flat spot in power band at 5000rpm

[rsfeller]

I think this had gotten a bit worse over the last year, mostly this summer.

1997 986 w/75K

After properly warming up and going for a spirited ride I always notice when in 2nd or 3rd gear and hammering the accelerator that my car pulls great from 4K-6k+ rpm. There is always a small drop (I hate to use the word hesitation) in the power around 5k-5200 rpm. You can just tell that the power drops momentarily w/o a feel of a miss. I really don't think it's ignition or fuel related from the seat-of-the-pants feel.

Anyone else experience this or know if this is a symptom of anything?

This car is well maintained with filters, belts, ect but I have not done the plugs yet and would assume they are original. If I recall the service period is 100K for the plugs but I'm curious to other opinions.

Thanks.

Shawn

[White987S]

Plugs should have been done at 60K. This is one of the cheaper DIY projects.

[White987S]

Actually, you should have changed the Spark Plug twice already. At 30K and again at 60K. The poly rib drive belt is due at 60K too.

[Tool Pants]

On a 2.5 the spark plug change interval with the oe plugs is 30k miles. Someone must have replaced the plugs over the years.

[rsfeller]

MBybe it's goodl got the interval wrong then. I've got a reason for theflat spot. I got the car @ 64K So I'm guessing the plugs werl done once at least. Is the 2.SL engine sensati ve when it comes to plugs? I've direct ignition cars the are hyper sensative.

go tribe

[juniinc]

Porsche recommend every 30,000 miles.

MaintenanceSheet9962_06_1_.pdf

MBybe it's goodl got the interval wrong then. I've got a reason for theflat spot. I got the car @ 64K So I'm guessing the plugs werl done once at least. Is the 2.SL engine sensati ve when it comes to plugs? I've direct ignition cars the are hyper sensative.

go tribe

[rsfeller]

So any other thoughts on my original question or is it just going to be a 30K plug pile on for this replies!?!?

thanks all! I got it, 30K...

Shawn

[perleman]

When the MAF sensor is on the blink it can exhibit this behaviour - flat spots in acceleration - not a particularly cheap part @ £150 UK, but apparently quite an easy self-fit part. However, I'd get the codes downloaded & checked 1st, as otherwise you could waste time & money.

[White987S]

Try running a couple tanks with fuel injector cleaner too. The Chevron product with techron is very good.

I agree with Perelman too on the MAF.

[Glyn]

Im with pearlman on this.

Most likely to be the MAF.

Disconnect the electrical connection on your MAF and try it, The management system will go into its limited opperation strategy and in built approximates will govern the fuel/air mixture, however the hestitation will be gone - confirming the MAF needs to be changed

[rsfeller]

When the MAF sensor is on the blink it can exhibit this behaviour - flat spots in acceleration - not a particularly cheap part @ £150 UK, but apparently quite an easy self-fit part. However, I'd get the codes downloaded & checked 1st, as otherwise you could waste time & money.

Perleman and gang. I have no codes or CEL at this point.

How could it be in failure mode if there are no triggered codes yet? Does it do this before it offically gives up the ghost?

[Chris_in_NH]

it's the MAF. i had the exact same thing happen to me. cleaning the MAF worked for a while, but the hesitation came back after a while. eventually the MAF starting sending crazy information to the DME and it was time for a new MAF.

to test your MAF, you can disconnect the MAF's wiring harness, then reset the DME by disconnecting the neg battery cable for at least a minute (have your radio code handy), then re-connect the neg battery cable. this will reset the DME (ECU) back to default values without any input from the MAF (this will not hurt your car/engine by running MAF-less). see if your hesitation goes away. i'll bet you $1 it will.

if you want to be sure, plug the MAF back in (and reset the DME again by disco'ing/reconnecting the neg battery cable) and then drive the car. see if the hesitation comes back. if it does, you know for sure that it's the MAF.

i think www.AutohausAZ.com has the cheapest prices on MAFs these days.

[mee]

How flat is flat? I think these cars all have a flat spot right around 5K. This might be due to when the VarioCam changes. If you look at dyno charts of Boxsters, they all have a HP flat spot right about there. Now, if it's really getting worse, then I might suspect something, but it could just be that you're more sensitive after you've noticed it. Since yours is a 2.5L, it's not a sticking resonance valve which might be something in a later model

[Loren]

VarioCam is deactivated at 5120 RPM. I think he has a bad MAF.

[rsfeller]

VarioCam is deactivated at 5120 RPM. I think he has a bad MAF.

Thanks all, sounds like the MAF is where I'll start, do the plugs too when I do my seasonal oil change in the coming weeks.

