Engine light with rough idle

Hello,


I have a 98 Honda Civic LX w/manual trans. Just yesterday, the engine light came on. Car runs fine upon start up. Once warm though, the idle becomes a little rough and the RPMs' drop to almost nothing at times causing the engine to shut down. Car starts up again right away, and with a little gas at the stoplights to hold the RPMs' up it holds just fine. Runs fine when driving.
One notable thing, the car has not had a muffler for almost a year now. Wondering if maybe the lack of backpressure on the engine is causing some of this. The engine light never came on during that whole year with no muffler until yesterday.
Wondering what to look for hoping that this may be a simple trouble shoot.
(I know, ya right...) Honda has been pretty easy to work on when I know what is wrong.

Thanks for any and all input that is provided. I try to keep the car out of the shop whenever possible.

UPDATE:

While driving this afternoon, the Civic was trying to give me a little more info. I was in 2nd gear when she started to lose power; sputtering, and not able to go over 40 MPH even in 3rd and 4th. I came to a stoplight and upon green, she accellerated ok and was able to go whatever speed I requested. (problem didn't occur for the remainder of the drive.)
Just some more symptoms in attempt to get a diagnosis.

I will get the code from the computer hopefully soon. Autozones here in Minneapolis area don't rent scanners like I have heard of elsewhere.:banghead:

My first suspicion would be O2 sensor, but you need to find out what the code is....

My first suspicion would be O2 sensor, but you need to find out what the code is....

My second suspicion: This sounds exactly like a problem I was having. It turned out that the spark plugs were back out about a full turn. I tightened them down, disconnected the battery for 30 mins to reset the ECU, then started her up and let it run for 15 mins (for ECU to re-establish it's settings). It works fine now and my take off speed has increased significantly. I thought it was a strange, but I also bought an '86 Nissan pickup one time that had the same issue (loose spark plugs)...:shakehead

Thanks for the reply TNRabbit! I will check these things out. The car is due a little tune up anyhow as she has 112,000 miles logged.
I appreciate your response!:grinyes: