One of the most over-looked tests, when troubleshooting a hard to solve misfire, is the engine compression test.
Over the years, I've run across many situations where the customer (or another tech or mechanic) just about replaced everything on their Ford 4.6L or 5.4L V8 and still the car (or SUV or pick up) continued to misfire!
This tutorial will explore the compression test as one of the key tests to troubleshoot a misfire code or codes (P0300, P0301, P0302, P303, P0304, P0305, P0306, P0307, P0308).
This article is a part of a series of articles that explore the different tests that are done to find the exact cause of a misfire condition on the 4.6L and 5.4L equipped Ford and Mercury vehicles. The series starts with the article: How To Diagnose Misfire Codes Ford 4.6l & 5.4L 8 Cylinder (P0300-P0308).
Contents of this tutorial:
You can find this tutorial in Spanish here: Cómo Probar La Compresión Del Motor (Ford 4.6L, 5.4L) (at: autotecnico-online.com).
Tools You'll Need:
- Compression Gauge Tester
- Engine Oil
- A Helper
- Pen and Paper
Symptoms Of Low Or No Compression
Engine compression issues can indeed fall into one of two main categories:
- Low Compression in Some but Not All Cylinders: This means that certain cylinders within the engine are not generating the necessary compression pressure. This can be due to various factors such as worn piston rings, leaking valves, or a blown head gasket. When compression is low in some cylinders, the engine may still run but with reduced power and efficiency.
- 0 PSI Compression in Some or All Cylinders: This indicates a severe compression problem where one or more cylinders have lost all compression. This could be caused by catastrophic failures like a cracked engine block, severe valve damage, or a broken piston. When there's zero compression, the affected cylinders won't contribute to engine power at all, and the engine may not start or run very poorly if it does start.
Regardless of whether the engine has low or zero compression, there are several common symptoms you might notice, which will impact the engine's performance, reliability, and drivability:
- Hard Starting or No Start: Difficulty starting the engine or it failing to start altogether, especially if compression is too low to ignite the fuel-air mixture.
- Poor Engine Performance: The engine may run roughly, lack power, or feel sluggish due to inadequate compression in the cylinders.
- Misfiring: Misfiring occurs when the air-fuel mixture in the cylinder fails to ignite at the proper time, often leading to rough idling, hesitation, or jerking while driving.
- Reduced Power and Performance: Due to inefficient combustion, the engine may struggle to produce the power needed for acceleration or maintaining speed.
- Excessive Oil Consumption: Compression issues can lead to oil leaking into the combustion chamber, causing increased oil consumption and potentially fouling spark plugs.
- Audible Engine Noise: You may hear unusual noises such as knocking, tapping, or rattling, which can indicate problems with compression or related components.
Which Compression Tester Should I Buy?
There are lot of engine compression testers to choose from and many places to buy them. I'm gonna' make two recommendations to you:
1) Which one to buy: The engine compression tester that I have always used is the Actron CP7827 Compression Tester Kit. My only complaint about this engine compression tester is that it does not come with a case to store it in.
2) Where to buy: You can buy an engine compression tester just about anywhere, but you'll end up paying more for it (especially at your local auto parts store). The above links will help you comparison shop. I think you'll agree it's the better way to save money on the compression tester!
TEST 1: 'Dry' Engine Compression Test
Each cylinder, besides needing air, fuel, and spark, to contribute to engine power, also needs to be in good mechanical condition. If either the cylinder head valves or the engine piston rings are worn and letting the compression created on the power stroke escape, then that cylinder will misfire.
This test article assumes that the engine starts and runs and that you're testing a misfire condition. Also, the test applies to both the ignition coil pack and Coil-On-Plug ignition coil equipped 4.6L and 5.4L engines.
NOTE: The engine should NOT be HOT (normal operating temperature) and yet, it should be not be COLD. So, if you've just turned off the engine, after an extended run time, let it cool down about 30 minutes. If the engine is cold, start'er up and let'er run about 15 minutes. If the engine in your vehicle doesn't start, then don't worry about it being warm or not for the test, since a compression test can still be done.
Let's get started:
Disconnect all 8 fuel injectors from their electrical connectors. This will prevent fuel injectors from injecting fuel into the engine cylinders.
Disable the ignition system.
If your specific 4.6L V8 has the two coil pack ignition system, you must disconnect both coil packs. This is important, since the ignition system has to be disabled and unable to produce spark. If your 4.6L V8 engine is equipped with individual Coil-On-Plug (COP) ignition coils you don't have to worry about this.
Disconnect and remove all of the 8 ignition coils (if your vehicle is equipped with one individual Coil-On-Plug ignition coil for each spark plug).
Remove all of the 8 spark plugs.
Thread the engine compression gauge into the spark plug hole for the number 1 engine cylinder (this is the spark plug hole closest to the drive belt). Hand tighten the compression gauge only! Do not use any type of tool to get it tight.
When everything is ready, have your helper crank the engine. Keep your eyes glued on the compression tester gauge.
Once the needle stops climbing, have your helper stop cranking the engine. Repeat this test twice, and record the second reading on a piece of paper.
Repeat steps 5 thru' 7 on the remaining 7 cylinders.
Let's examine your test results:
CASE 1: All eight cylinders had 0 PSI. This test result tells you that the engine has an internal mechanical problem.
The most common cause of this condition is a broken timing chain or a blown head gasket.
Your next step should be to check the condition of the timing chain and perform a blown head gasket test.
CASE 2: One or more cylinders had a low compression value compared to the others. This could be normal or it could be causing a problem.
To find out if the compression values are normal or not, go to: How To Interpret The Engine Compression Test Results.
CASE 3: All eight compression values were similar and above 120 PSI. This lets you know that a compression problem is not behind the no-start or misfire problem you're trying to troubleshoot.