elantraGLS2

So I replaced the Ignition Switch and that wasnt it. Thought it was since I drove a day without a hiccup, but by day 2 the problem came back.



As a next troubleshooting step, I have a Bluetooth ODB scanner and the "Torque" Android app, so i got the idea to plug it in and turn on logging and have it log a data file while I drove and experienced the cutout. It logs stuff like speed, rpm, voltage, sensor data, etc of the car every 1/10 of a second. So I started driving, and I noticed that when it cuts out, the Bluetooth ODB scanner loses connection and has to reconnect. So the logs arent helpful since the ODB scanner loses power when it happens. But the fact that the scanner loses power for that split second during the cutout is very good troubleshooting information I think! I think this means that the ECU itself is losing power, which is why the ODB scanner loses power, since the ODB connection is what powers the Bluetooth scanner. This tells me one of two things. Either the ECU itself has some type of issue, or it seems there is a bad connection to the ECU. I say this because I dont lose power anywhere else when this happens (radio, headlights, dash) and normally the bluetooth ODB scanner has power when plugged into the ECU even if the car is turned off with no keys in the ignition. In other words power is always going to the ECU, and power is always going to the ECU's ODB port. But when the cutout happens, it seems that power goes away for a half second. Bad ECU? Loose connection maybe? Wiggling the harness and any wires under the steering column doesnt replicate it. I will say that about a month or two before this started happening, the voltage regulator on my alternator went bad and was actually putting out too much voltage to the car. I got a check engine light and looked up the code online and it said something like "Voltage exceeded 17 volts for more than 3 seconds". So then i tested with my multimeter and it was 16.8 which is higher than the 13.5-14.5 range it should have been in. So I put in a new alternator and that fixed it. So one possibility I'm thinking is that the high voltage could have damaged something in the ECU, though I'm not really convinced of that. But still contemplating getting a used one off ebay just to see what happens.



So I think this must be an electrical issue and has to do with the ECU or power to the ECU. As i think I said in my first post, I've already tried swapping the ECU relays and fuses, so its not that. Any thoughts?

187sks

Inspect and/or replace battery to chassis ground and engine to chassis ground. Try to shake ECU gently while the car is running. Pull on and shake connections to fuse box under the hood while it's running.

Stocker

For sure check the grounds especially if there is one for (and nearby) the computer. Have a good look at the connector on the computer also. Maybe a bit of corrosion got in there? Connect and disconnect the computer connection(s) a few times, maybe it's "just" a bad connection.*



I'd be tempted to swap the computer with a known-good one or at least a junkyard one, but it's not my parts money. The voices tell me to beware of a junkyard computer not starting because the key has the wrong interlock code but I don't know about your model in particular. Can the computer on a 2004 be opened? If so, maybe a peek inside would be in order. Maybe there's (e.g.) a capacitor in there on its last legs that could be visibly bad. It's common to see 16V rated caps in automotive stuff generally. It's possible it took a hit from your bad alternator.



*no-fun anecdote: I had a car that would intermittently die like the key had been turned off, but would restart right away with the key to the Start position. Then it started really running like sh*t. The ECM was in the engine bay, and the engine had been power washed. The cable harness connector to the computer was all a mess of conductive corrosion by-products. With that cleaned up, it ran like new again.

tibbytib

Damn! Thought the ignition switch was gonna work for ya. Still sounds similar to my problem. I'd check grounds like others have stated.

elantraGLS2

Last night I took the ECU out. It was a freakin pain to get to all the bolts that needed taken out. But I finally got it out and took the cover off. It looked spotless inside. Circuit board looked perfect, no leaking caps from what i could tell. The connection to the ECU also looked perfect. No corrosion, tight connection, wiggling everything had no effect. I also cleaned up the ground from the battery negative to chassis (sanded more paint off, etc). Though it looked just fine. Did the same for the ground from Engine to chassis. I put everything back together and test drove it. No change. As another test (running out of ideas), i swapped my battery out for the battery in our other vehicle. They're almost exactly the same size. The battery was 4 years old, so its probably gonna need replaced in the next year or so, but it has been working perfectly, starts find, holds a charge fine. THough like i said in an earlier post, apparently an alternator putting out too high voltage can fry a battery to where it kills it and battery acid leaking out. That never happened, but maybe by some chance it has done something weird internal to the battery like shorted some cells out. I have no idea, just running out of ideas so trying anything at this point. Havnt drove it enough yet to know if the battery swap has made a difference.

Stocker

fingers crossed, intermittent faults are the worst.

tibbytib

The wierd thing is your entire ecm looses power. Thats proven when you cut out, the obd Bluetooth dongle has to reset and you have to reconnect to your phone/tablet, correct? Still sounds like you are either loosing power or ground to the ecm.