Tips For Diagnosing Possible Head Gasket Issues

I wanted to put this article together to help you diagnose a possible blown or leaking head gasket. The particular symptoms we are talking about here are an engine misfire, typically more pronounced on cold startup or after idling for a while and unexplained loss of coolant. Please keep in mind that there are many more symptoms of head gasket issues, such as water in the oil, oil and/or combustion pressure in the cooling system, external head gasket leaks (as seen in this article) etc, but these are just the ones we will be covering in this article and I will do further articles looking at other symptoms as the cars/engines become available to me.

Ok, so this particular vehicle, a 2010 Subaru Forester had the top tank of the radiator split and an overheat not so long ago. The radiator was replaced and not long after a misfire was noticed upon cold startup. This turned into an intermittent misfire during driving.

A quick note before we go any further – in their wisdom Subaru decided to omit the temperature gauge in Forester’s from around 2009 onwards and instead opted for a light on the dash that stays blue until the engine has reached operating temperature, then switches off, and then flashes red in the event of the engine getting hot. In years gone by we used to call theses ‘idiot lights’ and for good reason (these where first seen in early Holden’s and the like)

Typically a driver will get used to where a temperature gauge sits normally, I’m sure a lot of you even with just a casual glance at the dash would realise when the temp gauge needle is above where it normally sits and this action would typically give you time to pull over, slow down, whatever, before the needle edges into the red zone. Now I don’t know exactly what temperature the coolant has to be for the light to go red on the Subaru but suffice to say you are going to be blissfully unaware of a cooling system issue until the dreaded red light comes on. Typically with a gauge the rising needle is an indication of trouble ahead, no such luxury with an idiot light, by the time the light comes on and you take the necessary action the damage is already done. So if you own a 2009-onwards Forester without a temp gauge do yourself a favour and get an aftermarket gauge fitted or an OBDII interface and smartphone app that can monitor the coolant temperature as you are driving. Get a phone cradle too, don’t want to be seen touching your phone while driving 🙂

Ok, back to the article. So we have a Subaru that has an occasional misfire after an overheat, experience tells me that those symptoms alone would be enough to suspect a head gasket or two but let’s confirm this before spending lots of the customers money on a mis-diagnosis.

First things first, let’s get all the spark plugs out and have a look. The passengers side plugs look fine (remember this is a flat four, two cylinders per side in a flat configuration), however the drivers side plugs tell a different story.


As you can see the insulator on the spark plug on the right is clean compared to the insulator on the left. Also the tip of the electrode is not worn down as much on the plug on the right. Both of these could be signs of coolant in the cylinder – the insulator is clean because of the cleaning action of the water and coolant and lack of wear on the electrode could indicate that this plug has not been firing correctly, once again possibly due to the presence of coolant in the combustion chamber.

Again this points to a blown head gasket, but there is one more test we can do to be 100% sure.

A quick note about this next test – This needs to be done with the engine at operating temperature. Yes it makes getting the spark plugs out all the more interesting on a hot engine but the head gasket leak may close up when the engine is cold and the cooling system will be pressurised at operating temp also, which is important for a correct result.

With the spark plugs out of the engine we want to turn it over and see if any coolant is pushed out of the cylinders (remove the EFI fuse so you don’t have the fuel pump operating and the coils sending spark dancing around the engine). If you have an assistant to help you with this great, I didn’t so I’ll show you what I did. I rigged up a switch from the positive side of the battery to the solenoid on the starter motor.



And set some white printing paper up just out from each bank, you can usually find a way of keeping it in place easy enough.


With the paper in place over all cylinders hit the switch, or have your assistant hit the switch for 10-15 seconds. Then remove the paper and see what you’ve got (make sure you remember which piece of paper corresponds with which cylinder). As you can see below the rear cylinder on the drivers side of our Subaru has a leaking head gasket, there is undeniable evidence now!


And just for interests sake, this is what I found when the heads were removed –


Wet, discoloured valves and a section of the combustion chamber clean of carbon.


And a corresponding clean section of piston and bore. Also from this picture you can see why these engines in particular suffer from head gasket issues after even a slight overheat, the amount of block surface that the gasket has to seal against is minimal, with the majority of the area being made up of coolant jackets. This is also a really good reason not to use an abrasive wheel on the block surface when removing the gasket, a single-sided razor blade will remove all of the old gasket and won’t cause any hollows or low points on the block like an abrasive wheel can.

Anyway, I hope that has helped you diagnose your possible head gasket woes or you at least found the article interesting 🙂

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