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Discussion Starter #1
I lease a 2019 Sonata 2.0T. Since I have owned the car, it has intermittently bucked when accelerating when I start to drive. About two weeks, it also started to accelerate slightly when driving, again, only when I start to drive. Then, when I was driving it (engine was completely warmed up) and slowing for a light, the car began to vibrate badly, and the MIL light came on. I turned the car off and started it again, and it drove normally for the time it took me to get it to a local Hyundai dealership (not where I leased it).

The paperwork I got from the dealership says that the code was P030100 (cylinder one misfire). They monitored the cylinders and found #1 and #4 misfires. They "swap these coils still misfire at #1 and #4." They removed the spark plugs and found "a lot of blow by (dark)." They apparently replaced those two spark plugs and test drove the car without any misfires. When I came to pick up the car, the service advisor told me that they think that I got some bad gasoline, although I only buy top-tier gas at Costco. I have never put anything but Costco or Mobil gasoline in the car.

HOWEVER, two days after I picked up the car, I had the same problem with the car bucking when I start driving it and slightly accelerating. I think that the dealer was not able to pinpoint why a car with only 17,000 miles on it and which is not even two years into its lease is behaving this way. I am planning to bring it the dealership where I leased it for further evaluation. Any suggestions as to what is wrong with the car and how to make them fix it under warranty? TIA.
 

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I know, but the dealer doesn't seem to be able to pinpoint the problem and fix it.
I understand what you are saying, but I am not sure anyone here is going to be able to help. You just need to take it back to the dealer and if they can't fix it insist on a replacement vehicle for your lease. There should be wording in the lease docs regarding a defective vehicle. Additionally, it could just be a bad engine like many of the Theta II's are and it would be replaced for free. It sucks but at least the car is under warranty so you are only losing time and gaining a headache from all of this haha
 

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Discussion Starter #5
I understand what you are saying, but I am not sure anyone here is going to be able to help. You just need to take it back to the dealer and if they can't fix it insist on a replacement vehicle for your lease. There should be wording in the lease docs regarding a defective vehicle. Additionally, it could just be a bad engine like many of the Theta II's are and it would be replaced for free. It sucks but at least the car is under warranty so you are only losing time and gaining a headache from all of this haha
OK, thanks for your input.
 

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They should do a pressure test for those cylinders. It's very easy, I did it on my cars.
IMO that's the only way you can get "blow by" - busted piston rings.
 
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You know, I heard the service advisor tell me on the phone that they had done a "compression" test. Maybe he did say pressure test. But he said that the test didn't show any problems. And how does a car with such low mileage end up with busted piston rings?
 

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Compression test is what is called, sorry.
 

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Pretty much has to be either or a combination of
1. low compression - blowby oil fowling spark plug or just low compression due to rings or valves
2. bad spark plug - you already had it changed so it is unlikely you have another, but the plugs could be ruined by any of these other problems
3. bad fuel injector
4. bag ign coil

You are under the 60,000 so you are covered bumper to bumper. Take it to the dealer , demand a loner car and get it fixed.
 

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The one thing about this that is "nice" is that at least this is a lease and not a brand new car you own...you're gunna turn this thing back in at roughly 3 years/36k miles. So it just has to work well for that amount of time!
 

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Discussion Starter #11
Thank you, this is the information that I was hoping to get so that I can have a more informative discussion with the service advisor: "did you check the ignition coil? the fuel injector?" Otherwise, I'm just blind.
Pretty much has to be either or a combination of
1. low compression - blowby oil fowling spark plug or just low compression due to rings or valves
2. bad spark plug - you already had it changed so it is unlikely you have another, but the plugs could be ruined by any of these other problems
3. bad fuel injector
4. bag ign coil

You are under the 60,000 so you are covered bumper to bumper. Take it to the dealer , demand a loner car and get it fixed.
 

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Discussion Starter #12
The one thing about this that is "nice" is that at least this is a lease and not a brand new car you own...you're gunna turn this thing back in at roughly 3 years/36k miles. So it just has to work well for that amount of time!
Yes, that's the advantage. I normally don't lease because I don't want to be restricted as to mileage, but I had just bought a house so I was a little short on cash, and I hate to finance a car. But the downside is is that I'm stuck with the car until the end of the lease. Florida has a pretty good lemon law that I can invoke -- and I will -- if they don't get this fixed soon.
 

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. . .
Additionally, it could just be a bad engine like many of the Theta II's are and it would be replaced for free. . .
Not knowing the OP's driving habits over the 17k miles, I'd lean toward the GDI plague: carbon buildup on the intake valves. I'd request the tech inspect the back-side of the intake valves via borescope or pulling the intake, etc. Or, research/discuss 'seafoam' treatment benefits.

I doubt he has a bad motor since that issue was basically resolved in 2017's and later builds. However, this is a good starting point for his mech to document issues ICO a possible future failure.
 
