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Engine light on. I used to able to clean the EGR valve and it would go off for about a month. This time stayed on and the engile idle is rough. I tried changing the EGR valve with a second hand one cleaned up but no change. I tried changing the TP, IAC, Front O2 and MAP sensors, plugs, plug wires and coil and no change with any of those. I tried running the engine and taking off one plug wire at a time and then tried removing the wires to the fuel injectors one at a time and the result seemed to be the same for each. I cleaned out the Fuel injectors and still no change. I measured the ohms on all the injectors and they are good. I tested the temp sensor but it seems to be working fine. I could not measure the conpression as I do not have a device to do this. Unless there is a simpler way? I did change the fuel filter as it has the fuel regulator built in for the 02 Saturn SC1 but did not change the actual fuel pump in the tank, I assume. Still no change. I did not change the Knock sensor. The car has 299,000 KM on it and I am scared that it might cost more to get it diagnosed than the car is worth. Car drives fine and other than the VERY rough idle and engine light on, it gets around. The issue is I will soon need a Emissions test in order to get the stickers renewed. Any suggestions on other potential causes?

Origional code reading before all of the changes: P1128 and P0172

After all of the changes I ran the car for about 2 weeks before I had the chance to put it back on the meter. Then I got: P0455 & P0304

I got some interesting freeze frames:

AS SOON AS STARTED: Fuel sys1- CL, Load_PCT 42.4%, ETC-117deg C, SHRTFT1= -2.5%, LONGFT= -6.3%, MAP- 13.3KPa, RPM 1482?, VSS- 30km/h, TP- 0% and I also took Sys Status as soon as I started: MIL status-ON, misfire monitor- OK, Fuel sys Mon- OK, Comp component-OK, catalyst min-INC, Htd catalyst- N/A, Evap system mon- ok, Sec Air system- INC, Oxygen sens Mon- Ok, Oxygen sens Htr- ok, EGR system- ok.

AFTER SEVERAL MIN: I will only type the ones that changed: for the Freeze frame: ETC was now at 47deg C, SHRtFT1 was now at -2.3%, LONGFT1 stayed at -6.3%, MAP was now at 45KPa and stayed there, VSS was now at 49km/h. For the system status: Oxygen sens Mon was now INC, and EGR system was now INC. everything else was the same.

Any help or suggestions would be appreciated.

Vehicle: 2002, Saturn SC1, SOHC.

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P1128 Upstream 02 sensor swapped.

P0172 Fuel trim too rich

P0455  Evaporative emission control system leak detected (gross leak)

P0304 Misfire number 4 cylinder

Map sensor is low.The fuel trim is a total of -8.6, this make it a running rich engine,Verify compression, Try this this will give you a indication,disable the spark or ignition so the engine will not start, crank the engine as it is cranking the sound should be rythmic, example cranking sound ta,ta,ta,ta,ta if the engine has low compression the sound will be somewhat like this ta,ta,da,ta,ta da, A low compression on a engine will result in a uneven rythimic condition, The map sensor is low due to a misfire, P1128 is not a valid code for this year, It used as a lean bank code, and having a rich condition at the same time.There is a problem in the valve train,
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Thanks for the response. I tried the test u mentioned and it is a perfect rhythm. There did not seem to be a compression issue. One thing I should mention though is that certain times, and it happened again today, I can start the car and the light will not be on. Today I started it and immediately drove away. 40 min later I turned off the highway and the car came to an idle and within a min the light came on. That has happened before. Also, on times when I am driving I feel a slight studder in the engine. It doesn't do much but it is enought to feel.
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One more question: Could a vaccume leak cause this?  If yes, where should I look for one?
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Thanks for the info. I bought a new temp sensor and also took the EVAP Solenoid off and tested it. It worked fine so I put it back on making sure all the vacuum lines were sealed.  I ran the car for three days a now and NO LIGHT!!! Thanks again.
 
The car still idles pretty rough though and sometimes shakes the car.  Any ideas on what could be causing this? 
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Rough idle and sometimes shake, rough idles are misfire, what needs to be check are sparkplugs and sparkplug wires, There is a possibility the ignition coil is problem, If the vehicle stumbles on acceleration the ignition coil should be tested ,a replacement may be required for the solution to the shudder.

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