How much misfire is normal for a Pontiac Grand AM with 70k miles?

[Logan U]

My girlfriend has a 4 cylinder 1.6L engine in her 2000 Pontiac Grand Am. The engine has about 70,000 miles on it. At idle, the engine shudders and rocks heavily back and forth on the mounts once or twice every 10 seconds. Inspecting the tailpipe, the pipes *shake* briskly, lacking the smoother "vibration" of the exhaust system other 4 cylinder cars have. There are also several large poofs of exhaust gases once every couple of seconds; evidence of unburnt fuel passing into the catalytic converter, I assume. The service engine light is not on. It does not blink during idle, acceleration, or any driving conditions. The plugs, boots, and air filter were changed less than 2000 miles ago. The misfire has remained consistent over time and did not improve after the tune-up.

Comparing this behavior to a 4 cylinder 1993 Honda Civic that I am familiar with, it would seem as if this engine is in need of repair, however, as I said before no lights are on...

Is this normal behavior for a GM engine of this age?

 

[AnnaBean]

Just because the service light isn't triggered, doesn't mean there isn't anything wrong.

That doesn't sound like normal behavior for any car. I had a 94 Maxima with 286,000 miles on it. It didn't sound like that.

Misfiring isn't normal at all. It could be a few different things, some more expensive than others. You should find a mechanic you trust to get in there and figure it out before it becomes a huge issue.

And if it comes down to it, scrap it and get a different car. Sometimes repairs end up costing more than the value of the car.