Slight engine hesitation

[Boxstard]

1997 Boxster with 2.5 and 5-spd manual, when I am fully winding out through 2nd and 3rd gears, I feel slight hesitation/ flat spot just around 6,000 rpm before hitting red line. Is this just normal or something to look for? Engine runs very smooth, new plugs/ O2 sensors/ MAF sensor, etc. and no CEL since I fixed SAI check valves over the winter.

Also I searched here and elsewhere about upgrading with a larger 1999 996 throttle body. It seems more beneficial for larger 2.7/ 3.2 motors where TB may be more of the bottleneck, but could not find much about how effective with 2.5 motor. I'm wondering if the flat spot at 6,000 rpm may improve with the larger TB, hopefully without costing low end? I usually keep the rev above 3,000 rpm so probably appreciate mid/ high-end more.

Thanks for sharing thoughts!

[thstone]

A flat spot at 6K rpm is definitely not normal. And rarely (never?) will a performance modification eliminate a foundational problem (more often than not, the upgrade exacerbates the problem).

Check the ECU for any codes (like a misfire) that may have been detected but did not illuminate the CEL. After that you'll have to do some detective work to sort through the usual suspects like plugs, fuel filter, air filter, fuel pump, coil packs, MAF, etc. to find the culprit.

[tommy583]

I had a similar problem. It ended up being a totally clogged charcoal canister. Put in a new used one and haven't had a hesitation since. I just took all the tubes off of the canister and tried to blow through it. Could be something to look into.

[Boxstard]

Originally Posted by thstone

A flat spot at 6K rpm is definitely not normal. And rarely (never?) will a performance modification eliminate a foundational problem (more often than not, the upgrade exacerbates the problem).

Check the ECU for any codes (like a misfire) that may have been detected but did not illuminate the CEL. After that you'll have to do some detective work to sort through the usual suspects like plugs, fuel filter, air filter, fuel pump, coil packs, MAF, etc. to find the culprit.

No codes stored, engine runs fine 99.8% of time, and most stuff you mentioned are replaced last year within n the last 8K miles, except fuel pump and coils likely original at 77Knmiles. It is just slight hint of around 6,000rpm for fraction of a second before it continues to rev into red, so I do not think fuel starvation or ignition coils.

I agree that upgrading won't make sense until you fix whatever wrong first, I guess I'm just thinking if this momentary flat is more like by design?

[Boxstard]

Originally Posted by tommy583

I had a similar problem. It ended up being a totally clogged charcoal canister. Put in a new used one and haven't had a hesitation since. I just took all the tubes off of the canister and tried to blow through it. Could be something to look into.

Interesting, but I thought the canister gets basically purged at idle when vacuum past the throttle body is high, and at 6,000 rpm the purge valve is closed so the flow through the canister is irrelevant. Anyway I'll check it out, first try to blow through it.

[thstone]

Originally Posted by Boxstard

... I guess I'm just thinking if this momentary flat is more like by design?

My Boxster spends the majority of its life on the racetrack where literally every shift is made at redline (around 500 upshifts per race weekend) and I can assure you that there is no flat or dead spot at 6000 rpm (and I am on my 4th engine so I can say that this is true for more then just my current engine).

I agree with your logic regarding the fuel pump and coils. My only other thought is to check the throttle cable just to be sure that its smooth and linear and not hanging up momentarily somehow (a bit of a long shot but I'm running out of ideas!).

[Boxstard]

Originally Posted by thstone

My Boxster spends the majority of its life on the racetrack where literally every shift is made at redline (around 500 upshifts per race weekend) and I can assure you that there is no flat or dead spot at 6000 rpm (and I am on my 4th engine so I can say that this is true for more then just my current engine).

I agree with your logic regarding the fuel pump and coils. My only other thought is to check the throttle cable just to be sure that its smooth and linear and not hanging up momentarily somehow (a bit of a long shot but I'm running out of ideas!).

Okay I appreciate your feedback, very minor issue but keep thinking. By the way I understand that the variocam kicks in at about 4,000 rpm but does it shift the phase again at higher rpm, maybe around 6,000 rpm that I may be feeling?