How To Test For A Blown Head Gasket (1990-1995 5.0L V8 Ford Mustang)

If you suspect that one or both of the head gaskets on your 1990-1995 5.0L V8 Ford Mustang have failed, there are four tests you can do to find out. Two of these tests don't require any tools whatsoever and can be done in under five minutes.

The other two tests that I'm going to show you're a compression test and a block test. The compression test, of course, requires a compression tester, and the block test is done with a combustion leak detector.

All four tests are explained step by step, so you can quickly and easily figure out if you've got a blown head gasket on your hands.

APPLIES TO: This tutorial applies to the following vehicles:

• 5.0L V8 Ford Mustang: 1990, 1991, 1992, 1993, 1994, 1995.

Symptoms Of A Blown Head Gasket

A head gasket failure on your 5.0L V8 Ford Mustang is usually the result of the engine severely overheating. If this happens and one or both head gaskets fail, you're going to see one or more of the following symptoms:

• Overheating: The engine runs hot, and you can't find an obvious reason for it.
• Oil and coolant mixing: The oil turns a milky color —looks like coffee with way too much creamer— from coolant leaking into the crankcase.
• White exhaust smoke: Thick, white smoke coming out of the tailpipe from antifreeze burning inside the cylinders.
• No-start condition: The engine will crank, but it just won't fire up.
• Cooling system overpressure: The radiator gets overpressurized and starts pushing coolant out whenever the engine idles or after you shut it off.

TEST 1: Oil The Color Of 'Coffee With Too Much Creamer'

One of the most common results of a blown head gasket is coolant leaking into the crankcase. When this happens, the oil mixes with the coolant and starts looking like coffee with way too much creamer.

So for our first test, we're going to pull out the engine oil dipstick and check the color of the oil sticking to it.

If the engine oil is milky white, then it's confirmed that you've got a blown head gasket on your hands.

If the oil looks normal (the usual dark and dirty color), then the next step is to go to TEST 3 and see if combustion gases are escaping into the coolant system.

Alright, here's what you need to do:

1. 1

   Open your Ford Mustang's hood.

2. 2

   Pull out the engine's oil dipstick.

3. 3

   Check the color of the oil.

4. 4

   You'll see one of two things:
   
   1.) The engine oil will be a creamy tan/off white color and as thick as syrup.
   
   2.) The engine oil will be a normal color and viscosity.

Now, let's find out what your result means:

CASE 1: The oil is a light tan or creamy white color. This is a sure sign that one or both head gaskets on your 5.0L V8 Ford Mustang have failed and will need to be replaced.

CASE 2: The oil looks its usual dark color. That's the normal and expected result, but it still doesn't completely rule out a head gasket problem.

The next thing to check is whether the engine is forcing combustion or exhaust pressures into the cooling system. To do that, go to: TEST 2: Exhaust Gases Shooting Out Of The Radiator.

TEST 2: Exhaust Gases Shooting Out Of The Radiator

One of the jobs the head gaskets have is to seal the pressures created within the cylinders as the engine cranks or runs.

And by pressures, I'm talking about compression, combustion, and exhaust pressures produced during the combustion process.

Now, when the head gaskets are OK and doing their jobs, none of these pressures are allowed to escape into the engine's cooling system. But when one or both head gaskets fail, this seal is lost, and these pressures get pushed into the engine's cooling system.

You and I can easily check if this has happened by simply removing the cap from the radiator and then cranking the engine. We can expect one of two things:

• The coolant in the radiator will get pushed out forcefully while cranking the engine —confirming that one or both head gaskets have failed.
• The coolant in the radiator remains undisturbed —which is the normal and expected test result.

CAUTION: Make sure your Ford Mustang's V8 engine is cold or completely cooled down before doing this test. Otherwise, you run the risk of serious burns from escaping coolant. If you need to speed up the process of cooling the engine down, you can place a fan over the engine.

Here's what to do:

1. 1

   Remove the cap from the radiator.
   
   Make sure the tank has enough coolant. If it's low or empty, add water or coolant until it reaches the proper level.

2. 2

   Have your helper crank the engine while you watch the coolant level from a safe distance.

3. 3

   You'll see one of two things:
   
   1.) The coolant inside the radiator shoots up or bubbles out.
   
   2.) The coolant level remains undisturbed —cranking the engine has no effect.

Here's how to read the results:

CASE 1: The coolant either bubbles up or shoots out. That's a clear giveaway that one or both head gaskets on your 5.0L V8 Ford Mustang have failed.

CASE 2: The coolant stays calm and undisturbed. That's the normal and expected outcome.

If your Mustang starts and runs but overheats for no obvious reason, go to: TEST 4: Using A Chemical Block Tester (Combustion Leak Tester).

If your Mustang starts but runs with a misfire (or rough idle), go to: TEST 3: Doing A Compression Test.

TEST 3: Doing A Compression Test

One of the results of a blown head gasket that I've seen in these V8 engines is when one of the two head gaskets fails in a section right between two side-by-side cylinders.

When this happens, those two side-by-side cylinders produce zero compression. The end result of these two "dead" cylinders, when you crank and start the engine, is your Mustang running on only six cylinders.

The way to diagnose this particular type of head gasket failure is to do an engine compression test on all eight cylinders and see if you've got two side-by-side cylinders with 0 PSI compression.

