TEST 3: Checking The Cylinder's Compression
Since you have now confirmed which cylinder is dead and that it is getting spark, we're now going to test its compression. We're also going to make sure that the dead cylinder's spark plug is not broken or damaged in any way shape or form that would keep it from sparking.
I can tell your from personal experience that testing the compression of the dead cylinder is usually one of the most overlooked test when trying to diagnose a rough idle condition.
The compression of the dead cylinder has to be within 15% of the highest compression value obtained from all 6 cylinders.
Does this mean that you have to test all 6 cylinders? -Yes and no. To get a more accurate test result, if I were in your shoes, I would test all six cylinders. But it will suffice to test the two adjacent cylinders of the dead cylinder.
The following tutorial provides a more in-depth explanation of the compression test and how to interpret its results.
Alright, these are the test steps:
- 1
Disable the ignition system by disconnecting the ignition coil from its electrical connectors. This will prevent the ignition coil from sparking during the test.
- 2
Disable the fuel system by disconnecting the two throttle body fuel injectors. Disabling the fuel system will prevent fuel from being injected into each cylinder when the test is performed.
- 3
Remove the spark plugs from a slightly warmed up engine (if it starts and runs). Remember, the engine can not be hot.
When removing the spark plugs, be careful not to drop any of them on the floor, or you run the risk of having the spark plugs porcelain insulator crack and then you'll have a misfire on your hands.
IMPORTANT: Before removing the spark plugs, label the spark plug wires with their corresponding cylinder number so you won't lose their firing order. - 4
Thread the engine compression gauge into the spark plug hole for the number 1 engine cylinder.
NOTE: Hand tighten the compression gauge only! Do not use any type of tool to get it tight. - 5
Have your helper crank the engine till the needle on the compression gauge stops climbing.
- 6
Now, record on paper the value at which the needle stopped and the number of the engine cylinder on a piece of paper.
- 7
Release the pressure on the gauge and repeat steps 5 and 6.
- 8
Repeat steps 4 through 7 on the remaining five cylinders.
- 9
Multiply the highest compression reading by 0.15. Now subtract the result from the highest compression reading you got.
So, let's say that cylinder #4 gave you the highest reading of 170 PSI. Well 170 X 0.15 gives you 26 (25.5 rounded off). Now, the next step is to subtract 26 from 170, which gives us 144 PSI. So then, 144 PSI is the lowest possible compression reading that any one of the rest of the engine cylinders can have. Any reading at 144 or lower and that cylinder is dead.
You can find a more detailed explanation on this here: Interpreting Your Compression Test Results -How To Test Engine Compression (1988-1993 2.8L Chevrolet S10 Pickup, GMC S15 Pickup, GMC Sonoma).
Let's take a look at what your test results mean
CASE 1: Compression of the dead cylinder is OK. This test result tells you that the misfiring cylinder does not have compression issues/problems.
CASE 2: Compression of the dead cylinder is below 15 percent of the highest value. This is not good and let's you know that there's an internal mechanical problem causing the misfire.
I suggest that you do a wet compression test to find out if the problem is due to worn out cylinder head valves or worn out compression rings. You can find the wet compression test steps in this tutorial:
TEST 4: Testing The Distributor Cap
If the spark plug wire of the dead cylinder did not spark in TEST 2, then there's a good chance that either the spark plug wire itself is bad or the distributor cap is bad. Yes, it's a very common problem to have a spark plug wire stop transmitting spark. It's also very common for a distributor cap tower to stop transmitting spark to its spark plug wire.
Thankfully, there is a very simple way to find out without having to run out to the local auto parts store to buy a new set of wires and a new distributor cap.
The test itself involves connecting the HEI spark tester to the distributor cap tower (of the dead cylinder) using the same small vacuum hose that you used in TEST 1 (to do the cylinder balance test).
In the illustration above, of the distributor cap, you can see the HEI spark tester attached to a distributor cap tower with a small piece of vacuum hose. The vacuum hose is used just to keep the HEI spark tester in place while the engine is being cranked.
These are the test steps:
- 1
Disconnect the spark plug wire of the dead cylinder from its tower on the distributor cap.
- 2
Connect the HEI spark tester using a small piece of vacuum hose to the distributor cap tower as shown in the image above.
IMPORTANT: The spark tester should make metal-to-metal contact with the distributor cap tower. - 3
Connect the spark tester to the battery negative (-) terminal with a battery jump start cable.
- 4
Have your helper crank the engine once the HEI spark tester is set up and Grounded.
- 5
The spark tester should spark as the engine is being cranked.
Let's take a look at your test results:
CASE 1: The distributor cap tower is sparking. This test result tells you that the spark plug wire is bad and needs to be replaced.
Now to be a bit more specific: because in TEST 2 the spark plug wire did not spark but the distributor cap tower did spark (in this test section), we can conclude that the spark plug wire is defective. Since it's defective, it can no longer transmit spark from the distributor cap tower to the spark plug.
I recommend replacing the entire set of spark plug wires.
CASE 2: The distributor cap tower IS NOT sparking. This tells you two very important things. Number one that the distributor cap is bad and number two that the spark plug wire is OK.
Replacing the distributor cap should solve the misfire in problem. Now if the spark plug wires are as old as the distributor cap, I recommend that you replace them too.
More 2.8L Chevy S10 Pickup, GMC S15 Pickup, And GMC Sonoma Tutorials
You can find a complete list of tutorials for the 2.8L V6 Chevrolet S10 pickup, GMC S15 pickup, and GMC Sonoma in this index:
Here's a sample of the articles, you'll find in the Index of Articles:
- How To Test The Fuel Pump (1988-1993 2.8L Chevrolet S10 Pickup, GMC S15 Pickup, GMC Sonoma).
- How To Test Engine Compression (1988-1993 2.8L Chevrolet S10 Pickup, GMC S15 Pickup, GMC Sonoma).
- How To Test The EGR Valve (1988-1993 2.8L Chevrolet S10 Pickup, GMC S15 Pickup, GMC Sonoma).
- How To Test The Starter Motor (1988-1993 2.8L V6 Chevrolet S10 Pickup, GMC S15 Pickup).
If this info saved the day, buy me a beer!
