Fuel Trim question

Hello guys,

Have a 2107 Hyundai Veloster Base model, no turbo, simple 1.6L GDI engine. 118k miles. No CEL.
Bought the car last October, had one major issue, cat converter (fixed that, put new OEM one), other than that, just minor things. I have an OBD reader, started reading fuel trims. At first, LTFT was 9-13% under load, 6% idling. Took a guess it's not a vacuum leak, put 2 new O2 sensors, new MAP sensor, nothing changed.
Replaced the fuel filter, LTFT dropped a little, 10.9% max under load, 5.5% idling. Getting warmer I guess, replaced the fuel pump. And now we're here, 6.3% idling, 7.8% under load, if I let off the gas pedal 7%. Also, now under load LTFT doesn't budge, like it just goes 7.8% and stays there, where as before, it would jump around a little.
I know the number is not bad at all, but I'm pretty new to reading fuel trims, and I feel like it can get better. This is my personal car and I want to take care of it. What would you guys suggest should be the next step to diagnose and bring LTFT numbers even lower?
My guesses would be the fuel injectors and a high pressure fuel pump?
Also, it was doing this weird thing when I bought it, not so much now, but every time after I drive, when I'd turn it off, there would be this sound coming from the exhaust pipe underneath, cracking noise like when the heated metal cools down (I hope that makes sense). Since the car has 2 cat converters, but the sensors are only monitoring one, and I've replaced one of them, do you think it's the second cat that makes the noise, and could it not working influence the trims?
Thank you, very curious if anyone has any advice in which direction should I investigate.

Welcome to the community! It's good that you are interested in learning more about fuel control on your car, and I completely understand wanting to give it the best chance to run well for a long time.
In my opinion however, you are just spending money. If there's no check engine light and no drivability issue, there's nothing to fix.

+1 on what Noah states.

What is the total/overall fuel trim?

If you are to go ahead further with your quest for "better" numbers the consider an injector cleaning instead of a change. Leave the high pressure pump alone at this time..

What would you guys suggest should be the next step to diagnose and bring LTFT numbers even lower?

If this were a customers car, I don't think I could recommend anything. You're past the point of diminishing returns, IMO. There's other services to do and parts to replace, sure. But I couldn't 100% guarantee lasting results.

BUT, if you're enjoying learning and doing repairs, then who am I to discourage you? :silly:

I didn't see it mentioned, but have spark plugs been changed? Intake valve cleaning been performed? Both of those are good maintenance items at this mileage. I'd consider doing those if you haven't already, even if they might not have an impact on your trims.

I wouldn't replace the high pressure pump based on trims alone. Injector cleaning is an option, like Juergen suggested. Can't say I see too many Kia/Hyundai GDI injector issues, but it's worth a shot.

Since the car has 2 cat converters, but the sensors are only monitoring one, and I've replaced one of them, do you think it's the second cat that makes the noise, and could it not working influence the trims?

I wouldn't sweat the noise, honestly. It's probably exactly what you think - hot metal contracting as it cools down. Most all exhaust systems make noises like this to some extent or another.

The only way I see the downstream cat causing trim problems is if it's restricted. But if that were the case, I'd think you'd notice a lack of power. Plus, restricted exhaust on speed density systems usually results in extreme negative trims under load, in my experience. Your trims are slightly positive.

Thank you, thank you! Like I said, I understand that my numbers are not bad. But they did become better after replacing the filter and the pump, I'm just asking, let's say there was a "1" right next to my LTFT, and it was 17.8% instead of 7.8%. What would you check next, would it be the fuel system or something else?

What is the total/overall fuel trim?

You mean, LTFT and STFT? I know for sure, when idling LTFT is 6.3% and STFT is 0. Under load STFT can jump around a little, but if not mistaken it's +-3%. On my third screenshot you can see it shows 9.4% under load, but it never stays that high consistently.

As far as injection cleaning, you mean take the injectors out and ultrasonic clean them at some shop? Because, I've ran some chemicals to clean the system, including BG 44k, Lucas and Valvoline fuel system cleaners through it. My question was, do you think it would be the right component to diagnose (fuel injectors and HPFP)? I know there's a pressure drop test for injectors, for example. Or there's another direction could be taken?

have spark plugs been changed? Intake valve cleaning been performed?

Yes to both, spark plugs are NGK OEM ones, and I did do the Seafoam treatment before replacing the cat converter. It was a little weird, all the other videos I saw on YT doing it, their cars would smoke During the test itself and then after, where they would take it for a spin. Mine didn't smoke at all during, and then after I took it on a highway, it didn't smoke at all for some time, and then it stumbled for a second and BOOM, there was a HUGE cloud behind my car, like it dropped a smoke grenade or something I felt so bad for the guy behind me :pinch:
There are other ways to do it too, like walnut blasting or there's some BG treatment, maybe I'll look into it, if the price is right. Or just do it myself, idk.
Also, thank you for answering about the second cat, how restricted exhaust affects the trims, is what I wanted to find out, and you're saying it would go negative, so we should be good then.
For the amount of fuel system cleaners I've ran through my tank, I'd think that if they were clogged at some point, I cleaned it lol. And the only way they can malfunction is just to physically be bad.
But generally, do you think the fuel system is the one to keep digging in? And by digging, I mean checking parameters not replacing lol. I think there's a fuel pressure drop test for injectors.

