Blown Head-Gasket: A Mk-III Supra Need-to-Know.

Note: This article is intended as a guide for stock or near-stock condition engines. Engine modifications may introduce factors into the head gasket equation that are beyond the scope of this article.

Introduction

Any Mk-III Supra owner should know about Cylinder Head Gaskets. These model Supras with 7MGE and 7MGTE engines are notorious for blowing head gaskets. Blowing a head gasket is an engine crippling event, and can cost a lot of money to repair. It is, within reason, a preventable accident, but only if caught within time.

What is a head gasket?

A cylinder head gasket is a wafer of fibrous material that is sandwiched between the two halves of your engine - the head (at the top) and the block (bottom). The block is the largest single component of the engine, and forms the cylinders. As part of its manufacture it also has water and oil channels which carry coolant and lubricant around it and into the head. The head sits on top of the block and contains the valves, cams, spark plugs, and forms the roof of the cylinders. The gasket’s function is to seal the gap between the two halves, and prevent oil water and combustion gasses from escaping from their designed areas.

Click HERE for a view of a new head gasket

Click HERE for a view of a cylinder head (top view)

Click HERE for a view of a cylinder head (bottom view)

Click HERE for a view of a cylinder block

What is a blown head gasket?

A Blown Head Gasket (BHG) is where the gasket fails, and stops sealing. There are many reasons for this, and many effects of it failing. In simple terms the gasket breaks down, and allows the combustion gases to leak through into the cooling system and/or the lubrication channels. A BHG may also allow oil and/or water to be sucked into the cylinders during the period in which the cylinder is in vacuum.

Click HERE for a view of a blown gasket (wide shot, one cylinder)

Click HERE for a view of a blown gasket (close up, same cylinder)

Are all Mark-III Supras vulnerable to BHG?

All cars are vulnerable to BHGs due to various factors - a head gasket is a fallible component like any other part of your engine. The 7MGE & 7MGTE engines in the 3 litre Mk-III Supras are VERY prone to blowing gaskets. The primary cause of this is due to incorrect assembly during manufacture - the specification for the torque applied to the head bolts in the factory was approximately 20-30 lbs-ft too low, meaning normal running of the engine will eventually cause the gasket to fail. Toyota have never admitted this to be a problem, but did retrospectively change the torque specifications after the fault became known.

What can I do to prevent a BHG?

If the gasket in your engine is still in good condition, a BHG can be near permanently prevented by re-torquing the head. The procedure for doing this can be found HERE.

This is not a guaranteed fix for the problem, particularly if your engine has been driven for some years without this being done. It will generally prevent the problem from occurring for a lot longer, and in some cases will help extend the running life of the existing gasket if it has already begun to fail.

How do I know if I’ve blown a gasket?

The most usual means of detecting a BHG is by noticing your temperature gauge skyrocketing. A BHG does not directly cause the engine to run hotter - the break in the gasket allows combustion gasses to leak into the cooling system, forcing radiator water out into the overflow bottle. Eventually too much water gets forced out and the engine cannot keep itself cool. The result is an overheating engine. In this case your overflow bottle will most likely be full to overflowing, and the auxiliary electric cooling fans on the radiator will be going full blast.

The symptoms of a BHG can many and varied. Other possible indicators are:

A bubbling noise heard from behind the dashboard (air in the cabin heating system)
Strange coloured slimy deposits on the inside of the oil filler cap
Water in the sump
Steam coming out of the exhaust
Very discoloured radiator water
Bubbling and/or foaming in the radiator or overflow bottle
Misfiring or car refusing to idle
Sudden loss of power (if driving when gasket blows)

BHG’s can sometimes be hard to prove. There are many factors which may cause an engine to exhibit BHG-like symptoms, and you don’t want to have to be pulling your engine apart should it be simply a case of a dead radiator cap. Some of the ways to test for BHGs are as follows:

Exhaust gas test: A BHG will normally leak exhaust gasses into the cooling system. This makes the radiator water acidic, and this can be detected by a chemical test. In New Zealand the piece of equipment to do this is called a Tee-Kay tester. To use it you run the car with the radiator cap off, and suck the water vapour from the radiator through the tester. A BHG will show up as a colour change in the test liquid. This test is very sensitive to exhaust gasses, and will easily prove the existence of a BHG. However it is also prone to returning false negatives - showing no problems when in fact the motor has BHG’d.
Radiator pressurisation test: Under normal conditions your cooling system should pressurise to the release pressure of your radiator cap. This pressure should be maintained for as long as the car remains warm (ie running or soon after shutdown). Similarly if you artificially pressurise the system while the car is cool, it should hold that pressure for some hours. Using an air compressor you can raise the pressure of the cooling system to a bit above normal running pressure, and leave it to see whether the pressure drops. If it does drop significantly within about an hour, then it is likely that there is a BHG or a cooling system leak. If the pressure does drop, start the car after leaving it pressurised for some time - a BHG will allow coolant into the cylinders, and this should produce a large amount of steam out of the exhaust as soon as you start the engine.
Cylinder pressurisation test: This is the reverse of the radiator pressure test. It involves pressurising each cylinder through the spark plug hole, and looking to see whether the level of the radiator water rises. This is not a particularly accurate test, but will show up a badly blown head gasket. To do the test effectively each cylinder must be at Top Dead Centre to ensure the valves are closed - if they are not the compressed air will just pass out through the intake or exhaust.

What’s it going to cost me to fix?

That all depends on whether you get the job done professionally, or whether you do it yourself. Typically a garage will end up charging between $1500 and $2500 for the whole job, parts included. This is a fairly variable figure, as a lot of the cost will depend on what work needs doing to the engine once it is apart.

The major costs of doing the job yourself are as follows:

The head gasket kit: Part number 04112-42023. Approximately $300+GST retail from Toyota. This includes the head gasket and all other upper engine gaskets and seals necessary to do the job. It also includes valve stem seals which should also be replaced if the head is coming off

Machining the head: Should there be marks or pits in the surface of the head or the head has been warped due to excess heat, then it will need to be professionally milled. This is not a particularly complex process, but requires that nearly everything is removed off the head before it is milled. This should set you back no more than ~$150. It is advisable to avoid milling the head unless it is genuinely required - removing material from the surface of the head will raise the compression ratio of the car, and make detonation/knock more likely to occur.

Machining the block: Similar to the head, the engine block is susceptible to corrosion and wear and may need resurfacing. This is not particularly common on 7Ms however, as the cast iron block is usually quite resistant to pitting or warping. The cost of machining the block is variable, and requires that the block is removed from the vehicle first.

What can I do to prevent another BHG?

If the BHG repair is done correctly the first time, you should never have further problems with it. An engine that BHGs once is not any more likely to BHG in the future unless it has been damaged in some way (eg warped head or block). Continue BHGs are a sign of other significant problems with the engine.

Once the BHG repair is complete, there are a couple of good rules to follow to ensure that the gasket seals correctly:

For the first 1000Kms after the repair:

Use little or preferably no anti-freeze in the cooling system.
Drive the vehicle very gently. A good rule to follow is that if you see your boost gauge going over the zero point you need to back off.

After 1000Kms:

Retorque the engine as per the article mentioned previously. For a stock gasket you should be retorquing the head to 70-75 lbs-ft. The procedure should be carried out with the engine stone cold, or as close as possible to it.
Drain and refill the cooling system, adding anti-freeze in the usual quantities.
Preferably do an oil and oil filter change to ensure that any impurities such as gasket debris are removed from the oil system.

And if you're really in love with your engine:

Retorque again after a few months to ensure all slack has been taken up on the head gasket.