GMC Repair: Check engine light, gm motor, cylinder misfire

Question
QUESTION: Hello,

I realize you are listed as a GMC expert but I am hoping you can help me with a Saturn issue since it is a GM motor?

I have a 1999 SL1 with a 1.9 engine. The check engine light is on and my scanner reads a code P0301 - Cylinder 1 misfire. The vehicle has new plugs and wires as well as a new Idle control valve but I still have solved the problem.

The primary symptom is a pretty rough idle, both in park as well as in gear, although slightly worse in gear. However when you accelerate or are at speed on the highway or in town, the car seems to run quite well. The problem only exists at a stopped position.

At this point, I am suspecting perhaps an EGR problem, but the code again refers to a cylinder misfire, no mention or code for EGR? I had another vehicle do the same thing and it ended up being an EVR but this vehicle does not appear to have one.

Any chance you can help?

Thanks in advance for your time and your help!

Todd

ANSWER: Hi
If the original code was 301, and the tune up did not repair, I would next look and at the fuel injectors. If they can be easily swapped, I would swap the #1 with another cylinder, and see if the misfire follows the injector. IF so the injector is at fault, and it may be able to be cleaned, or replaced.
If the misfire does not follow the injector, It is most likely then a mechanical issue. A burned valve, poor compression, etc... a compression test and cylinder leak down will verify either one.
USUALLY, and again I have not worked on alot of Saturns, but usually a faulty EGR will set multiple misfire codes, or just a P0300, as it will affect all cylinders, not just one. I would also think it would set an EGR code, but thats notalways so, as you stated.
I really hope this gets you fixed up.
Thanks

---------- FOLLOW-UP ----------

QUESTION: Ok, I finally was able to get some time with the vehicle to check the items you mentioned. Here is what I have done.

As you recommended, I swapped the injector position and the code still read Cyl 1 misfire.

I swapped spark plug position and then plug wire position and still we have a Cylinder 1 misfire.

We performed a compression check on all 4 cylinders and had approx 150 lbs per.

The only thing I did not have the ability to do was the leak down test.

As I stated before, the vehicle seems to run pretty well with RPM's but at idle, runs really rough, really bad!

I am concerned that there may be internal damage despite good compression...should I be concerned about that?

I am also wondering if the cat converter may be plugged. Could this cause a constant Cyl 1 misfire??

Really at a loss at this point and I am overdue for smog.

Answer
Hi
At this point I would HIGHLY suspect a valve or head gasket problem. You may now want to take it and have a cylinder leak down test performed, and also have the cooling system checked for exhaust gasses.
You can take it to a muffler shop, where they can test the converter professionally, usually for little cost.
I have replaced NUMEROUS converters that were plugged, and all acted opposite of your problem, running smooth at idle, and when accelerating, they would stall down/loose power.
Thanks

I forgot....
You MAY also want to check the coil  for the 1 and 4 cylinder, ALSO check the spark plug wire.
The ignition module COULD also cause a misfire on just one cylinder.