Rough idling and poor accelleration


I own a 2000 subaru impreza rs 2.5. i was having misfires in every cylinder and fixed it. i replaced the spark plugs, valve core gaskets and grommet and spark plug wires. ran the codes in a obd2 and now the only thing that comes up is a knock sensor. can that be the cause of my rough idling and poor accelleration?


The knock sensor changes the timing. It could very well cause both of your problems. Check to see what the price is to replace the sensor. If it is not expensive I would start there. If it is expensive, then I would find an experienced mechanic who can determine if it is the sensor or something else.


is there a way to disconnect the knock sensor?

if it is causing this poor performance, then see if it is worse when disconnected. if there is no difference then the knock sensor s bad, if there is a difference when the knock sensor is disconnected then the knock sensor is not the problem.


Some suggestions for other boards is , those guys/gals live/breathe Subaru of all years.


You’re using premium gas, right? I believe that this is a premium required car-- if you’re using regular gas, you can expect rough idling and poor accelleration!


You can check for a bad knock sensor.

Make several reference marks around the crankshaft pulley with a piece of chalk or something like that. Connect a timing light. Start the engine and while pointing the timing light at the reference marks that were made on the pulley, tap on the engine next to the knock sensor with a screwdriver handle. If the knock sensor is good, you’ll see the reference marks change position each time you tap on the engine. If the reference marks don’t change position when tapping on the engine, the knock sensor isn’t functioning.



That’s a HUGH assumption that the knock sensor is causing, “…rough idling and poor acceleration.” What ARE the DTCs for the knock sensor circuits? The codes available are: P0324 through P0329 for one knock sensor. What does the scan tool show?
I’ll emphasize: The trouble code(s) the engine computer sets (and, it also turns on the “check engine soon” light) is to show that a trouble exists for a particular circuit which has a sensor (or, another component) within it. Troubleshooting should be of the circuit and include the sensor and related components.
The alternative is to just change some parts and hope that Lady Luck is looking favorably upon you. Is she?


ok…update. the code that it says is p0325 and it says its bank 1 or something like that. yes i am using premium gas and im using lucas gas treatment with it. when i tap on the knock sensor it doesnt change still runs rough. havent tried disconnecting it yet. mostly cause i thought the car needed it to advance the timing and make sure everything was running smoothly. guess it wouldnt matter though if it wasnt working properly though. guess i will try that.


It’s not likely at all that the knock sensor has anything to do with your problem. The knock sensor does not advance the timing; it sends a signal to the ECM which then electronically retards the timing to some degree in an attempt to get rid of a pre-ignition rattle.

Some more info would help; such as how many miles on the car, bought new or used, any overheating episodes, did the problems surface gradually or quickly, etc.

JMHO, but the first thing I would do is run a compression check followed by using a vacuum gauge on it. This would tell if engine compression is low, catalytic converter is clogged, etc.


The knock sensor is NOT the problem at idle. There are other things which are causing the problem(s). Open your repair manual to the section headed Troubleshooting. Start there. Have you changed the fuel filter, air filter, pcv valve? Have you checked the fuel pressure? Have you done a resistance and voltage check on the engine coolant sensor (cts); the throttle position sensor (tps); the idle air control valve (iac)?


it has 172000 miles on it. i bought it used and it doesnt have over heating issues. it surfaced gradually when i was in a bind trying to get the parts to stop the miss fires and finding time to fix them. when i did this came up right after. ive changed the air filter. so other then valve cover gaskets, grommets, spark plugs, air filter, nothing else has been done to it. and the only thing that comes up is the knock sensor on the code thingy.


Low fuel, pressure, vacuum leaks, incorrect idle air control valve flow, incorrect signals from the throttle position sensor, coolant temperature sensor, MAF (Mass Air Flow) sensor, won’t set trouble codes for the code scanner to pick up. That’s why they need to be read with a digital multimeter, or a full scanner (which has that capability).


how do you use a multimeter to check those…is it something i can do or too complicated? im an electrician and have a multimeter. also, now when i stop itll sometimes stall and when it idles, steppin on the gas it seems like it sticks cause itll take it awhile to go back down.


It’s a 9 year old vehicle with 172,000 miles and was purchased used. In a shop the first thing that should be done is to run a compression test. There is no sense in throwing a truckload of parts at it if the problem is mechanical in nature.
One always should determine if the engine is mechanically sound before replacing anything.


this is my first post but in my opinion if you let those misfires go to long before fixing them as you suggest here i would suspect you have a plugged converter and have a back pressure check done on the exhaust system. i could suggest that you take the #1 02 sensor ( that would be the one before the convertor)on that bank out and start it and see if the idle improves if it does odds are you have a plugged exhaust but im not sure exactly what is in the vicinity of the 02 sensor on this car and dont want you to burn up something when the exhaust comes out of the hole the 02 sensor goes into so you would have to make that call with good sense otherwise take it to someone who has the proper tools to do this test . the knock sensor code could be setting because of the way the car is running and it cant retard timing enough.