Hello and welcome to 2CarPros. Wanted to jump in and point out there is a service bulletin for your condition where a head replacement may address your issue. Here is the bulletin
Collision
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2004 Subaru Impreza
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F4-2.5L SOHC
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Engine Controls - Rough Idle/Misfire Diagnostic Tips
Vehicle Powertrain Management Technical Service Bulletins Engine Controls - Rough Idle/Misfire Diagnostic Tips
ENGINE CONTROLS - ROUGH IDLE/MISFIRE DIAGNOSTIC TIPS
basics, basics, basics
The Techline receives calls on rough idles and misfire codes on a daily basis. Before you call us, we suggest/ask the following:
Check the ECM reprogramming sheets available on the Subarunet. Com website to see if the particular model you are working on is eligible for a reflash for rough idle/misfire.
If your particular vehicle is not listed or the ECM calibration identification number (CID) matches the one shown on the sheets, then reflashing is not an option. NOTE: Reflashing will NOT address/fix a car that has an actual mechanical issue causing a rough idle. You will need to investigate the problem further to determine the cause. If the car idles fine and just has the misfire code(s), there is a good chance the reflash will fix it.
Does the engine idle fine, but, have a misfire code? If so, it might be a fuel mixture problem. Check the vehicle history to see if the car has been in before for this problem. In some cases, you'll find it has been. If so, was the misfire on the same or another cylinder? If the code was on another cylinder, it might be a fuel related problem. Hopefully you DIDN'T clear the code(s) so you can use the Subaru Select Monitor to pull up the Freeze Frame Data. You'll want to look at the Short Term Fuel Trim (STFT) and the Long Term Fuel Trim (LTFT) readings in the data list. These will give you an idea of what the Air/Fuel Ratio Sensor (A/F) is doing in determining the fuel trim delivered to the cylinders. If the A/F sensor is functioning properly, the numbers you should be seeing for the STFT & LTFT should be somewhere between a +5% to a -5%. Slightly higher readings are ok but if you have a reading in the double digits, typically T2% or higher, it usually indicates there is a problem with the A/F sensor. Trouble shooting of the A/F sensor should be performed to determine if it is functioning properly or another condition (vacuum leak etc.) Is influencing the A/F sensor readings. Our A/F sensor readings will max out at 37.5 %. If the number is a positive (+) the car is running lean. This is usually when we see the misfire and rough running concerns. There is simply not enough fuel getting into the cylinder to support combustion. This is referred to as a lean miss. A negative (-) number is of course a rich mixture and is not usually a cause for a misfire.
If the car has an actual rough idle, there are many things that you can do to track down the cause. Check the plug wire(s), coil(s), plug(s), and injector(s). If the code is always on a particular cylinder, try swapping that injector with a known good cylinder and see if the code follows the injector. Then swap a plug, then coil. This is what we'll suggest if you call us. If the code moves with the injector, then you have a bad injector. If it doesn't follow the injector, there is most likely something wrong internally with that particular cylinder however, the pins in the injector connector will need to be checked to see if they might be the problem. If everything external has been inspected and ruled out as the cause, the problem must be something internal to the engine. It could be a carbon buildup on the valves or varnish in the valve guides. It could also be a valve adjustment that is too tight. Further tests are necessary.
Hook up a vacuum gauge to a source of engine vacuum (below the throttle plate). A good running engine will have between 16" to 22" of intake manifold vacuum depending on the altitude. The needle should be steady on the gauge and not bounce around. A slight quiver to the needle is acceptable. If the needle is bouncing around 1" or more, it shows you there is a cylinder sealing problem (valves or rings). Most often it is the valves causing the problem. However, this test doesn't pinpoint the exact cylinder(s) at fault. If there is a misfire code, you can be reasonably sure that the code indicates the problem cylinder(s). If there is no misfire code, you'll need to perform a compression and cylinder leakdown test. We always recommend that a compression test be followed up by a leakdown test as we've seen many cases where an engine might pass a compression test but, fail the leakdown test. Most importantly an accurate leakdown test will help identify where you should look for the leak:
intake valve, exhaust valve, or rings. When you perform a leakdown test, always follow the procedure recommended by the equipment manufacturer to assure that the test results are accurate. Refer to the 04/95 Tech TIPS (available on the STIS website) for additional information concerning compression and leakdown testing.
If your testing has determined that the valves are the problem and you've checked the adjustment, the head(s) will need to be removed for further inspection. If the cause is then determined to be a carbon buildup, usually just cleaning the valves and the guides is all that is necessary to repair the problem. If the valves are reinstalled into their original positions (and they should be), everything should be fine, but you can lap the valves in by hand if you wish to insure a tight seal. We do not recommend cutting or grinding the valves or their seats at any time.
 In closing, there are other things that can cause rough idles and misfire codes. These are just the most common ones we deal with here on the Techline and the methods we suggest you use.
Thursday, September 26th, 2019 AT 11:47 PM
