997TT misfires, no CEL
09-19-2018, 12:27 AM
#1
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997TT misfires, no CEL
Hey guys. I thought it would be a nice vacation to road trip from Texas to California, and spend some time in California. Tonight I noticed that my car misfires under heavy acceleration, but only if I absolutely floor it, not if I roll into it. There are also no codes showing up for misfires but I know one when I feel one. Any ideas??? Most likely not fuel, I have been using Torco when filling up as a safety cushion for crappy California gas. One thing to note is the car had a catless exhaust installed a couple of days ago. Any ideas would be GREATLY appreciated, it’s a stressful start to my vacation to say the least!!
I will I’ll add that my primary suspect is spark plugs. I think they need changing but I don’t know if they would cause misfires without a code.
I will I’ll add that my primary suspect is spark plugs. I think they need changing but I don’t know if they would cause misfires without a code.
09-19-2018, 08:57 AM
#2
RL Community Team
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If it's a new problem, the first place to check is the last thing worked on, so in your case, the catless exhaust.
To run catless without throwing CEL codes, the tuner must have done something to prevent the DME from looking at the O2 sensor codes, or at least the secondary ones. Unfortunately, the DME uses the info from the O2 sensors to adjust the fuel trims and timing and it sounds like you've got a problem with one or the other or both. It could be causing predetonation of the fuel and the knock sensor may be pulling timing to protect the motor which may feel like a misfire. Who tuned the motor after removal of the cats and do you have printouts for the fuel mapping throughout the rev range, especially at the RPMs where you're feeling the misfires. You need AFR and MAF readouts at a minimum. Personally I'd go to high flow 200 cell cats (high quality ones like HJS) and reenable the secondary O2 sensors, and I bet your problem goes away. If the car is leaning out at high RPM, that is quite bad for the motor.
To run catless without throwing CEL codes, the tuner must have done something to prevent the DME from looking at the O2 sensor codes, or at least the secondary ones. Unfortunately, the DME uses the info from the O2 sensors to adjust the fuel trims and timing and it sounds like you've got a problem with one or the other or both. It could be causing predetonation of the fuel and the knock sensor may be pulling timing to protect the motor which may feel like a misfire. Who tuned the motor after removal of the cats and do you have printouts for the fuel mapping throughout the rev range, especially at the RPMs where you're feeling the misfires. You need AFR and MAF readouts at a minimum. Personally I'd go to high flow 200 cell cats (high quality ones like HJS) and reenable the secondary O2 sensors, and I bet your problem goes away. If the car is leaning out at high RPM, that is quite bad for the motor.
09-19-2018, 09:11 AM
#3
Rennlist Member
When you say "no codes" do you mean no stored codes, no CEL, or no misfires noted in the log?
I'm betting that there are misfires being noted, just not enough to record a code or trip the CEL. Under boost, good plug and coils are critical. I think you are on the right path with the plugs.
Not buying the O2 theory. IIRC, the secondary sensors only serve to monitor cat health. The do not impact drivabiltiy. Am I wrong about that?
I'm betting that there are misfires being noted, just not enough to record a code or trip the CEL. Under boost, good plug and coils are critical. I think you are on the right path with the plugs.
Not buying the O2 theory. IIRC, the secondary sensors only serve to monitor cat health. The do not impact drivabiltiy. Am I wrong about that?