TEST 3: Engine Compression Test
Sometimes, when an engine overheats, it can burn the head gasket between two adjacent cylinders (remember the image at the beginning of this guide?).
In my experience, even when this happens, it’s possible for the engine to still start and run. However, because these two side-by-side cylinders don't produce compression (and basically 'dead'), the engine will run, but with a noticeable misfire.
The best way to diagnose this specific type of head gasket failure is by performing an engine compression test. And that's what we're going to tackle in this section.
NOTE: You can find the engine compression test and how to interpret its results explained in more detail here: How To Test Engine Compression (1997-2007 4.6L, 5.4L V8 Ford E150, E250, E350).
OK, these are the test steps:
- 1
Disconnect the ignition coils from their electrical connectors. This is will disable the ignition system and is an important safety precaution.
- 2
Disconnect all spark plug wires (from their spark plugs).
- 3
Remove all of the spark plugs.
- 4
Thread in the compression tester by hand, on the first spark plug hole you're gonna' start with.
Do not use any tools to tighten the compression tester. Hand tightening the compression tester is more than enough to get the proper results. - 5
Have a helper crank the engine as you observe the compression tester.
- 6
When the gauge's needle stops climbing, have your assistant stop cranking the engine.
- 7
Write down the reading and what cylinder it belongs to on a piece of paper (you can use the illustration above to help you identify the cylinder).
- 8
Remove the compression tester and repeat the above steps in the remaining cylinders.
Let's examine what your results mean:
CASE 1: All cylinder compression readings where normal. This is the correct test result and it confirms that the head gasket isn't burned at a spot between two side-by-side cylinders.
If you still suspect a blown head gasket, go to: TEST 4: Using A Chemical Block Tester (Combustion Leak Tester).
CASE 2: Two side by side cylinders had 0 PSI compression. This test result confirms that the head gasket is burned at the point between those two cylinders. Replace the head gasket.
TEST 4: Using A Chemical Block Tester (Combustion Leak Tester)
There are times when one of the head gaskets gives out but doesn’t cause any of the issues we looked for in the first three tests.
In these cases, the engine starts and runs, but then it overheats within minutes, and it’s not clear why—especially when all the cooling system components (water pump, fan clutch, thermostat, etc.) are in good shape.
In situations like these, the only solid way to figure out if the overheating is because of a blown head gasket is to do a block test using a chemical combustion leak detector.
This is how the combustion leak detector test (block test) works:
- The combustion leak detector tester is filled with a blue liquid chemical (see photo above).
- The radiator cap is removed (you may have to drain some of the coolant in the radiator since this tool needs to 'gulp' some of the air inside the radiator).
- The engine is started.
- The tester is then placed on the open radiator neck.
- The rubber bellow is then squeezed to suck in the air up through the two fluid-filled chambers. As the air bubbles up through the fluid, it will cause a chemical reaction.
- If the blue chemical turns yellow (for gasoline engines), then combustion gases are entering the radiator. This in turn confirms a blown head gasket, a cracked block, or a cracked cylinder head issue.
- If the blue chemical doesn't change color, then you can conclude that you don't have a blown head gasket, a cracked block, or cracked cylinder head issue.
You can shop for a block tester here:
Should I Replace Both Head Gaskets?
If you've been wondering why auto repair shops refuse to replace just one head gasket. Or if you're thinking about doing the head gasket job yourself and wondering if you should replace them both, I want to share with you two important reasons for replacing them both.
Both Head Gaskets Feel the Burn: Overheating is the number one reason head gaskets fail, affecting both sides of the engine. The overheating that caused one head gasket to fail has likely damaged the other one too, only a matter of time before it fails.
By replacing both gaskets, you ensure that each one can handle the typical strains and stresses that the engine subjects them to. If you leave one old gasket in place, it's just a matter of time before you're back in there replacing it too.
Labor And Time: Removing a cylinder head is no small feat—it's a labor-intensive and time-consuming process. But once you've removed the components to access one cylinder head, you've essentially done 90% of the prep work for removing the other.
Since you've already got the engine partly disassembled, and because removing the second cylinder head isn't significantly more effort, it's a smart call to replace the other head gasket too. This approach will help you avoid having to tear all down again in the not too distant future (if you decide to just replace one).
To sum up this section, replacing both head gaskets together is a wise decision that's gonna save you time, money, and stress down the line, making sure the 'job doesn’t come back to haunt you' in your Ford E150, E250, or E350.
More 4.6L, 5.4L V8 Ford E150, E250 And E350 Tutorials
There are quite a few 4.6L and 5.4L Ford E-Series van 'how to' tutorials and wiring diagrams that you can check out here:
F-Series Pickups: You can find all of the available tutorials and wiring diagrams for the 4.6L And 5.4L V8 F-Series pickups in this index:
- 4.6L, 5.4L Ford F150, F250, And F350 Index of Articles (at: easyautodiagnostics.com.com).
Crown Vic And Grand Marquis: You can find all of the available tutorials and wiring diagrams for the 4.6L And 5.4L V8 F-Series pickups in this index:
If this info saved the day, buy me a beer!
