Weird issues with 02 Elentra

[FDF12389]

I told my buddy I would help him with his car misfiring, only a few things it could be right? WRONG! We replaced plugs, wires, coil pack. Still happening. All cylinders misfire, but not all at once. and it happens consistently, but not frequently. The CEL blinks on, car studders and then the CEL is gone, I have not been able to get a code. I did more trouble shooting, and nothing was making sense. He took it to the private garage he usually goes to. They had it for a week, could'nt figure anything out or get a code. They replaced the MAP sensor.

Issue persists. I tested fuel pump, ran a compression test, replaced O2 sensor. Still happening. his dad got fed up with me and took it to the hyundai dealer. 380 dollars later they replaced the O2 sensor and flex pipe. Issue persisted.

I told him I would look at it again. While I was looking it all over again the speedo needle starts dancing around, car was in park. A few days later his brake light and battery lights are on. I tested battery and alternator, both are fine. Brakes are fine. When he rolls down the drivers side window(Power window) the headlights will flicker.


I hate electrical problems with a passion, so now I turn to you guys for some help! Where/what else should I look at?

 

[exdeath]

Have you considered an exorcism?

 

[FDF12389]

Have you considered an exorcism?

I've considered testing my new 12GA pump on it a few times 🙂

 

[exdeath]

On a more serious note, CEL does not come on instantly and then go away when there is a misfire, or any other obd issue for that matter. OBD codes and CEL is a controlled and deliberate event. What you describe sounds like momentary interruption of power and ecu reboot, but since the key is in run and there is rpm it just resumes without missing a beat (see wut I did thar?)

Look at amps and volts on a scope trace and see what happens when it stumbles. At this point with so many problems in unrelated systems and the CEL behavior you describe, I would suspect a major ground point like the main battery cable or fusebox. Might mess around with the ignition switch and see if that causes it to manifest, same with beating on the computer housing, etc. Problem like this will require shooting in the dark until you get lucky.

 

[FDF12389]

On a more serious note, CEL does not come on instantly and then go away when there is a misfire, or any other obd issue for that matter. What you describe sounds like momentary interruption of power. Look at amps and volts on a scope trace and see what happens when it stumbles. At this point with so many problems in unrelated systems and the CEL behavior you describe, I would suspect a major ground point like the main battery cable or fusebox. Might mess around with the ignition switch and see if that causes it to manifest.

I looked at all the ground points when I checked the battery and alternator. And I had a multimeter hooked up to the car, it seemed to misfire, then drop from 13.8v down to ~12

Edit: at least to me it seemed light Misfire, then voltage drop. I will run it again with someone else to make sure its actually not voltage drop, then misfire.

The car has only died about two times in the last month, would the car died more often if it was loss of power?

Thanks again for your help.

 

[exdeath]

You need a scope, or something that can show a waveform over time and is very fast, a multimeter is pretty useless for fast glitches like this.

But with your last post, it's obvious that you have a major power supply connection issue. Note that even if the engine is idling, if power is cut for a split second, field current in the alt goes away and the alt stops charging regardless of rpm, that is when you only see 12v at the battery posts.. This same incident also causes the CEL coming on like when the key is in the ignition with the engine off, but then going back out when it resumes right away, etc. Field current to alt goes though the main relays tied to the ignition switch, etc.

Just brainstorming here, you really need to see a current and voltage waveform. Or just start looking at relays, ignition switch, etc.

 

[exdeath]

The CEL flickering and voltage at battery posts dropping to 12v is the key here.

Ill say again, CEL does not light without a code for a systematically raised OBD error, but it does come on any time the engine is stopped (stopped does not mean zero rpm mechanically, but efi system state), either by the key being turned off on purpose, or something else doing it.

Misfire seeming to be on random cylinders is just whatever cylinder is firing the moment electrical power is removed.

 

[FDF12389]

You need a scope, or something that can show a waveform over time and is very fast.

But with your last post, it's obvious that you have a major power supply connection issue. Note that even if the engine is idling, if power is cut for a split second, field current in the alt goes away and the alt stops charging, that is when you only see 12v at the battery.

Just brainstorming here, you really need to see a current and voltage waveform.

That's making more and more sense. I think I over complicated the issue, thinking that if it was something like that one of the two shops it was in for a total of 15 days would have figured it out...

 

[exdeath]

If this was my car, having thought about it here, It'd be looking over all main relays and wiring junction and fuses between the battery, ignition switch, and ecu, up to and including the ignition switch itself.

Problems like this are very difficult and easily overlooked, because it's one of the foundation systems that is a prerequisite for anything even working at all, so you just expect it to work and never consider otherwise. Anybody who looks at it will start off the same way you did, hunting a genuine misfire. Like I said, the CEL flickering and battery dropping to battery volts only is the key here.