I just got a flowmaster for my 91 GTA, on Tuesday, and ever since, it seems like the engine is running about 5 or 10 degrees hotter than normal. In other words it's acting like it's stuck in traffic or something, like its working hard, even when it should be cooler. (But it's not overheating.) I also notice that when I really floor it, the car acts like it is passing out: the level of the electricity goes down and the lights dim. It only does this when I accelerate really hard. Sometimes it seems like it suddenly got cooler... As soon as I take my foot off the gas, the electricity comes back. I'm wondering if the same effect that's giving me really great mileage is also causing this problem? I just took it in to my regular mechanic today (not the muffler dude) and they thouroughly checked the electrical components, and nothing came up. They did however give me a new starter (lies!) and fixed a fan motor on the radiator (which I knew was not working.) (All about $200) I thought that would do it, at least cool it like it used to before the muffler, but it's still acting up, same as before. Is this a common problem, possibly caused by the exhaust? Or maybe I forgot something when I had it done? I'd hate to have to go back to my stock exhaust. Or maybe my car is just getting old.

Putting on a new muffler won't do that.

Agreed. Unless they touched other stuff.. Simply bolting on a muffler wont do what you described.

Yup. 5-10* warmer, I hope you have an aftermarket gauge, cause I don't see how you can tell from the factory one.

maybe the stock guage just decided to work correctly

ur lights are dimming in ur car when u hit it? thats exactly the opposite whats supposed to happen... something in draining ur alt.. and eatin up the power, but the alt spins up higher RPM so it puts out up to its max... i can only think that ur alt is to small (in power, not in size) and ur car is askin for a lot when u floor it and the alt doesnt give enuff juice..... look what kinda alt u have... it might need to be replaced... thats all i can think of

The alternator just went out about a month ago, but the new one seemed to work just fine. I don't think it's the wrong size. I'll get it checked out though.
As far as the temp gauge goes, I was just guessing of course, but its pretty reliable.

Well, it seems like the electrical problem was caused by the alternator after all. It was new and I didn't think it had gone bad, but I went back to the guys who put it in, they decided it was defective and they replaced it (for free.) (But the car is still running ...warm-ish.) Thanks to everyone for their advice! I'll try to find help in a different topic.

Might wanna go to a lower tempature thermostat too. The stock of 195* is alot and really too much.

195* is too much? Man running down the high way in 80* temps with my air dam in and a less than clean (probably) cooling system and i still never go about 160-170 and only hit 220 after about 10-15minutes sitting in traffic at which point i kick on the fan and it runs on down to 160. 195 is perfect if you are going to be in an area were the winter temps get down there pretty good. especially if its going to be below 30* on a regular basis during the coldest bits.

i had a 160 degree thermo in mycar last winter...worked OK but took 45min to warm up. also with the 160 my fan never came on...even if i sat in traffic for 15min it wouldnt go past the 130mark or so. i switched to a 180 works well and heats up fast enough even in winter.

Yeah, it does matter where you are. I'm thinking from where I am.