TEST 1: Testing For Spark At The Spark Plug Wires
The starting point of your Misfire or No-Start diagnosis is testing for spark at the spark plug wires. Now, you might already have a specific cylinder you want to test first (if you have a specific misfire diagnostic trouble code: P0300, P0301, P0302, P0303, P0304, P0305, P0306, P0307, P0308) or you just don't know where to start. Well, my recommendation is to test all of the spark plug wires for spark regardless.
I want to stress the importance of using an HEI spark tester to perform all of the spark tests. Here are a couple of other important suggestions:
- Do not use a regular spark plug in place of a spark tester.
- Do not pull the spark plug wire off of its spark plug while your helper cranks the engine to verify spark. This will damage the ignition coil, if it isn't fried already.
OK, here we go:
- Disconnect the spark plug wire from its spark plug.
- Attach the HEI spark tester to the spark plug wire.
- With a battery jump start cable, attach the HEI spark tester to a good ground point or to the battery negative terminal.
- Have your assistant crank the engine while you eye-ball the spark tester. If you're testing a misfire condition, the engine will start... so be careful.
- Repeat the test for all of the remaining spark plug wires (if applicable).
- You're going to see only one of two results: either spark jumping across the HEI spark tester's air gap or No spark.
Let's take a look at what your test results mean:
CASE 1: If you got spark on all of the spark plug wires: This is an indication that the ignition control module (ICM), ignition coil, distributor rotor and cap are OK.
Having all of the spark plug wires firing off spark eliminates all of these components as the cause of your misfire codes or No Spark- No Start Condition. For some suggestions as to what could be the cause of the misfire condition and/or misfire codes, go to: TEST 8: Other Causes Of A Misfire.
CASE 2: If you got NO spark from any (none) of the spark plug wires: The next step is check that the ignition coil is creating and feeding spark to the distributor cap. You'll accomplish in TEST 3. Go to: TEST 3: Testing The Ignition Coil's High Tension Wire.
CASE 3: If you got spark on some but not all of the spark plug wires: The next step is check for spark directly on the distributor cap towers that feed spark to these wires that did not fire off spark. Go to: TEST 2: Testing For Spark At The Distributor Cap.
TEST 2: Testing For Spark At The Distributor Cap
It's a common thing, in this type of GM Ignition System, for one or two spark plug wires to go bad or for one or two distributor cap towers to go BAD and cause a misfire by not feeding spark to the spark plugs.
In this test step we'll find out if the spark plug wires that did not fire off spark in TEST 1 are BAD or not.
This test step will also help you to see if the distributor cap is BAD or not.
Important: this test is only for when one or several (but not all) of the spark plug wires did not spark!
OK, to get this test going, this is what you need to do:
- Un-plug the spark plug wire (that didn't spark) from its tower on the distributor cap and place the spark tester directly on the tower.
- With the same battery jump start cable you used in TEST 1, ground and hold the spark tester to the tower as shown in the photo.
- Also, you MUST use a small piece of vacuum hose to attach the HEI spark tester to the cap tower being tested (please see the photo above).
- Ask your helper to crank the engine while you observe the HEI spark tester.
- You're gonna' get one of two results: spark or No spark.
- Repeat this test with the others that did not fire off spark (if applicable).
You'll see one of two results: the HEI spark tester sparking or No spark. Let's analyze each of these results below:
CASE 1: If you got spark. Then the spark plug wire is BAD, replace all of them as a set. This will solve your misfire condition and misfire codes lighting up your CHECK ENGINE LIGHT (CEL). Now, if the rest of the distributor components (distributor cap and rotor) are as old as the spark plug wires you're replacing... I recommend replacing them too.
Here's the why of the No spark result: As the spark plug wire gets older, its normal resistance to spark increases to the point that the it can't and doesn't channel spark to the spark plug. This will either cause a misfire, or a lack of power, or a No Start Condition. The spark plug wires don't last forever, especially after-market ones (average life-span is 3 to 4 years).
If you need/want to save money on these parts, check out the section: Where To Buy The OE Parts and Save $$$.
CASE 2: If you got No spark. This is a clear indication that the distributor cap is BAD. Replace the distributor cap and distributor rotor as a set. This should solve your misfire problem. Now, if the rest of the distributor components (spark plug wires and spark plugs) are as old as the distributor cap and rotor you're replacing... I recommend replacing them too.
Here's why: As the distributor cap ages, the terminals that transmit the spark to the spark plug wires corrode. This corrosion increases the resistance to spark and over time (as more corrosion is created) this same corrosion stops the spark from passing thru' to the spark plug wires.