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The dealer did the easiest repair.

2 misfire codes = 2 replaced spark plugs, easy peasy!.

Away you go!

A few weeks later you state same problems.

You did not mention if the "check engine" light came on again,
I assume it did,
and I assume for the exact same reasons.


Either way, this is not your problem.

Bring it back!

Hyundai watches.
2 warranty repairs for the same customer complaint / DTCs.
It looks bad on the dealer.
It looks VERY bad on the service advisor.
It looks bad on the service tech.

'Betcha they try and disguise the 2nd repair as a different problem.
 

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Discussion Starter #15
The dealer did the easiest repair.

2 misfire codes = 2 replaced spark plugs, easy peasy!.

Away you go!

A few weeks later you state same problems.

You did not mention if the "check engine" light came on again,
I assume it did,
and I assume for the exact same reasons.


Either way, this is not your problem.

Bring it back!

Hyundai watches.
2 warranty repairs for the same customer complaint / DTCs.
It looks bad on the dealer.
It looks VERY bad on the service advisor.
It looks bad on the service tech.

'Betcha they try and disguise the 2nd repair as a different problem.
The check engine light has not (yet) come on, but the car is doing the same lurching when I start driving it in the morning, and the same surging on acceleration when it's cold, that it did before I did get the check engine light and the misfires. That seems to indicate some kind of fuel/air balance, right?

Of course, the dealer does the least amount possible. That's a given. Several years ago, I had an Elantra and I had to move heaven and earth to get them to replace the power switch on the driver's side door when it shorted out periodically. The dealer ADMITTED to me that the motor was shorting out, but it took multiple complaints and the involvement of a newspaper columnist who wrote a car column before they finally agreed to do the repair. I was probably an idiot to get another Hyundai after that, but it's priced better than a Camry, so I did.

If I take it back to the dealer without a code, I know that they will tell me that they can't find anything wrong with it. That seems to be SOP.

I wonder if Technron fuel injector cleaner would be of any help?
 

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I lease a 2019 Sonata 2.0T. Since I have owned the car, it has intermittently bucked when accelerating when I start to drive. About two weeks, it also started to accelerate slightly when driving, again, only when I start to drive. Then, when I was driving it (engine was completely warmed up) and slowing for a light, the car began to vibrate badly, and the MIL light came on. I turned the car off and started it again, and it drove normally for the time it took me to get it to a local Hyundai dealership (not where I leased it).

The paperwork I got from the dealership says that the code was P030100 (cylinder one misfire). They monitored the cylinders and found #1 and #4 misfires. They "swap these coils still misfire at #1 and #4." They removed the spark plugs and found "a lot of blow by (dark)." They apparently replaced those two spark plugs and test drove the car without any misfires. When I came to pick up the car, the service advisor told me that they think that I got some bad gasoline, although I only buy top-tier gas at Costco. I have never put anything but Costco or Mobil gasoline in the car.

HOWEVER, two days after I picked up the car, I had the same problem with the car bucking when I start driving it and slightly accelerating. I think that the dealer was not able to pinpoint why a car with only 17,000 miles on it and which is not even two years into its lease is behaving this way. I am planning to bring it the dealership where I leased it for further evaluation. Any suggestions as to what is wrong with the car and how to make them fix it under warranty? TIA.
I don't know why anyone failed to see this. You stated misfires on 1 and 4 and then switching coils 1 and 4. That won't tell you anything. Sounds to me like 2 bad coils. switch 1 and 4 coils to cylinders 2 and 3 and see if your misfires move to 2 and 3. If so you have 2 bad coils,.
 

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What's the spark plug gap? My Gen Coupe turbo they list the wrong gap. .040 is recommended but .028 is what works best.
 

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faulty trans temp sensor? Quite a common issue with sonatas. Also don't assume you're stuck with the lease. There is ways to get out such as selling to carvana, vroom, etc.
 

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I don't know why anyone failed to see this. You stated misfires on 1 and 4 and then switching coils 1 and 4. That won't tell you anything. Sounds to me like 2 bad coils. switch 1 and 4 coils to cylinders 2 and 3 and see if your misfires move to 2 and 3. If so you have 2 bad coils,.
I don't think anyone missed this...I assumed it was worded wrong and they swapped it correctly. No one could be that stupid!
 

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I don't know why anyone failed to see this. You stated misfires on 1 and 4 and then switching coils 1 and 4. That won't tell you anything. Sounds to me like 2 bad coils. switch 1 and 4 coils to cylinders 2 and 3 and see if your misfires move to 2 and 3. If so you have 2 bad coils,.
I understood the OP narrative to state that they,
"swap[ped out] these coils [and] still misfire at #1 and #4."
believing no logical-reasoning tech would swap around two assumed bad coils as a process of elimination... But, maybe tech misunderstood the GDS.
 
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