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Discussion Starter · #1 ·
I've been trying to track down an issue (or issues?) that first manifested itself as surging at highway speed and has now progressed to a subtle misfire at idle and low-load situations. I'm not getting any misfire codes and the only DTC that come up in VIDA are related to the clutch position sensor. In the past several months I have cleaned my air filter element, cleaned my MAF, replaced my TCV, replaced my MAP sensor, and cleaned and re-gapped my plugs (Denso IHK22 with 30k on them.) My coils are relatively new with 30k on them as well. Based on several threads here I also just checked the VVT behavior and the actual valve angles track the target valve angles pretty well for both intake and exhaust. Somewhere along the way the highway surging went away but I'm not sure what fixed it. The subtle misfire is more heard than felt, and sounds like a low bass drum or distant fireworks and is most prominent around 2000 rpm under light or no load. I hear it when parked in my driveway with the car in neutral so it isn't a suspension issue masquerading as a misfire.

I have now turned my attention to my oxygen sensors since neither has been replaced as far as I know and the car now has 226k miles on it. I hooked up a generic bluetooth OBDII reader to quickly check my O2 sensor readings and I got the results below on the left after the car was completely warmed up. For comparison I did the same with my wife's 2010 XC70 and got the results on the right. Note the front O2 sensor is on the bottom of the images while the rear sensor is on the top.



The trace on the right from the XC70 is more what I would expect from the sensors - a smoothly varying front sensor going from rich to lean and back again with the rear sensor following suit. I'm not sure what is going on with my S40 (on the left) and wonder if one or both of the sensors are going bad, or if they are reading correctly and there is something else happening with my motor that could be causing these readings. One other possibility I've thought about is a faulty injector but I haven't done anything to diagnose those yet.
 

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I think I had a similar issue a couple months ago. At cold start my engine felt like it had a miss until it came off high idle. Low idle was good. Driving between 2000-3000 RPMs it would surge. This was more pronounced in lower gears. Acceleration was still very good. There were no codes. I connected a vacuum/boost gauge. The needle was steady at low idle, wound bounce a bit on high idle. Driving between 2000-3000 RPMs it would bounce around like I was playing with the throttle. Full throttle boost was steady at ~9 psi.

I thought about injectors, O2 sensor and other stuff then I thought about the throttle body. There is a YouTube video where a guy tears down an S60 TB and describes how the position potentiometer can develop dead spots. https://www.youtube.com/watch?v=6-2Ui1PXQPc So, I went to the local bone yard and pulled a TB off a 2007 S40 T5 (same part number as my 2005). Bingo! problem solved for $35.
 

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Discussion Starter · #3 ·
Thank you for your suggestion. I have the Porsche 70mm TB installed and my original on the shelf so perhaps I should swap it back. I'm not sure I'm experiencing the same issue, though. I'm hearing misfires at idle and at steady throttle in the 1500-2500 rpm range. The highway surging seems to have stopped. I went ahead and ordered new O2 sensors since as far as I know they are original and have 226k miles on them and it's probably time to replace them anyway. I already loosened the front sensor in preparation for installing my new downpipe so I should be able to get the front one installed fairly quickly and then I'll go from there.
 

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Discussion Starter · #4 ·
It was a bad plug!

Just as a follow-up, I was able to track down the misfire and it turned out to be a bad plug in cylinder #2. It looked a little funny when I removed, cleaned, and re-gapped the plugs but since there was no noticeable change in behavior before and after I started looking elsewhere. A misfire was being registered in VIDA in the misfire counter but I didn't see a way to narrow it down to a particular cylinder. It wasn't bad enough to generate a code so I couldn't trace it to a specific cylinder. I also was using a ELM327 WiFi code reader and found a function - non-continuous monitors - that was quite helpful in that it specified which cylinder was experiencing the misfire based on the misfire counter. It shows number of misfires for that driving cycle as well as an average of the last X number of cycles. VIDA probably has this function as well but I am just not familiar enough with the program to find it. Anyway, installed 5 new Denso Iridium Power IKH22 plugs and all is well.
 
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