Summary: Mopar 4.7 engines used in many Jeep and Dodge vehicles share a common problem. Misfires which are accompanied with coolant loss, rear heater not working properly and engine temperatures running a higher than normal range, could have a leaking head gasket. There’s a couple of simple ways to check for a blown head gasket, one is by using a block test kit, another way is to use a cooling system pressure tester and removing the spark plug in the misfiring cylinder.

It’s best to eliminate the simple possibilities first when diagnosing problems with any vehicle. Coolant loss? Pressure test the cooling system and see if there’s any external leaks. But what if there are no external leaks? Then there’s a possibility that the coolant may be leaking inside the engine. Due to the combustion chamber pressure being higher than the cooling system pressure, coolant can leak into the cylinder when the engine is turned off. When the motor is running the coolant that leaked into the cylinder, will be burned in the combustion chamber and in the catalytic converter, leaving little indication of a problem since tailpipe emissions may seem normal. The 2002 Dodge Durango featured in the images had not only coolant loss, but also had a history and a stored misfire code for the number three cylinder – P0303. Someone previously had replaced the #3 spark plug in hopes of curing the problem with the misfire… needless to say it wasn’t going to be that easy. With the spark plug out and pressure testing the cooling system the #3 cylinder filled with coolant quickly. So much for a simple repair.

Off with her head! The image above with the driver’s side cylinder head removed reveals an obviously washed #3 cylinder. Notice there is no carbon build up on the top of the piston. When coolant gets into a cylinder, the tell tale sign of washed piston and valves is the result.

The head gasket in the image above shows where the leak was. The triangle shaped area for a coolant passage has missing black coating where the coolant was entering the cylinder.

Again the washing is evident on the affected cylinder. But what was the cause? The most common cause of a blown head gasket usually is from an overheated engine from a coolant leak. The cylinder head can warp due to the excessive heat and the gasket will not seal any longer. However, this Durango had no other coolant leaks.

After further inspection, a pitted area between the coolant passage and the combustion chamber was found. Pitting like this can be caused by electrolysis – an electric charge in the cooling system. A reading may actually be taken with a meter in the coolant. Over time the electrolysis can eat away at the aluminum until the gasket can no longer seal. A cylinder head like this can be machined as long as the pits are not too deep. If the damage is too deep replacing the cyl. head is the only option.