Euphoria wrote:

...I'm just asking, let's say there was a "1" right next to my LTFT, and it was 17.8% instead of 7.8%. What would you check next, would it be the fuel system or something else?

On a speed density system, I'd have to stay with the fuel system. Poor injector patterns, fuel quality, or just a genuine lack of fuel delivery into the cylinder. Speed density system rarely have genuine lean conditions, in my experience. Because air measurement is pretty foolproof, the only direction to go is fuel. :silly:

Yes to both, spark plugs are NGK OEM ones, and I did do the Seafoam treatment before replacing the cat converter. It was a little weird, all the other videos I saw on YT doing it, their cars would smoke During the test itself and then after, where they would take it for a spin. Mine didn't smoke at all during, and then after I took it on a highway, it didn't smoke at all for some time, and then it stumbled for a second and BOOM, there was a HUGE cloud behind my car, like it dropped a smoke grenade or something I felt so bad for the guy behind me :pinch:

Yeah, that happens. :lol: The cleaner pooled at the bottom of the intake, then got sucked in all at once. Most four cylinder engines like yours will do that after an induction service.

There are other ways to do it too, like walnut blasting or there's some BG treatment, maybe I'll look into it, if the price is right. Or just do it myself, idk.

The BG GDI service is the only chemical solution I've found that has any impact. The Seafoam probably just polished up the hard carbon deposits. :silly:

IMO, if the goal is to have clean valves, you gotta clean them manually. Either rig up a walnut blaster or scrape them with a pick and carb cleaner.

As far as injection cleaning, you mean take the injectors out and ultrasonic clean them at some shop? Because, I've ran some chemicals to clean the system, including BG 44k, Lucas and Valvoline fuel system cleaners through it.

If you could find a shop that'll clean them, sure. Most (around here, anyway) don't own the equipment. You could find someone to send them off to? They also sell ultrasonic cleaners for reasonable-ish prices on Amazon and such.

I don't find that in-tank additives will do much for a fuel system problem. I've run several in my personal vehicles and never noticed any kind of improvement. Not real evidence, I know, just my personal experience.

I know there's a pressure drop test for injectors, for example. Or there's another direction could be taken?

...

And by digging, I mean checking parameters not replacing lol. I think there's a fuel pressure drop test for injectors.

There is manual injector pressure drop testing that can be done. You'd need an injector timer tool (or make one yourself), a low side fuel pressure gauge, and some way to run the low side fuel pump. You'd also need a wiring diagram to make your connections at the injector harness. There is no automated/scan tool injector flow test on this YMME that I know of.

The problem with flow testing this way is that it's done with low side fuel pressure only (say 50-100 PSI), and not true rail pressure (500+ PSI). Plus, it won't find injectors with poor spray patterns.

Ok cool, thank you so much for this invaluable info, I've never really messed with fuel trims before (perks of having a Toyota lol reliable as a tank), and got a little overwhelmed with the amounts of variables that could cause positive fuel trims.
I'll try to find a shop that tests/cleans fuel injectors, I feel like it could be just a good maintenance thing to do, regardless of any trims or anything. Hopefully, there is someone that can do that where I'm at.
I guess I'll try to clean valves too, this time with taking off the intake manifold, to see the before and after results. Funnily enough, there's a video from Nate's Interactive something, where he used an intake valve cleaner from STP, that according to him made those valves look almost brand new. I was really skeptical seeing that, since he didn't take off the manifold in the video, he just inserted a "before and after" pictures. Maybe I should record my own video, with proper proof, to see if it works as well as he claims, or not. And then do a proper manual cleaning, if the valves are still dirty.
My last question would be, do dirty valves have an impact on your trims at all? Or you just recommend it as maintenance?

Euphoria wrote:

I guess I'll try to clean valves too, this time with taking off the intake manifold, to see the before and after results. Funnily enough, there's a video from Nate's Interactive something, where he used an intake valve cleaner from STP, that according to him made those valves look almost brand new. I was really skeptical seeing that, since he didn't take off the manifold in the video, he just inserted a "before and after" pictures. Maybe I should record my own video, with proper proof, to see if it works as well as he claims, or not. And then do a proper manual cleaning, if the valves are still dirty.

Interesting. :huh: I hadn't heard of Nate, so I went looking. Was this the video?

My last question would be, do dirty valves have an impact on your trims at all?

Depends on who you ask! :silly: Some techs swear they've fixed lean conditions by cleaning intake valves. I've never seen it. A few percentage points here or there, maybe, but nothing substantial. If there are others reading along that can offer their input on that, please do.