Knowing how to do a manual cylinder balance test (when trying find the misfiring cylinder on your Quad-4 equipped GM vehicle) is gonna' save you a lot of time and frustration when the PCM refuses to give you a specific misfire trouble code.
Why? Because the most important first step, in diagnosing a misfire, is knowing which of the 4 cylinders is ‘dead’.S
Aside from the fact that a cylinder balance test isn't hard to do... another cool thing about doing a manual cylinder balance test is that it doesn't require any expensive diagnostic equipment.
Cylinder Balance Test
A manual cylinder balance test involves disconnecting the one fuel injector at a time (from its harness connector) while the engine in your Quad-4 equipped car runs.
If the engine cylinder (who's fuel injector you just disconnected) is working and doing its part, then unplugging the injector will cause the engine's RPMs to drop. Not only will you hear the effect (of this RPM drop), but you'll also see/hear the engine idle rougher.
If the cylinder is ‘dead’, unplugging the fuel injector will not cause any change whatsoever in the RPMs and you won't notice any change in the hum of the engine's idle.
To get a more accurate test result, what I do is connect a vacuum gauge to a vacuum hose (on the engine) that has vacuum (when the engine is running) with a plastic T. As I disconnect and reconnect each fuel injector, I can see if the vacuum gauge's needle reacts or not. Now, this step is not necessary/critical, so if you don't have a vacuum gauge or can't get a hold of one, you don't need to worry about it.
OK, this is what you need to do:
Remove the plastic air filter/cleaner assembly from the throttle body... so you can have access to the fuel injectors.
- Connect a vacuum gauge to any available vacuum hose.
- You can use a plastic ‘T’ connector to connect your vacuum gauge to the fuel pressure regulator's vacuum supply hose.
- NOTE: Remember, using a vacuum gauge is not an absolute must.
Crank and start the engine..
- NOTE: Take all necessary safety precautions while working around a running engine.
Disconnect and reconnect one fuel injector at a time.
- When you disconnect the fuel injector, you should hear and feel a drop in the engine's idle (RPMs).
- If you also have a vacuum gauge connected, you'll see the gauge's needle drop.
- If the cylinder was ‘dead’ to begin with for any reason (not getting spark, low compression, etc.), you won't hear/feel a drop in the engine's idle (RPMs).
Repeat the test on the same cylinder.
- You can repeat this test as many times as you need to make sure of your test result.
Reconnect the fuel injector and test the next one.
- Perform the test on all of the 4 fuel injectors.
Interpret the results. After testing all cylinders, now you need to interpret the results.
CASE 1: Unplugging the fuel injector caused a drop in the RPMs. This tells you that the specific cylinder you tested is OK and not the cause of the misfire (miss or ‘dead’ condition).
More specifically, this test result lets you know that the fuel injector, spark plug, spark plug wire are OK for that specific cylinder.
CASE 2: Unplugging the fuel injector DID NOT cause a drop in the RPMs this tells you that the specific cylinder you tested is ‘dead’.
The cylinder could be ‘dead’ for a number of reasons. For example:
- Bad spark plug (broken, severely worn out, carbon tracks, etc.)
- Bad spark plug boot.
- Bad plastic ignition coil cover.
- Low engine compression.
- Bad fuel injector.
Don't let this big list of ‘possibles’ worry you, because one of the most important things you've accomplished with the cylinder balance test, is to find out which cylinder is the ‘dead’ one and this narrows down and focuses your troubleshooting efforts.
You'll find a list of articles that will further help you track down the problem that's causing that specific cylinder to misfire here: Tutorials to Help You Find the Cause Of The Misfire.
NOTE: Every now and then, I've seen cases where the engine has a rough idle condition and I mean it's shaking all over the place... When I've done the cylinder balance test, unplugging each fuel injector did not cause any change/drop in the RPMs. Why? This is because whatever is causing the rough idle or miss is affecting all cylinders evenly.
Although it's beyond the scope of this article to test such a condition, I can tell you that the most likely culprit behind this type of problem is a leaking intake manifold causing the air/fuel mixture to lean out. Among other things:
- Fuel pressure regulator is leaking fuel into its vacuum hose/line.
- EGR valve stuck open (if the applicable).
- Fuel pump that's outputting insufficient fuel volume/pressure.
- Major vacuum leak.
- Low engine compression across all cylinders.