NOTE: If you need a more detailed explanation of how to do the compression test and interpret its results, you can see this tutorial here: How To Do And Interpret An Engine Compression Test (1990-1995 5.0L V8 Ford Mustang).

OK, here's what you need to do:

1. 1

   Disable the ignition system.
   
   You can easily accomplish this disconnecting the ignition coil's electrical connector (if your vehicle has a distributor) or the ignition coil pack's electrical connector.

2. 2

   Disable the fuel system. It's important that fuel not be injected into the engine.
   
   You can do this by removing the fuel pump relay or the fuel pump fuse.

3. 3

   Disconnect all spark plug wires (from their spark plugs) and then take out all of the spark plugs.
   
   NOTE: Before you remove the spark plug wires, label them so that you'll be able to easily reconnect them to their proper original locations.

4. 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. 5

   Have a helper crank the engine while you observe the compression tester.

6. 6

   Once the gauge's needle stops climbing, have your assistant stop cranking the engine.

7. 7

   Write down the reading and what cylinder it belongs to (you can use the image in the image viewer to help you identify the cylinder) on a piece of paper.

8. 8

   Remove the compression tester and repeat the above steps in the remaining cylinders.

OK, let's take a look at what your results mean:

CASE 1: All cylinder compression readings were about the same. These results confirm the head gasket is doing its job and hasn't burned through between two cylinders.

If you still have reasons to suspect a blown head gasket —especially if your Mustang overheated recently— the next thing to do is a block test with a combustion leak detector. Go to: TEST 4: Using A Chemical Block Tester (Combustion Leak Tester).

CASE 2: Two cylinders side by side showed 0 PSI compression. This reading confirms the head gasket has burned through at the spot between those two cylinders. Bad news here —the head gaskets will need to be replaced.

TEST 4: Using A Chemical Block Tester (Combustion Leak Tester)

Over the years, I've run across many vehicles with a head gasket in the early stages of failure. So, when I checked the engine oil, it was OK —it wasn't that milky white color like coffee with too much creamer.

Also, combustion, exhaust, or compression pressures weren't being pushed into the cooling system. And, of course, compression was good across all cylinders.

But the vehicle was overheating within minutes of starting its engine (and for no apparent reason). In these cases, the only way I was able to confirm that the issue was due to a blown head gasket was by doing a block test with a combustion leak detector.

And let me tell you, this is the go-to test that most shops do when you take your car to them to diagnose a blown head gasket. And it's something you can do yourself.

This is how the test is done:

• A blue chemical solution is poured into the tester (see photo above).
• The tester is set on the open expansion tank neck. You may have to drain a little coolant to let the tool pull in radiator air.
• The engine is started.
• Squeeze the rubber bulb to draw air through the fluid chambers. The bubbling air reacts with the chemical.
• If the fluid changes from blue to yellow (on gasoline engines), combustion gases are present in the engine's cooling system. That means a blown head gasket, cracked block, or cracked cylinder head.
• If the fluid stays blue, then those issues are not present.

You can get the chemical and block tester at most auto parts stores or here:

Frequently Asked Questions

1.) How can I tell if the cylinder heads are cracked?

The heads have to come off for a proper inspection. If the damage is severe, the crack will usually be easy to spot with a visual check.

But when it isn't that obvious, a machine shop can pressure test the heads and let you know for sure if they're cracked or not.

2.) Do I need to remove both cylinder heads?

Yes —both should come off so they can be inspected and/or tested by a machine shop for warpage and cracks.

3.) Do the cylinder heads need resurfacing?

Yes, the heads should be resurfaced before reinstalling them. An automotive machine shop can take care of that, and it's especially important if the engine overheated.

The only time you can skip it is if you (or the shop) check the cylinder heads with a straight-edge and find them perfectly flat. If there's no sign of warpage, resurfacing isn't required.

4.) Should I try a head gasket sealer?

I don't recommend it, but I understand why people do.

Sometimes the sealer will hold for a short while if the problem is just a tiny coolant leak into a cylinder. But even then, the results are unpredictable.

The bigger risk is that the sealer flows through the entire cooling system, not just the gasket. It can plug up the radiator, heater core, or coolant passages (in the block and cylinder heads), which hurts the cooling system's performance.

And if you eventually replace the head gaskets (and you will), the hardened sealer becomes another problem. Cleaning it out is messy and can add to the repair bill.

I get it —a head gasket repair is expensive and time-consuming, even for a DIYer. That's why folks try the quick fix first. But if your 1990-1995 5.0L V8 Ford Mustang is already overheating or losing coolant fast, a can of sealer won't cut it. And if it does, it usually doesn't last.

More 5.0L V8 Ford Mustang Diagnostic Tutorials

You can find a complete list of 5.0L V8 Ford Mustang wiring diagrams and diagnostic tutorials in this index:

• 5.0L V8 Ford Mustang Index Of Articles.

Here's a sample of the diagnostic tutorials you'll find in the index:

• How To Test The Fuel Pump (1990-1995 5.0L V8 Ford Mustang).
• How To Test The Throttle Position Sensor (1994-1995 5.0L Ford Mustang).
• How To Test The MAF Sensor (1991-1995 5.0L V8 Ford Mustang).
• How To Do And Interpret An Engine Compression Test (1990-1995 5.0L V8 Ford Mustang).