Piston Slap: Roughing It in Tucson?

Steven writes:

Hi Sajeev,

I have a 2016 Hyundai Tucson 1.6T with 90K and I have an intermittent issue of a rough idle with the A/C on — it feels very jumpy and did not have this issue before. When you turn it back off the idle becomes smooth again. I’ve cleaned the MAP and boost pressure sensors as well as removed the throttle body to clean it — it had a little gunk at the bottom of it but it looked really clean. That seemed to clear up the problem, but the issue came back again. I know it’s due for another spark plug change (as Hyundai says every 45K) so I’ll get that done soon. but I’d like to trace this down while I’m doing the work under there.

Any ideas?

PS: And no, it’s not under warranty — as countless others have claimed.

Sajeev answers:

Any engine codes? It never hurts to scan for codes just to cross it off the list.

That said, I am leaning toward either a vacuum leak (not likely, but modern turbocharged and direct injected mills got plenty of plumbing) or a refrigerant pressure problem. If you used a cheapo A/C recharge kit (again, not likely) get a pro (or professional gauges) to test the actual pressures: maybe your expansion valve (some cars use an orifice tube) is blocked and causing pressure issues that’s driving the engine computer bonkers.

But this is a direct injected motor! Fouled spark plugs and coked up EGR passages/combustion chambers (though high mileage suggests otherwise) could exacerbate the poor idle when the A/C compressor starts churning. So replace the plugs, but put a scope in there before installation: do you see significant carbon buildup?

Hopefully not, but if the plugs give you no joy, maybe it’s time to de-carbon the engine as per Hyundai specifications.

What say you, Best and Brightest?

[Image: Autoguide]

Send your queries to sajeev@thetruthaboutcars.com. Spare no details and ask for a speedy resolution if you’re in a hurry…but be realistic, and use your make/model specific forums instead of TTAC for more timely advice.