The 2004-2009 4Runner equipped with the 1GR-FE, 4.0 liter V6 is prone to cold start misfires, particularly on cylinder number six in combination with coolant consumption (frequently with no external leaks present). The codes can be any P0301, P0302, P0303, P0304, P0305 but most common is a P0306. The engine will often run rough for the first few seconds, but otherwise run smoothly. The 2004 model year seems to be the most prone to the issue at high mileage.
Once the common causes have been eliminated (swap ignition coils, check spark plugs, etc.) and found to be solid, a DIY owner or auto mechanic should use a chemical test to check for head gasket failure. The kit looks like this:
On 4Runners with a failed head gasket, coolant is burned in the cylinders and exhaust passed into the coolant passages. Some of these exhaust gasses dissolve into the coolant, leaving trace hydrocarbons the chemical tester reacts with. By sampling the vapor in the radiator and pulling it through the tester and chemical, the hydrocarbons react with the chemical and change the color of the normally blue fluid to a light green or yellow. This is highly accurate, as normal coolant does not react with the chemical (don’t suck liquid coolant into the tester, it ruins the result). Combined with the random misfire and coolant usage this is enough to condemn one bank’s head gasket properly.
CARspec uses this procedure to diagnose 1GR-FE 4Runner head gasket failures, saving the owner money over doing a leakdown test that usually requires additional labor. As Eden Prairie and Chanhassen’s Toyota and Lexus repair specialist we hope this helps a 4Runner owner or auto mechanic diagnose this issue quickly.