Ive got an 06 cobalt, 2.2L, 190K miles. Started having a rough idle last week. I figured it was a dirty throttle body, because it ran great above idle, and I've had to clean it a couple times before.

A few days later, I notice it's down on power, making a terrible clacking noise, and stalls out at idle. Going to work yesterday, it went into limp mode. I cleared the codes when I left, and it drove home not in limp mode.

The codes are P0106, (map sensor) and P0300 (random misfire).

I cleaned the throttle body, it was super grungy, and bought a map sensor for it, but there was no change.

I'm pretty stumped, all this came on pretty quick, and the car has been super reliable up to this point. Any help is appreciated!

 

Map sensor on a Alpha N tuned car will cause all kinda hell at idle as it uses it for IDLE. Under driving it uses TPSxRPM for the tune and ignores the MAP. All in all, replace the map sensor first.

 

Makes sense, but I have a new map sensor on it, and while it does help, the idle is still crap.

I cleaned the MAF while I was there, and that helped a little. Also, I noticed I was almost 2 quarts of oil low, and there was oil on the underside of the engine/transmission. This car has NEVER leaked oil at all, so eventually I'll have to find and fix that leak!

So right now, with my new map, and cleaned MAF, it will idle, although rougher than a corncob, but the little driving I did earlier, seemed to run fine.

Plugs/air filter changed in the last 10K or so miles, as well.

 

I chased a misfire issue on a Buick a while back
Turned out it had pulled a rocker stud
I tell you this because you said you got some clatter
A compression test might be a good idea

 

I had exactly the same experience on my wife's old '99 Pontiac Grand Am. There was a P0300 and it ran rough. I did a compression test and found a lack of compression on the #5 cylinder. A rocker stud had pulled out and that caused the misfire. One trip to the machine shop later I installed the repaired cylinder head and the car ran great afterward.

 

A 4cyl might be doable on car
My buddy did the buick on car

 

Looking more and more like internal problems. Tried a new ICM, no change, so we pulled the coil off, put the plugs in to see if they had fire. They did, but I noticed one plug was a lot darker than the other three, and the gap was much smaller. I just changed plugs, and I know I checked the gap before putting them in.

So I regapped it, put it in and fired it up. The ticking, rattling clacking noise was MUCH worse, and it would barely rev up at all.

Thinking a compression test might be in order, at least see whats up with that cylinder.

 

Update: pulled the cam cover off, found the "follower" for one of the #3 intake valves laying on the other side of the head. Apparently the stud that it sits on broke off from the head. Not sure if those are pressed in or threaded in. Anyway, I think that's my problem! I have a very rare 15 valve engine!