A little background:
My CNG Civic has been idling just slightly rough for a long time, but holds steady at about 700-800 in neutral, about 900-1K in D with brake on. Dealer said it was a mount, but it didn't go away when I replaced it.

Last week it suddenly became kinda gutless. It takes longer to reach freeway speeds, and I lose speed on the slightest slope in the freeway. It seems weaker over 3500 RPM. I replaced the plugs one 1 coil, and got my MPG back, but not the power. Air filter is clean. Trans fluid is good, oil was just changed. Cooling is fine, no fluid loss, exhaust 'looks' clean.

The chassis is 250K miles, but the engine has about 85K and the trans has about 65K.

Any suggestions what else I should be looking at?

 

Sounds like you've looked at everything that I would think to look at... Might be time to take it to a mechanic that knows their stuff... good luck..

 

Does it make any funny rattles or noises?

 

Sounds? I've got one valve tapping a little, but it doesn't sound bad enough to crack things open.

New development today though. Trying to maintain freeway speed, I was just over 4.5K rpm for about 10-15 seconds and got a MIL. Codes were misfires detected across all cylinders. I cleared, and the codes haven't come back.

I hate throwing in the towel and going to my dealer, but I just can't find any better troubleshooting tips for this problem.

 

It may be that your catalytic converter is extremely clogged, and preventing the exhaust flow which could cause loss of power. Just throwing an idea out there, but that doesn't explain the MIL.