2017 Hyundai Elantra 2L - P0303 cylinder 3 misfire - Be careful of used ECMs! Part 2

[@kstatinet-wk2gh]

Thank you for your tutorial on the troubleshooting. Have an issue with P0303 on the same car model 17 Elantra SE 2L. I bought the car used with an odometer at ~62k miles back in January 2023 for my son to drive. Two weeks later after the purchase, the engine light came up and the code was P0303. I called the car shop that the previous owner was using and found out that he brought the car for the same reason at 55k miles and the tech replaced 4 spark plugs (Denso 3461) & one coil (Import Direct 230578). After checking spark plugs and coils (all clean no issues), I tried to switch coils btw 3 & 1 but there was still same issue - engine shaking and code present (after reset). As the last attempt I checked the cabin and engine filters. These were dirty beyond believe. Apparently, the previous owner drove almost 6 years w/out changing these once. Okay. So I replaced both and the engine code gone. Little victory. Yesterday and 23k miles later my son calls me about engine shaking and we got the same code P0303. I replaced the third coil with Delphi ignition coil bought at Walmart. The code is gone and I showed my sone how to do it. a) Should I replace the other 3 coils or should I give him spares since he goes to AL in about 3 weeks? b) I suspect I'm gonna get a call on this again. Should I replace ECU ? The new OEM ECU is around $500. Not sure if replacing ECU is something I can try. I have Autel MaxiAP AP200 OBD2 Scanner Bluetooth Wireless tool for this car. Would you recommend replacing ECU if the code comes up again or should we bring this car to the dealership for reprogramming? Not sure how much they charge on that.

[@MrXunoriginal]

I've been dealing with rough idle/engine shake for the past month or so and finally caught the engine light on with a P0304 code. Just changed an ignition coil and 4 spark plugs yesterday now I find out the T coils in my car are no bueno. Hoping it isn't the ECM, car has been running smooth again (so far) but thank you for putting this video out there. SLOWLY getting to the bottom of this pain in the ass car and it's issues.

[@eurbanautotech]

@MrXunoriginal It can be super frustrating indeed! Hopefully your car is all good for a while. This car here was totally good then would misfire like crazy. Solid dead misfire for 5-10 minutes or longer. Then it'd be fine for weeks. The misfire would just suddenly come on then just as suddenly leave. That's usually an indication of something like a bad ECM.

[@MrXunoriginal]

My car was an intermittent misfire as well, and did feel like it kicked on and off. Elantra is running smoothly now so far, next up is fixing an oil leak which an auto mechanic shop diagnosed as a bad Engine Oil Switch. $12 part which is cheap but not so easy to find. I'm having fun learning about cars while saving money doing the project myself. @@eurbanautotech

[@eurbanautotech]

@MrXunoriginal Glad to hear you're having fun working on your cars! It's an amazing time for it. There are so many resources freely available now. 20 years ago all you had were Haynes manuals and trying to dig through forums! Now you can get MUCH better educational content on KZbin then you'll ever get in any mechanic apprenticeship school... Trying to figure something out on your own car can get pretty frustrating at times but it can also be super rewarding. Plus you get all kinds of tools and knowledge to show for it! 😃

[@MrXunoriginal]

100% more rewarding then it is frustrating. Content like yours makes it easier to troubleshoot for sure. I guess car troubles are one way to pick up a new hobby@@eurbanautotech

[@eurbanautotech]

@MrXunoriginal Thanks! Diagnosing is my passion and it's awesome being able to pass it on and show some of the cool things I run into!

[@MarcosFigueroa-k4f]

Thnx so much

[@NoDayz0ff]

I have a customer that has this same exact problem, but cylinder 2 and 4, dealer did tsb (just ecu update) but stated ignition coils had to be paid for although it says to replace them in the TSB... she brought it to me and I replaced the plugs and coils, random misfire still present and im starting to think the ecu is bad as well

[@edrominger6165]

I just bought a 2017 and my number 3 is misfiring but only did it for a few min and shut car off turned back on ran fine but had it took and had the code checked since the engine light come on and it's showing misfire and cylinder 3

[@eurbanautotech]

