I have a 91 Civic Si with a B16. I bought the car with the motor in it so I don't know the history of the motor, although I assume it was imported.

I have had the car for a few months and it ran perfect up until yesterday morning. I will just list the symptoms. Hopefully a few of you can help out. I will say that I changed the spark plugs yesterday night and it seemed to fix it but this morning, after the car was all the way warmed up, it started hesitating again.

- Car runs and idles fine when cold. When it gets warm the symptoms start.
- The idle bounces around from 800-1,000 RPM
- The motor does not run smooth at idle. It is pretty rough and sometimes seems like the engine might die. It never does but it seems like it.
- When I accelerate there is no power and VTEC does not engage. It smoothes out a little as the RPMs get higher but there is no power and is never as smooth as it should be.
- The exhaust sound is not smooth. Rather, it sounds like "poof poof poof poof poof" really fast. Kind of like the "balloon effect" sound when you drive with the front windows down in some cars, just out of the exhaust.
- When I pulled the spark plugs there appeared to be some coolant around the plug. I can not say for certain that it was coolant but there appeared a green tint to it.

I think the head gasket might be blown and that when fluid gets into the cylinder the motor starts to bog down. What does everyone think?

Thanks for your help