I have a 2008 GTC that I purchased as a bit of a project. I've been working on it for a few months and have it drivable. It runs fairly well, but there's two things I've noticed:

1) Frequently at an idle the engine will lope a bit. If I touch on the brake once the motor smooths right out for 5 - 10 seconds and then goes back to lope-ing a bit.
2) Under acceleration it's noticeably louder than my 2004 GT, and vibrates some, seemingly down in the transmission area... about where the turbos are located. It somewhat feels like a driveshaft out of balance, but not quite... but seemingly similar to a plugged exhaust. If I back off the throttle, and maintain highway speed on a smooth highway, the vehicle is smooth and quiet.

Could a vacuum leak be the common denominator here? Are the turbos somehow vacuum controlled and if the vacuum is low, the turbo isn't responding correctly, and thus the engine is fighting the turbo? I have one engine code of a flat-lined right O2 sensor... which I'm fixing to change... if I can figure out how (short of pulling the engine).

I apologize about the typo in the title... I think I fat fingered a T rather than an intended F in "dysfunction".

 

Yes, vacuum manages the turbos. The dump valves (wastegate) are vacum actuated. No vacuum would keep the dump valves open and no boost. The trouble codes for a vacuume leak would surely show up. Short of a smoke test one could spray with caution starter fluid behind the engine and see if the engine picks up.

If the failed O2 sensor is pre catalytic converter, closed-loop engine management would suffer (warm engine). A failed pre catalytic converter O2 sensor can ruin the catalytic converter. Driving a vehicle with a bad O2 sensor could be an expensive mistake. The failed O2 sensor could be the source of the performance problem. Hopefully the catalytic converter is still okay. Check the IM readiness, this is the eight function test done at most smog test stations. Two of the eight IM readiness test are catalytic converter function and O2 sensor function, they will only be shown as either pass or fail. This test would only be an indication of a problem. A search online of IM readiness will be a better explanation than I can give. Depending on what type of scanner is used, this function may be available in addition to the standard trouble codes. The Ross Tech vcds may refer to it as "readiness".

 

Thank you. That's what I suspected. I haven't seen a trouble code of a vacuum leak yet, not that there isn't one... I just haven't come across it yet. I guess read and follow instructions (when all else fails).

I've ordered a smoke tester... I figured that would be the next logical step with this project.

Is the pre cat O2 sensor the 3 wire or 5 wire sensor? And what kind of a learning experience is it to change?

I suspect it's likely ok. I've only recently rehabilitated the vehicle to "drivable" and only driven it <2 miles. When I bought/received the vehicle it had sustained a light engine fire... the batteries where disconnected. The fire was confined to the right rear of the motor... cooked the right side 2 VVT wires, one cam sensor wire, the two right O2 sensor's wires and the bottom of the right air cleaner box. I believe I rescued the vehicle from likely a less glamorous life. I've repaired the burnt VVT wires and replaced the cam sensor and air box... and attempted to repair the O2 sensor's wires, but I'm guessing either my farmer fix of one of the O2 sensor wires wasn't good enough or the O2 sensor is shot.

Will do. Thank you for the insight.

 

Not sure which wire configuration on the O2 sensors. The left side (driver side USA) can be done with engine in, a [P.I.A.], requires disconnecting exhaust system and pulling out the catalytic converter. There is a video online, showing the procedure. The video was done by some British bloaks. The right side is probable reason to pull the engine?

Quite a project. Photos would be interesting to see.