@edrominger6165 On cars, if something is working totally fine most of the time then very suddenly becomes a problem then just as suddenly goes away... that's usually a matter of solid state electronics failing. Transistors, integrated circuits, etc. So things like sensors, ignition coils, computers, etc. Now saying that is getting way ahead of ourselves. Gotta start at the basics. 90% of all misfires are caused by ignition problems. Spark plugs, ignition coils, etc. Best thing to do is swap some stuff around. So if your problem is cylinder #3 then maybe swap the ignition coils between cylinder #2 and #3 and swap spark plugs between #3 and #1 say. Then clear the code, drive it for a while and see if anything comes back. If you now have a problem with #1 then you know it's a faulty spark plug. If you now have a problem with #2 then you have a faulty ignition coil. If your problem stays with #3 then you have something else going on. If you have a single cylinder misfire then changing the spark plugs and the ignition coil for that cylinder will fix your problem much more often than not. But as seen in this video, sometimes it's something different. Sometimes you need to use a labscope to really know what the problem is. Being able to use a labscope effectively is 1 part knowledge, 1 part experience and 1 part luck of having the car actually act up when you're hooked up to it! Good luck and let me know what you find!

[@labuser0111]

Hi, just curious to know if the new ECM fixed the issue. The P0303 started on my Elantra 2017 at around 50 000 KM. I changed the the spark plugs and the all 4 coils using Hyundai parts, now the issue is back at 55 000 KM. I hope it's not the ECM in my case :( .

[@eurbanautotech]

@Labuser01 Yup. The ECM was the fix for this vehicle. I'm not sure how common ECM failures are on these Elantras. The way this ECM failed definitely is not common. Normally when transistors fail they just don't close or they glitch out making a lot of electrical noise. Can be hard to spot too. Would be very hard to find without a labscope and even then I got lucky. I managed to have a time where it was acting up long enough for me to get my equipment setup. Before that I had looked at it many times over the course of months but it would not act up.

[@rodrigoccassiano]

I have the same thing on my car, but have not yet checked the signals as I don't have an oscilloscope. Question : what if you install a resistor (with high resistance) in between the ground and signal cable? We call this a pulldown resistor to ensure the signal is at 0V when the transistor doesn't switch to ON (voltage). Just a thought

[@eurbanautotech]

@Rodrigo Castilho That would also pull down the 5V signal. So, I don't think it would work.

[@rodrigoccassiano]

@@eurbanautotech not if you have high resistance in your pulldown resistor

[@eurbanautotech]

@Rodrigo Castilho I'm not an electrical engineer but I don't think that would work. The PCM transistor provides the 5V reference. In order for the 5V reference to cleanly pull the circuit all the way up it would have to be a significantly lower resistance circuit than the ground. So one more high resistance path to ground wouldn't affect anything because it already has a high resistance path to ground. That part is not faulty. In order to pull it back down to ground when the transistor is acting up you would have to have a lower resistance path to ground and that would affect the 5V circuit. I think the only way you could rig anything up would to build your own driver circuit. Have an IC that has its own 5V regulator and its own comparator circuit. Have it measure the control wire coming out of the PCM and when it saw a decrease of more than 2V then to turn on its own transistor providing a low resistance path to ground. Then when it sees a rise of more than 2V to turn off said transistor. The only worry with that is it might actually pull the PCM's 5V regulator circuit down affecting everything else. Plus it would be VERY involved. So ya, I really don't think there would be any easy "bypass" if you will. Still though, it's always good to think about these things. These kinds of thought experiments are how you learn!

[@deevee9077]

Hey did you try the ecm software update with the new coils before trying to replace the ecm im having the same issue with my 2017 elantra cylender 3 misfire. Just curious

[@eurbanautotech]

@DEE VEE Nope. I was lucky and was able to catch this in the act on the labscope. What it was doing can only be attributed to a failing transistor. Not a logic problem. Not sure if this is a common problem for this vehicle. It's certainly not a common problem in general. Sometimes transistors fail by being stuck closed, sometimes they let a lot of hash/noise through but most of the time they just don't ever close when they fail.

[@andys_youtube]

Did you resolve this issue?

[@eurbanautotech]

@andys_youtube Yup. This was a case of a bad ECM. Replacing it fixed the issue. Ya just gotta be careful with ECMs on these Hyundai's. An engine computer for a newer Hyundai can only have a VIN written into it once. Which normally means you can only install new ECMs into them. You actually can install used ECMs but you have to get them cloned which is a lot more effort.

[@khushpreetdhillon8923]

Where you are located Boss?