Recently replaced my CTS thinking it may be related to my poor fuel economy (roughly 12mpg), and all seemed fine for about a day, then I started to get a huge miss/stumble under light throttle conditions such as accelerating from a light in traffic. It was bad enough that it was extremely difficult to drive the car around town. I never got a CEL during all this, and of course I began to fear opti failure, lol. It lessened some on the freeway, but was still noticeable. It only seemed to happen once the engine warmed up and went into closed loop. For the first few minutes on a cold engine, it runs fine. Unplugging the MAF and MAP one at a time seemed to make it slightly worse. Tried unplugging the CTS, and the problem seemed to go away.

That said, is it possible that the sensor just failed that quick, causing all these problems, or is unplugging it just masking another problem?

 

I'd put it on a scanner and monitor what the sensor is doing. You can also check it using a multimeter by following Rob's diagnostic page at www.shbox.com

 

By pulling the CTS wire off (you do mean the one on the water pump, right?) the PCM drops out of closed loop. When you unplug the sensor, it should set a code and turn on the SES light. Are you sure the SES light is working?

As noted, scan it. There are several key codes that don't turn on the SES light.

Check your O2 sensors. Check for excessive misfires. Check for exhaust leaks before the O2 sensors.

 

Yes, SES light is working, comes on when key is turned to on, then off after engine starts. I was not getting the SES light while it was missing/stumbling before, but did get one immediately after unplugging the MAF. Plugged it back in, pulled the ECM battery fuse for about a minute and plugged the MAF back in. Test drive, got the stumble, pulled the connector off the MAP, got a light. Repeat pulling fuse, SES light out, unplug the CTS - got the SES light.

Also yes, CTS by water pump. I can't figure out where I put the scanner cable for the laptop (would have been far too logical to keep it with my AutoTap CD, lol). If I can't find it, or someone else locally with a cable/scanner, would it be sufficient to go the autoparts stores for the free scans they all have?

I did recently replace the header to head gaskets on both sides about a week ago, although it's odd that despite both gaskets being nearly completely blown out before that, it ran OK then, although it was loud. I was going to drop the Y pipe again this weekend to recheck all the header bolts.

 

Check the wires to the coolant temp sensor. Also remember that there is 2 of them, one for the gauge and one for the PCM.

I had issues with the wires for the one on the PCM and was getting an intermittent connection and it created HELL for me; took me forever to find it and caused similar issues to what you're describing. Mine was dropping out so the comp thought the motor was freezing cold and dumping fuel in, essentially flooding it.