What could it be? surging during moderate acceleration

I just recently picked up a 2009 Hyundai Genesis sedan with the 4.6 and 190k miles. I have noticed that during light/medium throttle to accelerate it has power surges. When it's happening the RPMS will pull for ~200 RPM, then pause for a second, then pull again, then pause, etc., almost like the throttle is like an on/off switch. It still shifts up and down between gears fine and at WOT there is no surging. It doesnt feel like the trans is slipping at all as when the RPM increases it does speed up and you can feel it pulling. It has no CEL so i have nothing to go off of there.

I have changed the fuel pump (due to an unrelated issue) which has the filter integrated into the assembly. I plan on cleaning the throttle body and MAF when my CAI arrives. I have also ordered a trans filter and fluid to do just a drain and flush, it's supposed to be a 'sealed lifetime unit' but at 190k miles it could probably use a new filter and a little new fluid.

Any idea what might be causing the issue? anything anyone has experienced before? I would think that if it was TB or MAF related I would receive a fault. I also am not ruling out the trans but I would think if it was a trans problem I would experience shifting issues or have some slippage. The trans is a ZF 6HP26 which is used in several BMW's and other euro cars if that helps at all.

If the surging rpms go up, might be a trans issue.  If the surging RPMs go down, its engine.

Sounds like an ignition issue to me.  I would take it on a long drive and continue to repeat the conditions until you get a CEL.  Does it take longer than normal to fire when you crank it?  If so, that makes me think plugs.

The rpms go up in small increments under steady throttle, it's not like the trans is slipping then grabbing again, it puts you back into your seat every time the rpms jump up. if it where on a graph instead of going at a smooth angled line it would look more like stair steps, which then smooths out when over ~2300 rpm (when manually shifting so it doesnt shift itself sooner). 

maybe surge isnt the right word as the rpms dont jump up and down, at idle or steady cruise the rpm stay solid, its only when accelerating that the rpm does its stair-step like increases.

It doesnt feel like an ignition issue as it's too long of a pause between rpm increases to be any kind of misfire atleast that i've ever felt before and it cranks and starts up just fine every time, nothing out of the normal, but i do have a new set of plugs so i plan on doing those anyways.

I’d suspect the “drive by wire” motor.  It sounds like a similar problem my crapbox Suzuki is having.  New throttle body unit is on the way and should fix it.

If you have one oxygen sensor that has biased a bit lean, it can cause that type of surge.  Sensor reads rich, it leans the mixture until it sees lean, then richens the mixture.  If its pulling it too lean, you get a surge.  Fuel trims can be a clue, but it takes a skilled eye to see.

Haven't seen it often, but it does happen.  

O2 sensor is my guess as well. Could this be an issue with the drive by wire potentiometer?

yea it is drive by wire, which would make sense if it was doing some wonky stuff and constantly adjusting the throttle blade at steady throttle. I have an ultagauge bluetooth deal so i can mess with it and see if i can read the specific throttle body value and see if it's fluctuating while driving, i can also take a look at what the o2's are doing as well. hopefully i can get something figured out.

In reply to Streetwiseguy :

That was my first thought. Lean surge

well i checked my ultragauge last night and this morning on the way to work. During the stair-step-like acceleration the throttle position stays pretty steady as it should so i dont think the TB is trying to do anything weird. I also monitored both upstream o2 sensors, they are narrow bands so not super easy to monitor but during acceleration they fluctuate between 0.7-0.8 to 0.1-0.2, seem to bounce back and forth pretty regularly with no long periods at either end that i could tell, which if i remember right is normal for a narrow band reading?

also all my stuff for doing the trans filter should be here today so i plan on getting to it tonight or tomorrow to see if that makes a difference at all.

In reply to KyAllroad (Jeremy) :

Tell me about this crap box Suzuki and what you're experiencing?  

well I finished changing the filter and fluid in the trans last night. The surging has gotten about 90% better, much much smaller surges that do the stair-step motion quicker (if that makes sense) and that's only at light, almost grandma, type throttle inputs. any throttle inputs over that it seems to have a smooth rpm sweep between gears. So i think I'm onto the problem.

Also the fluid I drained out of it was a dark red, as far as I'm aware there is no red fluid that is meant for these ZF 6 speeds, only blue and clear. Some people online say they've used some regular ATF's and its fine but most people say it's best to stick to the ZF specific fluid. So I'm guessing the old stuff in the trans is some sort of ATF since it looks like someone changed the fluid at some point due to the plastic drain plug being stripped. I plan to drive the car for a while then I will probably do another drain/refill just to help get as much new fluid into the system as I can.

pres589 said:

In reply to KyAllroad (Jeremy) :

Tell me about this crap box Suzuki and what you're experiencing?  

It’s a 2008 Forenza I bought for $150 because bad HG.  Replaced HG and got it running but at very part throttle it surges pretty badly.  More throttle and it cleans up and runs fine.  It’s pure rental spec, stripped down, white econobox sedan for the teenager to drive.  

KyAllroad (Jeremy) said:

It’s a 2008 Forenza I bought for $150 because bad HG.  Replaced HG and got it running but at very part throttle it surges pretty badly.  More throttle and it cleans up and runs fine.  It’s pure rental spec, stripped down, white econobox sedan for the teenager to drive.  

Sometimes when I'm going up a long hill in my Suzuki SX4 the car will feel like it's surging.  Manual trans so it isn't a torque converter clutch acting up, but that's kind of what it feels like.  Clears up if I drop to a lower gear, but the car has enough power to pull the hill in the gear it is in, this is just to get the RPM up.  I've wondered if it's the drive by wire throttle acting stupid or what.  Maybe I need to get something like the Torq app and my phone out to tell me what the throttle position is?  

Your car and mine seem to share very little engineering so maybe your experience isn't as relevant as I had hoped.  

Well, once again my “throw parts at it” technique of auto repair has failed me.  Installed new throttle body and it still surges, possibly at a higher frequency than before but still irritating.  It really is at light throttle, low rpm use.  Over 1/4 throttle or 3,000 rpm and it’s fine.

Maybe spray some electrical circuit cleaner in the gas pedal potentiometer.