Poor Performance at High RPM

Poor Performance at High RPM

> As a new member of the list I am taking the opportunity to canvass
> experienced 928 owners/enthusiasts about a problem I am having with my 89
> S4.
> The car idles and drives ok below 4000rpm but once above this level the car
> chokes and misses intermittently{slight miss, not full miss].
> This problem has dogged the car since purchase and has failed to be
> rectified despite full diagnostic check from the porsche garage.
> the car has been serviced and has had the plugs and leads changed.
> The coils have been checked and the injectors have been checked and given a
> full bill of health.
> I am losing confidence in the porsche people to diagnose the fault and the
> cost is rising.
> At top end the car pulls no more than 130 mph despite all the love and
> attention.
> Can anyone out there suggest a possible explanation before I lose faith in
> what is otherwise an excellent car.
> thanks in advance
> barry cattanach

Try *new* coils. They are about $65 each. I recall Jim's S4 doing about the same thing as you described (kinda like hi-rpm stutter). One ohm'd out okay but was dead anyways, zero reading on meter when running, similiar to my old coils. Installed new ones and...


Phil Tong

Have you checked the operation of the "tuning flap" in the intake? It may be stuck or bad or it may not have any vacuum available to the flap to actuate it.

Here are a couple of ways to test that it is functioning properly. Remove the round rubber cap that is in the very middle of the intake (on the top of the intake)

Test #1 - Note location of "circlip" that is visible, have someone start the car while you watch this circlip, it should rotate 45 degrees or so for just a second when starting and return.

Test #2 - Note location of "circlip" that is visible, with the car running (in neutral or park with parking brake on) rotate the throttle at the intake housing to rev up the motor very quickly (at least 4500-5000rpm) and watch the circlip to make sure it moves.

If the circlip moves during either and/or both of these tests then the intake tuning flap is working correctly and you have another problem. If it doesn't move then your performance problem is the tuning flap which is either stuck, not getting vacuum or the actuator is bad or disconnected.

Hope that helps,

David Roberts

> What is the "tuning flap" you mentioned? Is this a "tuned intake
> runner" device? I have noticed that sometimes my GT will hesitate
> briefly if I am around 3000-4000 RPM and punch the throttle. It seems
> to slightly bog and then immediately rev to redline. It revs smoothly
> to redline if I punch the throttle at a lower RPM. Is this normal or
> could my "tuning flap" be sticking or having a vacuum problem?

This sounds exactly like a tuning flap problem.

All 87 and up 928 intakes have a 2-stage intake system, the flap remains closed at lower RPMs to give it lower end torque and then opens up around 3500 or so. This is what gives the 87-up 928s such a great flat torque curve and on a dyno sheet you can actually see where the flap opens. For example, my 89GT (91K miles, stock w/RMB) has a minimum of 250-260ft/lbs at the rear wheel all the way from 2000rpm to 6000rpm with 2 torque peaks at around 3500 (flap opening) and around 4500rpm (torque peak of 295.5 ft/lbs at rear wheels). The guy who owned the local dyno shop (Dynojet 248C) I use was totally amazed when he saw my 89GT's torque curve on a dyno sheet for the first time. They were used to dynoing 650+ rear wheel HP blown Mustangs etc and everyone was gathering around to see the almost perfectly flat
torque curve.

Try the tests I mentioned before and also check to make sure your vacuum storage system is working properly. A simple test you can do for checking to see if you are "storing vacuum" is to leave your 928 sitting overnight after driving (or at least 4 hours ) and pull the vacuum check valve out from the 4-way splitter side (located near the engine side of the brake booster), you should hear vacuum escaping, i.e. "hiss". If not, you have a bad check valve or a vacuum leak in your HVAC system, cruise control vacuum line/actuator (87 and older only), Intake flap actuator, 4-way splitter, etc.

Hope that helps,

David Roberts

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