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The first generation Forte shares the same platform as the Hyundai Elantra (HD), though employing a torsion-beam rear suspension in place of the Elantra's multilink design.

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Possible engine failure or bad gas?

I have been looking at this 2011 Forte to see why it is running rough, and I suspect it's possibly a bad engine but just looking for a second opinion. Engine has 154k miles.

The first thing done was changing the spark plugs as a gamble. No fix, of course. Cylinders 3 and 4 also had a lot of oil on the spark plugs... More on that in the compression test numbers.

The first thing I checked for was codes about the problem. Regrettably, I forgot the code, but I saw a generic Cylinder 1 misfire code. To rule that out, I swapped coils, cleared the codes, and re-scanned. It's the same code on the same cylinder. I'm not too surprised being it uses Denso ignition coils.

The next thing I checked was the compression. This is where I begin to be leary of the engine. The other thing I noticed (but can't show, my borescope camera doesn't support image saves but cylinders 1, 3, and 4 have oil from bad rings; 3 and 4 being the worst, and also have the worst compression numbers.

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  • Cylinder 1 (left): 1,350KPa
  • Cylinder 2: 1,325
    • So far so good. Kia/Hyundai says there should be a 100KPa differential on this engine at most. A little deviation will happen with this many miles on anything, so nothing concerning.
  • Cylinder 3: 1,250-1,350KPa
    • 100KPa differential, ick. It wouldn't be worrisome if isolated and it's fine, just a death sentence with Hyundai/Kia when it's worse.
  • Cylinder 4: 1,100-1,300KPa, 1,200KPa on the 2nd test
    • 100-200KPa differential. Not great.

Cylinder 4 is all over the place. Cylinder 3 is borderline at best.

The only thing I have not checked is the ethanol and fuel injectors, yet. That's next.

In the meantime, I'm weary of the engine and based on those numbers and I do suspect the engine is potentially NFG given the inconsistencies with Cylinder 4 and Cylinder 3 being borderline. I've even tested Cylinder 4 4 times, the others were baseline run and checked a 2nd time. The only test I didn't check is the wet compression. I saw everything I had to see.

Update (03/08/24)

Update on the checks: Checked the gas, 10% ethanol. It's gotta go back under the microscope.

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Update (03/12/24)

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Swapped injector 1 to 2. Code moved.

Update (03/20/24)

And I'm back with yet another report. Sigh...

So I swapped the injector, and it had the same codes.

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I swapped the replacement injector into Cylinder 3 (and put the good one in Cylinder 2), same codes

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The only things I can see to try are the connector and if that doesn't work it looks like a PCM/ECU or engine failure.

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Bingo, we have a winner!

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@dadibrokeit It used to say Generic, not Kia. I guess clearing it 2+ times knocked it out of Generic.

Guess I was just a bit put off by 154k being the near end when I'm used to Toyotas routinely hitting near 200k miles before things break down and being fine when I saw the initial compression numbers. And it's usually not the engine outside of the 2AZ-FE; always something like the alternator.

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Hi Nick,

Having wrenched on internal combustion engines for several decades now, I'm only in partial agreement with you. Yeah, that engine has definitely seen better days, and the bad rings are eventually going to be the demise of that motor. However, the compression numbers, while not great, really aren't bad enough to be the source of the rough running, and besides, if the misfire was detected in cylinder 1, that doesn't match up with your compression numbers that are showing cylinders 3 and 4 being the worst.

Since you've already done a good job of eliminating the spark plugs and coils as possible sources, that pretty much just leaves fuel as the last thing to check. So yeah, swap out the injectors just like you did for the coils and see if the misfire moves as a result.

But all in all, based on my experience with import car engines, I'd say the motor is tired but not quite at the end of its life. My guess is it's got another 50K miles to go before it gets to the point where it'll either be smoking or losing power, so it's probably worthwhile to pin down the source of the misfire before resigning it to the scrap heap.

I'd suggest going ahead with the wet compression check just to verify that it's the rings and not the valves that are causing the lower compression numbers you're seeing. You never know; if they all come up you might be able to extend the life of that engine significantly with just a valve job, depending on whether you feel it's worth the investment. My 1996 Volvo is still plugging along nicely after I had to replace a broken exhaust valve. Of course, I had to spend a good $150 for tools and a head gasket set in order to replace a $10 valve, but it was worth it to me.

Update (03/06/24)

Just as a side note, I was reading an article about misfires and how the engine computer detects them. What it's doing is reading the rotational speed of the engine; every time a piston fires, there is a distinct jump in speed that can be measured, so when one cylinder isn't putting out as much power as it should, that difference is reflected in a smaller rotational jump and can then be flagged as a misfire if it exceeds a certain threshold.

I know, doesn't really help with anything, but I thought it was interesting to know how the misfire detection works.

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The excess amount of oil in Cyl 3 and 4 compared to Cyl 1 was the reason I suspected the engine at this point. Yes, the misfire is on Cyl 1, but 1/3/4 and having puddles of oil on the piston is offputting. I forgot to check Cyl 2 for oil with the camera this time, but I'm just going to assume it's no better given the other 3. If they were all close like 1 and 2 were being off by 25, I'd have looked at the injectors before running it past you guys, it wasn't cut and dry like well under the acceptable minimum on most of the engine.

The fuel test is first because of the timing, then the injector swap. And you realized you described my scan tools with the tools? At least tools are a one-time expense, then you do not have to buy them again. At least when it's time I can more easily inspect a high-mileage car like a Toyota if it comes down to it. My first scanner was pure garbage that could barely clear codes so I had to buy a rebranded Innova (BLACKTEC 440).

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Gas is fine and isn't over 10% ethanol. I have to check the injectors next; wouldn't shock me if they're bad at this point.

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My scan tool can't do a full system diag, and it's still there. Possible wiring or PCM issues, maybe the engine (gasket or short block).

I probably did condemn the engine prematurely looking back, I agree.

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UPDATE: I got my hands on a better scanner, and attempted to reset the throttle body, with the same issue. The issue remains even after cleaning the connectors in the fuse box and ECU with contact cleaner. Battery/variable reset didn't help yet, but it needs miles put on it to see.

The next target is to check the ignition coil resistance, as well as the wiring and injector resistance; I got the data from the SM. If this isn't it, it's a deep wiring issue I need an Autel scanner to put it against.

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Nick さん、ありがとうございました!
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