Sun Mar 16, 2014 10:20 am
Had a nightmare with my car yesterday. It had started misfiring at partial throttle and a check with an OBD-II scanner didn't show any faults or pending faults, but the live data showed the O2 sensor was a bit lazy so I replaced that on Wednesday.
Thursday I still had the misfire and it now showed cylinder 4 misfire on the OBD-II scanner. I left it parked Friday and went and got some parts. It's an 05 Subaru 2.5 RS and the plugs are down long tubes that go through the valve cover. I knew the seals on the tube for cylinder 3 was leaking so I got a complete valve cover gasket set and a set of plugs. I get the right hand side (1&3) done fine, both had been leaking a bit, but the plugs looked fine. The left hand side (2&4) was anther matter. Both were leaking and plug 2 was fine, but the first bit of bad news was when I unscrewed plug 4 and pulled out the socket. The socket came out with only the terminal from the plug and its stalk down the insulator. I used another socket that gripped on the insulator rather than the terminal and got the plug out of the tube to find the ground electrode had been melted away. I can't believe the thing only misfired sometimes, I'm seriously impressed by the power of the ignitor.
I thought this was bad enough but it was only just getting started, I try to put a new plug in number 4 but the thread is stripped. I always lubricate the threads and use a torque wrench with the reduced torque figure for lubricated threads so it must have been all the other damage that meant the threads were just done in. I got a helicoil set and did the grease on the tap with multiple cleanings trick to avoid having to pull the head so hopefully that's OK and didn't have any chips go through to the cylinder. I've got the helicoil in, new plugs, new wires and a new gasket set. I'm just waiting for the sealant on the helicoil to set before giving it a gently test this afternoon. Hopefully it's good otherwise it's going to have to go to a mechanic for at least the head off and maybe even a look at the piston.
Moral of the story is to keep an eye on those spark plug tubes regularly and replace the seals as soon as they show any leaks at all, and not wait a few months for the temperature to warm up when it is nicer to work on the car. I must have had enough oil leak to cause the lead to short in the tube, making it misfire occasionally but also heating and potentially setting fire to the oil enough to damage the plug and the plug threads when I removed it.
I had no idea it was that bad from the driving symptoms. Every other car I've had has been far more sensitive to the ignition system not being perfect. With the plug in that state I would expect it to barely if ever actually fire, but the idle was smooth and I only got the misfire after spending more than 5 minutes on the highway. Fingers crossed on my fix working...