Jumpy RPM while cruising
 

Hey there, recently picked up a 17 brz with 29k miles. When driving above 35mph till around 45/50mph the rpms sometimes bounce a bit up and down when giving light steady throttle. A lot less prominent if I use cruise control and completely goes away if I engage the throttle a bit more or disengage and let it coast. Just seems to happen when applying light steady pressure. The car had a catless exhaust on it from the previous owner, the dealer had to install a cat so it would pass emissions. Otherwise runs great and transmission seems solid. Just stumped on the jumpy rpms. Any insight would be appreciated.

Manual?

Yes, it’s a manual.

It's possible the previous owner was dumping the clutch. But this is just my guess.

Sounds like a slipping clutch. But the only thing that would do that.

It shifts smoothly between gears and I don’t notice any grinding,slipping of clutch. The rpms just seem to get erratic if you’re not giving gas or letting it coast, it’s most noticeable between 35mph and 45mph, usually 4th gear around 2600rpm. Acceleration is smooth when I don’t feather the throttle. If I put in cruise control sometimes it’s steady but other times it’ll act like it’s bogging.

There is a weird dead spot right at the top of the pedal that produces some problems you might be talking about. If your in cruis control and you lay your foot on the acccelerator you actually start to lose speed.

So the dealer installed the catalytic converters so they could sell the car. Normally one would get a tune with a catless exhaust. Did the dealer also revert the tune back to stock? If this wasn't done, you'll have issues.

Ya I’m having a local Subaru dealer do a full checkup on it tomorrow, hopefully all is well, I’m thinking at best I’m just going to replace the exhaust with a stock one or a better aftermarket setup where I know what’s going in. At worst I’m thinking they have a mild tune on it that I’ll have to attempt to reflash, supposedly the headers are stock but wouldn’t surprise me if they aren’t. Drives very similar compared to my old 17 brz but the jumpy rpms around 35 to 45mph are the only things bugging me, aside from the obnoxiously loud agency power exhaust.

id say its running lean, enough that its starting to break up a bit, but not enough, nor long enough to set a lean code.

Can you read data from the engine with a scan tool or OFT?

I did full exhaust and intake, drove around for about a month like that before getting a tune, I did not have the issues this OP is describing and I was for sure running lean(James @ HRI showed me).


OP should put it in fifth at a low speed and give it throttle to see if the clutch is slipping because that's what it sounds like.

yours wasnt lean enough to let it start to stumble a bit :)