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George H.
George H., Hyundai Technician
Category: Hyundai
Satisfied Customers: 18702
Experience:  Hyundai Gold certified, ASE Master tech 15+ years
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What could cause a misfire besides plugs, wires or coil pack Could a sensor cause interru

Customer Question

What could cause a misfire besides plugs, wires or coil pack? Could a sensor cause interruption to the primary voltage to coil pack? Thanks
Submitted: 5 years ago.
Category: Hyundai
Expert:  George H. replied 5 years ago.

Hello I will help you with your question,


The other possibility is a failing crank sensor or a computer if you are sure the timing light works well.


How does the engine run? Is it missing?



Customer: replied 5 years ago.
The engine runs ok and idles ok (sometimes hard to start). Colder weather will make this worse however it has cut off on my daughter particularly when she slows down to stop or turn. When I look at the rpm with my diagnosticscanner the rpms will jump around a bit (plus or minus 40-60 rpm). I know it is 4 cylinder but it does not have much power at all. The timing light does work well and I verified this with another vehicle to make sure. I will check/replace the crank sensor as well.
Expert:  George H. replied 5 years ago.


Do you get trouble codes for any particular cylinder or just the P0300 general misfire code?


Have you cleaned the throttle body and idle air passages?


If you hold the throttle open so the engine runs at 3000 rpm does it die off?


Has the fuel filter been changed?


There is a problem with rust in the fuel tank on these so you might want to take a look in there (under the back seat) and see. If there is rust the injectors are full and need to be cleaned/replaced.


None of that will make the spark weak though, make sure the coils are grounded and that the signal to the coils from the computer is solid.


Do you have any way to look at the electrical signal?



Customer: replied 5 years ago.

Believe it or not, no codes for misfire. Not sure if it hasn't got bad enough yet for it to be picked up.

Have not cleaned the throttle body or idle air passages. Will do.

Engine does not appear to die off at 2500-3000 rpm from what I can tell. I will try this again and hold it at that rpm longer to know for sure.

Fuel filter has been changed. I did that a few months back. There was some debris in it.

I will check the tank as you mentioned. Do I understand correctly that I gain access to it by removing the back seat (I don't have to remove fuel tank)? If I find rust I will check injectors.

What would I need to look at elec. signal? This would be an analog 12v wouldn't it? I don't have an o-scope.


I will add one other thing. I didn't mention this because I did not believe that it had anything to do with spark firing. I did have a po455 fault for "large evap leak. I replaced the gas cap first with an OEM cap. That did not take care of it. I pressure tested at 20"h20 and did not have a pressure drop but I did discover a few pieces of the carbon filter material had somehow gotten to the vent valve. That may have been keeping it open causing the code to show up. I cleared code day before yesterday. I do know that evap issues can effect performance but I thought I was having two separate issues and that I may have fixed one of them. I appreciate your all of your help. This is the first time I have ever had to get some back-up. Scratching my head a bit. Thanks


Expert:  George H. replied 5 years ago.

Ok if you have loose carbon in the vent valve you will need to replace the canister and blow the lines clear of any loose carbon. That should cure the P0455 if you don't have any rotted lines between the purge valve up front and the canister.


I doubt that is the cause of the miss but if the purge valve is being held open with carbon it could be, pinch off or plug the hose between the purge valve and the intake, the valve is on the left fender liner.


Still none of that accounts for a changing/weak spark. I have had to replace a couple of these computers for poor communication and strange readings when being scanned so I hope you don't have to go there. Without a scope it will be tough to see the signal to the coils and determine if it is poor signal or a bad coil. I was hoping you would tell me you have a Vantage pro or something similar.


You do open the fuel tank by removing the rear seat cushion, there is an access panel and the hoses are quick connects, be sure to cover the connection before you open it, I get fuel in my face if I forget this step. Keep the area clean so you don't cause a leak when you open the ring on the pump module.


Try cleaning the throttle body and see if that makes a difference, I would replace the crank sensor if the budget is not too tight, it is on the front of the block under the coils toward the front of the engine.


Let me know if any of that helps, and when you changed the wires were any of the coil towers corroded or full of white gunk?



Customer: replied 5 years ago.

ok to carbon filter replacement

I have pulled the purge valve recently (a couple of months ago) to make sure it was opening/closing properly/tightly. It was ok. If code comes back I will pinch line to isolate.


I may be able to borrow o-scope we have here at work. What should frequency look like?


If I remember right when I pulled the crank sensor out several months ago there was some metal dust (assume from normal wear) collected on the end of it. I remember having to clean it off. If this is a prox/magnetic type sensor would that interfere with signal and cause misfire issues?


No signs of corrosion inside the coil posts. They were clean.


Expert:  George H. replied 5 years ago.

All you will see at the coils is the computer closing the signal to ground to fire the coil, the crank sensor will put out a changing AC signal as the engine is running


The white wire is the signal to one set of coils and the green is the signal to the other. A clean regular signal into the computer from the crank sensor on either the yellow or the brown wire and a regular signal out of the computer grounding the coils.


If the coil signal is not pulled fully to ground look for corrosion on the ground for the computer and if none is found then the computer is failing.


A little "dust" on the tip of the crank sensor is normal but if there was fuzz pull it again and take a look, you may have a bearing failing.


Let me know what you find





George H. and 2 other Hyundai Specialists are ready to help you
Customer: replied 5 years ago.



will check ground


will check sensor again.


Thanks again and I will get back with you

Expert:  George H. replied 5 years ago.

I will watch for your post so let me know what you find and what I can do to help


Thank you