test on top of slow cranking still gives readings that cannot be trusted; the cranking speed has to be fixed first.
Incorrect: Cam timing is not the suspect when every cylinder is low by the same amount and the cranking speed is known to be low.
Question 5 of 7 · Lubrication and Cooling Systems Diagnosis and Repair · 18%Not answered
An engine has an oil pressure of 45 psi at a cold idle, but the pressure drops to 5 psi at a hot idle. The factory specification requires a minimum of 15 psi at hot idle. What is the most likely cause of this condition?
Explanation
Incorrect: A bypass valve stuck open sends unfiltered oil to the engine but has almost no effect on gallery pressure.
Correct: Thin, hot oil escapes quickly through worn main bearings, so pressure that is fine cold and collapses when hot points to excessive bearing clearance.
Incorrect: A relief valve stuck closed raises pressure, especially when the oil is cold and thick, and does not cause low hot pressure.
Incorrect: A restricted gallery starves the parts past it, but the gauge would not show a hot-only drop to 5 psi.
Incorrect: A miscalibrated gauge would read wrong cold and hot alike; it cannot explain a reading that changes with temperature.
Question 6 of 7 · Lubrication and Cooling Systems Diagnosis and Repair · 18%Not answered
To verify a suspected combustion gas leak into the cooling system, which of the following procedures is most appropriate?
Explanation
Incorrect: A running compression test shows how well cylinders breathe, not whether combustion gas is reaching the coolant.
Correct: A pressure tester on the cooling system while cranking shows bubbles or a rising gauge when combustion pressure is being forced through a breach.
Incorrect: Debris under the radiator cap says nothing about combustion gas getting into the cooling system.
Incorrect: A power balance test finds a weak cylinder but gives no evidence about the cooling system.
Incorrect: An intake vacuum reading reflects overall engine condition and cannot show gas entering the coolant.
Question 7 of 7 · General Engine Diagnosis · 40%Not answered
An engine has low power at all speeds. Which quick test, done without removing any spark plugs, gives the best first check of the engine's overall mechanical condition?
Explanation
Correct: A vacuum gauge on the intake manifold shows how well all the cylinders breathe and seal together, so it screens the engine's mechanical health in a minute.
Incorrect: Fuel pressure checks the fuel supply, not the mechanical condition of the engine.
Incorrect: An ignition timing check covers the ignition system, not the engine's mechanical health.
Incorrect: A compression test is mechanical, but it needs the spark plugs out, so it is not the quick first check described.
A steady 17 to 21 in-Hg at idle on a normal engine, per MOTOR. Expect less at high altitude, and read the needle's movement as well as the number.
How much leak-down is acceptable?
ALLDATA calls 5 to 10 percent on a naturally aspirated engine great, 15 to 20 percent not a reason on its own to condemn it, and 30 percent major trouble. Where the air comes out tells you which part is leaking.
How do you diagnose a blown head gasket?
With a chain of tests: a cooling system pressure test with no external leak, a combustion leak test at the filler neck, compression showing two neighboring cylinders low, and leak-down pushing air into the coolant or the next cylinder. The decision path above lists them in order.
What does a wet compression test tell you?
Whether a low cylinder is leaking past the rings. Oil sprayed into the cylinder seals the rings for a moment: a reading that rises points to the rings, one that stays low points to the valves or the head gasket.
Are engine tests on the ASE A1?
Yes. Manifold vacuum, power balance, compression, leak-down and PCV tests are tasks in general engine diagnosis, and the oil and cooling pressure tests are in lubrication and cooling. The questions above are written to those tasks.