All cylinder misfire and random misfire codes on 2004 manual TL with 220,000 miles. i went to change to spark plugs and one of the back spark plugs was covered in oil and had some oil in the tube above the orings. but all the old plugs looked fine, i still put in brand new ngk iridiums and i tested the coil packs woth a voltage and resistance meter and they all were fine. the engine does need a valve adjustment soon. but what could be causing these misfires??? it has an aftermarket exhaust and i’m smelling rotten eggs on hard acceleration also. but could it be valve gasket and spark plug orings causing the misfire? also i thing the rotten eggs is from my o2 sensor or cats

 

start pulling the connector on each coil until one of them doesn't change the RPM's of the engine, that's your faulty coil pack.

 

clogged cats can cause the misfire and smell...

 

pinpoint the misfire by pulling the power harness off each coil pack, one at a time.

if oil is getting into the spark plug tube, would check the seals in the valve cover to see if they are torn or out of place.

Which codes exactly are you getting?

 

Also, is your CEL flashing, or just staying on???

 

i tried doing that but they all had an effect on the rpms

 

it is always in but a lot of the time it will be flashing about 20 times and then go solid for a few minutes or it will flash the whole time

 

is there a way to verify it’s the cats. and how would i clean them?

 

Flashing CEL means you have a cat damaging misfire. It's likely due to unburned fuel. Your exhaust manifold will glow if you run it for too long.


A clogged cat will make your car noticeably sluggish. The simplest is to test the backpressure. Here's a video.

You could try cleaning it with some detergent, but I have never seen it work. The most common thing to do is to clear out of the restriction in the cat, but you will then have a P0420 or P0430.

 

Hello, speaking of code P0420...I'm reading a lot about that being a very common code and most likely is a faulty in-stream O2 sensor.
Can anyone confirm this? FYI - the '08 intake and exhaust system is all stock still...
Thanks in advance!

 

Usually P0420 is caused by bad cat. Confirm with thermal camera / IR thermometer . Find on YT how to do that.

 

Much more likely that its a bad cat.

 

Might be bad injectors as well.. but if you are getting a P0420, then it's very likely a bad cat.

 

hey guys so it’s still not yet fixed and i’m not getting a 420 code. all the codes for misfires are p0300 p0301 p0302 p0303 p0304 p0305 p0306. and no i haven’t tried fixing the leaking valve cover gasket or leaking spark plug tube seals yet. waiting on my tax refunds lol. so i’m hoping to get that done while getting a valve adjustment and maybe the misfires will go away 🤷🏼‍♂️. i also don’t think the cats are clogged cause i’m not getting any codes for it or for the o2 sensor. the smell has me puzzled for now. hopefully the misfires will stop once i get the valve adjustment and those seals replaced. because they were leaking oil into the spark plug well. what do you all think?

 

You could still have a clogged cat. You can easily tell (most of the time) if you put your hand at your tail pipe and if there isn't much pressure (compared to normal) then that's a easy indicator.
The best, and I can't remember the design of the 04 TL, is if there is a pre-cat O2 sensor and a post-cat O2 sensor; pull both and start the car. If the pressure coming from the pre-cat hole (where the 02 sensor was) is higher than the pressure coming from the post-cat O2 sensor then you know the Cat is clogged.

 

Obviously the oil on your spark plug and inside the cylinder head is causing your misfire. Oil contamination in your cylinder or spark plug means no spark, means no ignition and no combustion - hence misfire. I'm not exactly sure how a leaking valve cover gasket allows oil into the spark plug tube or cylinder head because there are seals in place to prevent oil contamination even if your valve cover gasket is leaking. A leaking valve cover gasket usually leaks oil onto your external engine parts which are hot and burn off, easily noticeable burning oil smell. Damaged spark plug tube seals, valve seals and piston o-rings allow oil into the cylinder head. You should also see white smoke exhaust if the amount of oil seeping in is severe. I would definitely replace your spark plug tube seal in that cylinder ASAP to fix the misfire. When you replace the valve cover gaskets, be sure to check the condition of the valve seals - if worn or damaged, oil will leak into your cylinder heads this way as well.

Speaking of the "smell"...un-burnt fuel due to misfire may cause a smell but it should smell more like un-burnt fuel and oil; however, that "rotten egg" smell usually (in most cases) is an indicator of coolant seeping into your cylinder heads causing the coolant to mix with the A/F mixture and resulting in the rotten egg smell after combustion. You must be sure that you DO NOT have a seeping head gasket (SHG). The coolant and A/F mixture will deteriorate the cleaning material inside your cat very quickly and cause your cat to fail very quickly. Conduct a cylinder compression test to confirm your head gaskets are not damaged. If you do not pass the compression test, then you may be facing a BHG or SHG. If the engine has a history of overheating or overheated till the engine froze or stopped, you may have damaged head gasket(s).

 

Try this