Loren, You don't think there is any correlation to the disengagment of the variocam at the given RPM, because if I had to guess exactly where the drop kicks in.

The MAF is an easy check so that is where I'll start this weekend. I just figured when my decided to go like everyone else's it would be slightly more dramatic!

What does MAF standard for, is it a air mass meter?

Shawn

[Loren]

I think if the Variocam were not working right you would get poor torque in the mid-range. The MAF is critical for high RPM since it measures the air flow.

[jporter]

I had the exact same problem (no codes either) until I replaced the MAF.

[mee]

VarioCam is deactivated at 5120 RPM. I think he has a bad MAF.

Loren, how you know/find all this is amazing! Where can one find this information?

Do you know if the 5120 RPM is the same or varies per engine 2.5/2.7/3.2? I would guess this question is just for the basic VarioCam since I think the VarioCam Plus (or some flavors of it) are continuously adjustable. Also, do you know what RPM the resonance/crossover tube operates? I read somewhere it was 4700 RPM (IIRC), but this could also be engine size/year specific.

[Loren]

Most of that is in the model year specific Technical Service books. You always need the first model year of each book as they give detail of just about everything new to the car. After that the next years book just lists changes from the previous year.

Too my knowledge they all turn off at the same RPM including VarioCamPlus.

Porsche calls resonance/crossover "boost" - the Resonance Flap.

I don't have detail on early the Boxster (perhaps Tool Pants has a MY97 Boxster Tech Book) but on 3.4 liter Carrera it works like this:

"An intake system divided by a resonance flap allows utilization of oscillations along the intake air column across a broad speed range (resonance supercharging). Due to the different working cycles of the individual cylinders, air is drawn from both reservoirs in the intake system alternately. The mass air flow in the intake system is excited by the alternating induction. In the case of resonance, the intake frequency of a cylinder line matches the frequency of the compressive oscillations in the pertaining reservoir. This frequency is determined by the geometry of the intake pipe, the resonance pipe, and the reservoirs. However, the overall length of the pipes from the intake cylinder to the subsequent intake cylinder, the division within the intake and resonance pipes as well as the reservoir depth in the flow direction are the deciding factors.

The DME control unit activates a diaphragm valve controlled by negative pressure. This valve opens or closes the resonance flap. The resonance flap is closed when de-energized, but is actuated and open whenever the ignition is switched on. If, after the starting process, the engine reaches the programmed idle speed, the resonance flap is closed again.

Switching points of the resonance flap:

Resonance flap not actuated between 700 rpm and 3,120 rpm and from 5,120 rpm.

Resonance flap actuated between 3,120 and 5,120 rpm, if the throttle is open by more than 30%."

[mee]

Switching points of the resonance flap:

Resonance flap not actuated between 700 rpm and 3,120 rpm and from 5,120 rpm.

Resonance flap actuated between 3,120 and 5,120 rpm, if the throttle is open by more than 30%."

Thanks, Loren. I did some digging in my files of interesting stuff that I run across, and it looks like the operation of the resonance flap is likely very highly engine/model-specific. I found, for the 987, that it is actuated above 4700 RPM for the 3.2L and above 5000 RPM for the 2.7L. This is quite different than the 3.4L Carrera (open only during a certain window) and likely different than the 987 3.4L and earlier 2.7L/3.2L 986es which is what I have (latter).

Do you know if the problem above (original poster) might coincide with the transition from closed loop (using O2 sensor feedback) to WOT open loop fuel metering? This would make a lot of sense if this change occurred around 5K RPM and the MAF was gunked up, it would be running too lean-- although it is probably more complex involving load and other things telling the ECU when to switch.

[Loren]

If the MAF is bad then the readings it is giving the DME are bad. So the fuel - air mixture will be bad at high RPM. You can disconnect the MAF and drive around all day and never feel a difference - as long as you stay under 4500-5000 RPM.

[Tool Pants]

I have the 1997 Boxster tech intro booklet. It also says 5,120 rpm.

I have a 1997 and several years ago I had a hesitation at about 4,000 rpm, then it would go smoothly to redline. I thought it was the VarioCam. When I was at the dealer for a smog inspection it turned out to be the air flow sensor.

[pkilleen]

VarioCam is deactivated at 5120 RPM. I think he has a bad MAF.

Loren knows what he is talking about. A couple of weeks ago, he advised that I had an MAF problem, and he was right. Just changed it with a part I bought cheap on line and car is back to normal greatness. Thanks to Loren and the others that help out on this forum. This one repair saved me more than my Durametric code reader cost. By the way, I had an identical power drop at 5000 rpm before I changed the part.