Hesitation at high RPM's
 

My 98 Boxster seems to hesitate after 5,500 rpm's. It doesn't always do it, but at full throttle it sometimes feels like it stumbles. The air filter is new. I had my indy change the spark plugs. He inspected the coil packs and said they were fine. Any ideas?

Fuel filter or fuel pump.

I forgot to add the the fuel filter is new also.

O2 sensors getting tired?

Well i bought the car last summer. It did the same thing before i put it away for the winter. Since then i have replaced the plugs, air filter, fuel filter, clutch, IMS bearing, flushed break fluid, replaced belt and replaced AOS. If it could be the coil packs, that would seem like an easy fix. The fuel pump seems like a pain. I believe it's in the tank, i hope i'm wrong about that. My Indy has a fair labor rate, but i'd rather try to do more myself before the wife kills me.

MAF??? My hesitation cleared up after I changed the MAF. At first there was no CEL but as symptoms got worse I got a CEL, and O2 sensor codes that pointed to a bad MAF.

MAF? Maybe, but so far I have never had a CEL. Then again I have never hooked up a reader just to make sure. But the CEL light does come on when I switch on the the key, just like all the other lights under the gauges. So I would think that it does work and no one has disabled it.

Did you retrieve any codes? Bad spark can cause the same, maybe one of the coils how old are the plugs? A can of techron couldn't hurt.

That is the first step in diagnosing these cars. Get it hooked up to a scanner and see what is going on.

The plugs are around a month old. The tech at the shop said that the coils looked fine when he changed the plugs. But one or more of them could have cracks he didn't see i'm sure. I'm thinking that even though i have never had a CEL i should have it hooked up to a scanner. Does it have to be a Durametric or will any professional scanner that a shop would have work?

Changing the fuel pump is not a pain. Just pay attention. After you do it once , You will say how easy it was to do. Good luck !!

I also forgot to mention that I have a chirping sound upon hitting the gas. It happens more when i let out the clutch ( under load ). If i just hit the gas in neutral ( wife helping ) then it's just a quick chirp, instead of a few chirps. I'm thinking after replacing the belt, I'm putting more stress on a pulley or the water pump. Or after searching the threads maybe the tech didn't snug down one of the spark plugs when he replaced them. Could any of this ( Original Post ) loss of power at higher RPM's and Chirping be related?

Yes.
Failing AOS, loose spark plug, and badly worn chain tensioners have been known to make a chirping sound. Your car is a possible candidate for all three.

Well the AOS was replaced last fall. It didn't make the noise before the belt and plugs were replaced, but it did hesitate from time to time before that. So they probably aren't related now that i think about it. I'm just going to have to hook it up to a scanner and see what the issue really is before i just throw parts at it.

A new fuel pump is one of the best investments you can make for $ 200.00 bucks. Well worth it !!!

Unless the one in the car is still ok. ;) Changing out good parts is never really a good investment. Get a clear diagnosis first. Measure the fuel pressure and if the fuel pressure is low, change the fuel pump. If the fuel pressure is normal, changing out the fuel pump might be fun but it won't solve the hesitation at high rpm.

+1 I thought by now the codes would be on the thread, why take the circuitousness hard way when you can get some clues as to where to start looking?

will be spark plugs for sure

Well the codes are not on the thread yet because I haven't had the chance to take the car to the shop. The check engine light is not on, nor has it ever been on since i've owned the car. I don't think i can just swing by Auto Zone and have them see if there are any codes can i ? It has been pouring rain here the last few days. I hope to get it somewhere to see if there are any codes in the next day or two. Thanks for all the suggestions so far.

Read the first thread - plugs have already been changed out by his indy.....