Engine Struggling for power / new battery
 

2004 6spd with 67k miles. Just recently replaced plugs, coils and wires. Have started getting the coolant light issue with the sensor going bad in the reservoir -- only comes on when car is driven at higher rpms or right after a redline then goes back out.

About a month ago got the P0661 CEL code for the shutter valve, but we've only seen it about 3 times. Dealership said shutter valve is probably sticking but not bad enough yet to be an emergency.

Two days ago battery suddenly died (car wouldn't turn over so we assumed it was battery) -- bought a replacement of the exact same battery. Installed it last night.

Drove car today and after freeway driving of about 45 mins, battery light and CEL light came on for a few seconds then went out. Engine has now lost power. Struggling to go up any incline and noticable power loss in all gears, but especially 2nd and 6th gears. Had to drive on interstate in 4th or 5th gear to get home because putting in 6th car can't maintain speed.

Battery checks to be fully charged so I assume alternator is keeping it charged. Nothing seems loose.

Any ideas? CAT? Engine failure starting? Powertrain warranty just expired 14 days ago.

so -- after more research I now think my car went into limp mode. Is this possibly just due to a new battery being installed the day before?

Maybe..
 

You're having a lot of different symptoms but for a noob on here I would say go with cleaning your SSV valve. I've replaced the battery twice in my Rx8 over about four and half years and had to unhook it multiple times and I've NEVER had a CEL from pulling the battery in and out. Searching "loss of power" on this site will scare the hell out of you, if it was my car, I would consider going right back to that first CEL code of the SSV. On here theres a DIY for "cleaning secondary shutter valve" Try that and see if your power returns. Not positive but limp mode might be triggered by SSV valve failing? possibly if the AIR/Fuel Mix is off to such an extent that it diagnoses an error code and puts it into Limp mode to keep you from running straight fuel with little or no air in the mix at high rpms?

One other thing, nothing seems loose on your battery? With 67k miles do you have any corrosion on the stock battery cable ends? Might be worth a shot to take them off, clean with a wire brush terminal cleaner, spray with some contact cleaner and make sure the Negative is tight on your car. I've fouled that before and not tightened the terminal on the battery well enough and had it come loose hitting bumps on the road.

Hopefully some of that helps, and I'm sure some more experienced guys on here will help you out quick. Good luck

I would start by getting the latest CEL code.