Hello. I had this problem for a long time now but still drove it for about 2 years with the same problem, the problem is that when the car is at red light or in a parking lot not being driven the idle fluctuates up and down. its jumping from like about 500 to 800 rpm, no problems while being driven tho, no stalling or any sort of thing everything is smooth when it's being driven, the only thing I would say that the car is accelerating pretty slow maybe but I don't know if that's the limit of the car or the issue may cause worse performance. And also the fluctuating idle begins when car warms up, when it is cold the idle is not jumping up and down, in the winter obviously it warms up more slowly but in summer now it's only for a short time when it is working fine, car will warm up and when you stop at red you can notice the jumping idle while it's in neutral gear, the car also vibrates when its happening. I scanned the error codes and there was 5 of them: P0300, P0301, P0302, P0303, P0304. I added a screenshot of the scan result with the torque app. And also here's a short clip how the engine looks like and sounds like when it's warm:

You Tube

So it only happens when the car is stopped, tried to press the accelerate in neutral gear, holding it from 1,2,3 thousand rpms continuously no shakes no vibrations everything is smooth and fine, its only when you leave it alone the rpm's fluctuates. What likely could be the cause of this? Does anybody have experience with such issues? I am on budget but I want to fix the car till it's not completely damaged from this issue. The car was driven for about 7k km's in the past 2~ years , all spark plugs were changed in 2020 when I bought the car, so the spark plugs have like about 7k~ kilometers on them so I don't know if they are the issue, recently the oil and air filters were changed but nothing else. What should I do? Any suggestions? I kinda want to have some ideas before going to a mechanic or a car repair service because in my past experience with services that they will look at unnecessary things and may not even fix the car, so if someone have some ideas that would be very helpful. Thank you.

 

That's... not good. Usually that's a problem with ignition system, since the spark plugs have been replaced, possibly coils. A bit strange that it's all cylinders but maybe it started at 1 or two while you were driving around.

 

If it was just the surging idle, I'd be suspecting a vacuum leak (this can cause high MAP/load reading, in turn causing over-revving, and then subsequent fuel cutoff due to high RPM while throttle closed, cycling over and over). Double-check the air filter installation, MAP sensor readings, and for air leaks generally. When the engine is cold it'll prefer a higher idle speed and richer mixture, which could mask the effects of the leak. Interesting that there are misfires on all four cylinders, though - could be a separate problem?

 

Took the car to the autoservice they couldn't find anything, just replaced all 8 spark plugs, picked the car up and it ran worse than before probably lol. But they recommended me a guy who worked with petrol engines for 25 years and has expensive diagnostic system/tools. Gave the car to him, couple hours later he called and he said it was fixed. I was very tired in that period with throbbing headache and couldn't remember what exactly he said but I think he just programmed something with his diagnostic computer? Maybe there was issue with the car computer not working correctly or something, also he found that after the fixing the car he said some strange issue came up with camshaft sensor, so he said if the engine light will ever come back because he wasn't sure if the sensor should be replaced if it works fine then there is no point to replacing but there may be a chance that I will need to replace camshaft sensor which costs about 40-80$ in the future. So yeah. It costed around 55$ from him to fix the honda. So the problem was in computer I guess. The car runs so smooth now, I thought the clutch was in bad shape but it turns out it was the misfires from all cylinders that affected the overall driving experience. Engine is also like 2 times more silent when it warms up, the idle is stable, there are no more shakings in 1-2 gears which I thought also was issue with clutch or gearbox. The only thing that I noticed that when the AC is on and the radiator fan turns every 1 mins the idle starts to fluctuate again but without check engine light, I don't know if its normal. So yeah.. 55$ fix and the car runs very smooth now. I thought about vacuum leaks, ignition coils but that guy did a good job with his diagnostic tools.. My only guess is that he programmed and trained the ECU to work correctly without misfires? I don't know much about this car because I bought it 2 years ago and it was ported from another country so there may be many causes why could that fix work, maybe the ECU was replaced or engine was replaced and it needed a correction with proper diagnostic tools.

 

Sounds like a good result so far - I hope it continues! I don't have any experience with replacing or reprogramming ECUs, but I do know the Fits are known for a syndrome of worn spark plugs -> coil failure -> crankshaft position sensor interference -> misfires and other engine troubles. The AC compressor is known for loading the engine quite heavily and sporadically...if you get the idle surging, try applying a tiny bit of throttle and see if it goes away. If it does, it might still indicate a small vacuum leak. MAP readings should be around atmospheric pressure with the engine off, and around 20-34 kPa at idle (0.7-1.1 V). The throttle position sensor should read around 10% when fully released.