w204nyc

Ive been having a number of CELs this past week and my mechanic nor the dealership can figure out what the problem is. Last week it was misfire in cylinder 1,2,5. They changed all 8 spark plugs and coils. Then on tuesday I got another CEL. This time it was a misfire in all but cylinder 8. They ran a test and cylinder 2 and 6 were acting up. So they ordered brand new spark plugs and installed them. Everything went fine. The next day the car was shaking while idling and the exhaust was acting weird. It felt like it was going to shut off. Yesterday another CEL came on. The mechanic ran the codes and it was an oxygen sensor that had a short circuit. The problem
Is that it never showed up before. The mods
On my car are an OE tune and removed secondary cats and resonator. I dont have o2 extenders and i had it retuned to turn off the secondary cats.

CarHopper

Who told you they turned off your secondary cats? I ask because never go back to them... The secondary cats have no o2 sensors or anything related to them other than cleaning up a little more exhaust. As far as the car is concerned, it doesn't even think they are there in the first place.

Did you check your MAFs? Make sure those are working correctly. Otherwise, it could be a bad o2 sensor(s) or cats. Probably wouldn't hurt to put the stock file back on the car while you sort this issue out either.

w204nyc

Yea when I had my car retuned, he told me he was going to turn the rear o2 sensors off since i didnt have sec cats. Not sure what the MAFs are! When he was checking the diagnostics, it showed my rear sensors and it just said no for both. Idk if they need to be changed or not. Someone said this isn't tune related but not sure what to do

CarHopper

Ah, okay. So the way it works is your primary cats have a sensor before them and after them. Not sure why they would turn off the rear ones though because once again your secondary cats are further down stream than even that rear o2 sensor they shut off.

MAFs are the things sticking out of your intake near the throttle body when you pop the hood.

Someone saying it isn't tune related could be right, but going back to a stock file eliminates one more option of what it could be. I'd also have your rear o2 sensors turned back on because that makes no sense to me.

w204nyc

I guess I can do that. Just flash back to stock. I was also thinking maybe I should put my secondary cats back on. Maybe whoever cut them out didnt do it properly. I'm not able to pass state inspection cause the monitors aren't ready!

CarHopper

That's because of your tune/programming, not the secondary cats. I'm in NY too so feel your pain.

Your secondary cats, again, contribute absolutely nothing to the car in terms of passing inspection, CELs, etc. You can leave them out. Your problem here is the fact that someone thought it was a grand idea to turn off your CEL and turn off your rear o2 sensors. This is what is preventing your monitors for setting to "ready"

Turn all your o2 sensors on, your monitors will set to ready. You CAN leave your secondary cats out, again, they do nothing.

If that doesn't fix your CEL issue, flash back to stock and drive the car.

If that doesn't work then I'd explore having either bad o2 sensors, bad MAFs, or possibly bad cats.

w204nyc

Yea I'm going to go back to the guy who updated my
OE tune and have him turn thm back on if they really are turned off! Thanks for your help brother

roadtalontsi

same tank of gas? bad gas? 11.3:1 compression doesnt like crappy gas. that's where i'd start.

w204nyc

I try and stick only to mobil gas stations!

roadtalontsi

its good to stick with a name brand station but any station can get contaminated fuel.
-on a side note, i love when people say they always put good gas in their cars, you know they use premium costco and kroger gas lol.

Gerry_M

Whats wrong with Costco Premium?

rentzington

Funny enough costco is top tier rated fuel now, so it should be good to go in theory.

i always try to stick to qt or chevron myself, except for my truck that gets whatevers cheap and will run on anything put in it.


curious to see what happens when restored back to the stock tune.

roadtalontsi

not sure if this is sarcasm. You get what you pay for. Had to replace a v12 -block 28k$ and pistons $600 each, lol. This is because of massive carbon buildup and detontation it caused, killed the cylinder walls causing misfiring and oil consumption. Knock sensors will retard timing to save the engine but long term time and time again damage is slowly done over 60k miles.

I am Jeff

Some can blame cheap costco gas, but if it's rated tier one, I don't see the issue with using it. The bigger issue with carbon build up is that people don't flog their cars enough. These motors need to run through the RPM ranges.

http://www.toptiergas.com/retailers.html

rentzington

I think costco is a fairly recent addition to the toptier list. I stopped buying their gas years ago because it was so awful. it should in theory be good now.