duwerke

I recently installed a set of catless headers from a known brand on my 2021 GT4 PDK. I simultaneously applied a tune from another very reputable source. The following issue exists both when the tune is applied and I’ve tried the stock file too, no change so symptoms are definitely hardware related. So here it goes…

Upon starting the car, I noticed the RPM bouncing. It does it at cold start, it does it at warm start, it does it at every stop light. Even more annoying than that, is I'm also experiencing surging under light throttle and on decel.

Videos:

Idle at stop light
https://photos.app.goo.gl/cWwsXam1GK33ZtBAA

Low speed surging in parking lot
https://photos.app.goo.gl/rJiTfoTeaeqZ8Wcb8

Light throttle surging during deceleration
https://photos.app.goo.gl/7ANi8gRN4jJ8yEZi8

I spoke with the tuner, who has been very helpful, but immediately stated this is a known issue on the 718 4.0 cars, even with 200 cell headers. Severity, he said, depends on the car and the batch of headers... some side-effects are livable, some aren't. I also spoke with my friend in the Porsche exhaust industry, who asked the dev team where he works, and they also mentioned it's a known issue on the 718 4.0 platform. Surprisingly, these are the only two posts I could find on the topic, and one just links to the other:

https://rennlist.com/forums/718-gts-...dle-surge.html

https://rennlist.com/forums/718-gts-...installed.html

Most of the commenters seem to have a bouncing idle at start up that goes away. Mine does not. None of the commenters mentioned the throttle surging while driving, which is the most unlivable side-effect.

My tuner suggested I swap primary and secondary O2 sensor locations (even though there's no cat and they're right next to each other). Reason being, the primary he said was too close to the collector where the three pipes meet and it's readings were biasing one cylinder, and by putting that a few inches downstream toward the flange, it may read better. This unfortunately did not fix the issue.

Further discussion with the tuner brought up the O2 sensor bung depth and angle. Now my headers resemble half a dozen others on the market, and they all have somewhat tall bungs welded in at an angle instead of shallow and at a 90. This begs the question, would this matter? If the end of the sensor isn’t perfectly in stream? Could it be as simple as drilling out these bungs and welding some in with more care to positioning?

I’m about out of ideas, but I’ve already spent money on the headers and tune and taken the time to install. In addition, the headers + my OAPS + the factory muffler is the best sounding combo I’ve heard for these stifled cars, and I’ve tried several, so I’d really like to make this work.

For those of you with headers, please let me know what headers you have and if any of this has been an issue. I’m really curious since it seems so known in the industry but almost zero customer feedback online.

DerekHK

I use JCR headers with cat at the OAP (I love this setup). I only get "bouncing" RPM when I first start the car and it goes away after a minute. Your videos above sure do look a bit strange. Hope you get your issue sorted out soon. Keep us posted as I'm really curious as to what is happening.

dnimi123

I have a tune and Kline 200 cell cat headers - never any issues or RPM bouncing.... works perfect. I think its the tune. Can you let us know which tune you are using?

duwerke

As stated, symptoms are the same with or without the tune applied. Tuner is well regarded.

6IXSPD

Have you inspected the gaskets between the headers and the motor, or headers and OAPs?

duwerke

not since installing last Sunday, but I torqued everything to spec at that time and new gaskets and hardware were used.

enduro

Mine bounces during the cat light off sequence, then settles down. Hasn't bothered me enough to do anything about it. (Dundon street + JCR valved silenced). Zero bounce as soon as cat lightoff is done.
Wonder if yours has a leak in the vacuum lines for the cutouts.

Westcoast

Since you have let the tuner off the hook and say it is hardware related, which headers?

BoxKing

I get the same little bounce, but it’s smooth… it fluctuates from 900-1100RPM at cold start, then everything is normal. Its very subtle, and hardly noticeable unless you’re the one turning the key.

Kline 200 Cell

ShawnM981

I started one of those previous threads.

I have fabspeed headers with 200 cell cats, fabspeed OAPS and softronic tune. Mine does the bounce for a minute or so on startup and then goes away, I’m considering going to another brand to try eliminate this, can’t imagine dealing with it all the time especially while driving.

One thing to note - my car got reflashed by Porsche last year while in for service, tune was gone and when I went to reinstall Scott said he needed the ecu to unlock it because they’d changed something. Since the tune has been reinstalled I’ve noticed the cold start surge lasts a little longer and I’ve gotten a check engine light related to emissions (0420) a few times which never happened before.

Could be something in the newest refreshed ecu tuning from Porsche that’s fighting the catless system more than before??

Angryinch

Sounds like you have a leak before the o2 sensor causing the fuel trims to hunt up and down.

Ramon Vennik

catless JCR headers on 21 Spyder ,very very little fluctuation , the needle stays above the 800 rpm.
Are you sure that there is no leaking somewhere?

MannyLon

A decat can cause lumpy idle. I had this on the 718 GT4, just ignored it..