Is check engine light on?
If so, need to get the code(s).
How many miles on the car?
We had such a problem this summer, car (2016 Forte 1.8) would die when hot, at idle. Later, Replaced gas cap, ccv, pcsv, mass sensor, gas tank pressure sensor... We were on a trip and didn't have my Scangauge to get the codes.
I'd guess yours has a problem with the vapor system too.

 

I have my mechanic likely, he connected the computer, no error codes.
The car has a new engine I bought one month ago the old engine seized because lost the cool liquid.
The old owner I think did around 5000 km I did 2000 since bought.

 

The odometer was 72.000 km anyway

 

plugs, correct type spark plugs, spark plug gap.

Ignition coil pack or packs, ignition coil pack insulation cracked, cracks, missing, deteriorated, wire harnesses loose/not seated connected correctly, wires pulled/pins making contact.

Check level of fluids.

Battery, alternator, battery cables, MAP/MAF, PCV, dirty filthy gunked up throttle body.

It can be several things.

Drive vehicle to local Autozone/O'Reilly's for free OBD2 scan if no OBD2 in tool box.

Can be bought from Autozone, O'Reilly's, Walmart & local FB marketplace.

 

Spark plugs, correct type spark plugs, spark plug gap.

Ignition coil pack or packs, ignition coil pack insulation cracked, cracks, missing, deteriorated, wire harnesses loose/not seated connected correctly, wires pulled/pins making contact.

Check level of fluids.

Battery, alternator, battery cables, MAP/MAF, PCV, dirty filthy gunked up throttle body.

It can be several things.

Drive vehicle to local Autozone/O'Reilly's for free OBD2 scan if no OBD2 in tool box.

Can be bought from Autozone, O'Reilly's, Walmart & local FB marketplace.

We cleaned the air intake and the air mass sensor as well.
Changed oils transmission (manual) Royal purple and engine with mobil1 0-30w right after bought, air filter as well.
Everything looks new I did not check the sparks yet since the car runs great maybe I will check them after winter, I do not want to mess too much now with the engine I'm going to face a very cold starts soon since I live in Toronto

 

Run a bottle of Techron Fuel System Cleaner through a tank of gas. It won't hurt it and might just help it.

 

I did it right after bought before the oil change, I put royal purple system cleaner despite I was looking for Red Line SL1.

Maybe went slightly better, I got it better by adding in the gas my UCL which has high cleaning power as well.

 

If you need important electronic sensors, use Kia ones (buy on-line). This is from experience.

 

Yep, OEM/Factory switches, sensors, modules.

Carid.com (sells oem)
Kiapartsnow.com or other authorized online (Mobis) & your from your local parts dept Ki/Hyundai dealership.

 

First I want to thank all of you for the prompt responses.
I will try to answer all of them.

 

The problem comes from the too low RPM at idle, just 100 rpm more would have been perfect.

 

Spray carb cleaner at hose connections etc, while car is running, to see it has leaks, cracked hoses and whatnot, a quick and easy test

 

Tonight seems is getting better by itself, maybe the air mass sensor was dirty and the computer is adjusting slowly or the UCL is unclogging some injectors let's see next days if continue to improve🤞

 

Today the idle is almost perfect, I can say that it is improving a lot, maybe the air intake and the mass air sensor were the problems and the computer is adjusting slowly.
The UCL smooths the idle too, as one of many positive effects on the engine.
My Idle goes like in the picture,I thought it was too low.

 

How did it work after that?

 

Fuel tank pressure sensor + KIA gas cap+ shipping $98.69
KIA CCV + shipping $32.38
KIA fuel filler hose + fuel pump O-ring + shipping $51.95

Along this journey, I also bought off-brand PCV valve, MAP, purge valve, gas cap, but ended up using either Kia parts, or reusing original parts.

We still get p0455, but after erasing it, now it takes a week or more to trip again. I don't care anymore because we don't have inspections in AL, and the car runs fine.

When we had major problems this summer, with car dying at idle, not starting after gas fill-up, etc, the only meaningful new parts were CCV and gas cap. After that, it ran ok. I ignore the p0455, and taught swmbo how to erase it.

 

From the FSM, for 2nd gen forte,

Idle speed with AC off 640 +/- 100
Idle speed with AC on 680+/- 100

 

So my idle is at proper RPM

 

You may want to switch gas stations. Water can leak into underground tanks, especially if the tanks are old. Sediment can be a problem, too.

 

Regular gas is ok just make sure its high tier and your furl system will stay clean

Fuel Stations - TOP TIER™

TOP TIER™ Licensed Brands prominently display the TOP TIER™ logo on the pump, pump handle, canopy or in a station window.

www.toptiergas.com

 

I think OM says to use Top Tier gas.
However, I've never noticed any difference with the brand of gas used.

 

I think OM says to use Top Tier gas.
However, I've never noticed any difference with the brand of gas used.

I have not noticed any difference either when I’ve had to use non-Top Tier gas. However, if it’s available, I prefer to use it.

Also, if you’re using non-Top Tier (or any gas for that matter), make sure you use a station that sees heavy use. Less chance the fuel is sitting around getting contaminated.