Rough Idle and Misfire Occasionally

I’ve got a 2007 Saturn Ion 2.2 liter engine and I’ve had it for about two years now (didn’t buy it new obviously) and recently, I’ve noticed that every so often - READ: not all the time - when I’m stopped at a stop light, the car will idle roughly. The tackometer is normally under 1K but sometimes it will quickly fall under 500 and then back up. When that happens, the car will normally misfire when it shifts from 1st to 2nd gear. I changed the spark plugs before taking it to a mechanic and that didn’t work. I’ve taken it to three mechanics - two guessed it was a cracked head and the third actually told me it’s not a cracked head since the symptoms would be happening on a regular basis. The third mechanic tested timing which was perfect. Compression test showed that the 1st piston was at 90 psi and the other three were at 150 psi. They did a fuel system treatment and Seafoam twice to remove a bunch of carbon that had built up (I’m guessing the previous owner was a city driver and I am as well). I got it back 5 days ago and it’s starting to do the same things again - idle rough and misfire OCCASIONALLY. Out of the 55 miles I’ve driven since I got it back, I would say it’s done it only a handful of times, not every stop. If it comes down to it, I’ll take it back to the third mechanic but I’m hoping I can get other ideas from here first since no one really seems to be able to help me.

That low compression on #1 is a problem. Compression should vary only 10% or so max between cylinders. You have a 40% variation there. Could well be the cause of the random misfire, possibly due to a burned valve. Unfortunately that’s an expensive fix.

You could also have the idle air control valve (IACV) checked. If it’s dirty or malfunctioning, that could also cause rough idle/misfire.

Another possibility is a bad fuel injector.

But I suspect it’s related to the low compression in #1 cylinder.

To your point about the low compression in cylinder #1, I forgot to mention that after the fuel system treatment, the noticed that the compression went back up to 130.

And another question, would the fuel system treatment and seafoam be run through the IACV? Or is that something different?

130 PSI is still too low when compared to the others being 150. The problem being intermittent would steer me away from the compression though.

The fuel system treatment and Seafoam would not pass through the IAC. Also, the IAC would not affect just one cylinder. The fuel system treatment does, however, pass by the valve stems. The fact that it improved the compression after using it may indicate that you have an intermittently sticking valve. The fuel treatment may have cleaned it up just enough to free it.

Another idea, you may have a sticky or broken valve (or spring) in number 1 and that’s why it works sometimes and not others. 90 psi compression is a big problem, and treating the fuel wouldn’t normally affect compression, except perhaps if you had a sticky valve. But it would have to be really sticky, and that isn’t a common problem. Much more likely to be something wrong with the valve.

Note: It could be the piston ring too as mentioned above. My guess is the valve.

If I had this problem I’d probably ask my shop if they could pressurize that cylinder to hold the valve in place, and replace the valve springs, valve stem seals, and keepers. Just for that cylinder. And check the valve clearances for all the cylinders while there.

Another cause for this symptom is a sticking egr valve. That’s fairly easy for a shop to test for.