kj217

When the rpm drops can you hear any other noise? I'm thinking some parasitic load kicks on. A/C compressor. Electric fans.
i hear an electric actuator humming in the front right behind the dashboard

Refresh my memory: Since this symptom pretty easy -- I take it -- to reproduce -- why haven't you taken the car to a Porsche dealer or a good indy shop for this symptom?
i had it to the dealer and they charged me $200 for nothing

Also, you were able to reproduce the symptom with the MAF disconnected at the wiring harness, yes?
that's right

kj217

how to eliminate the gas and pump since they are the suspects.

redridge

I would start eliminating the easier and less expensive first..... I believe the gas pump is in gas tank.... but you mentioned this only happens on idle at park or neutral....

could it be that the o2 sensor are not reading properly and the ecu is adjusting accordingly (false idling)

kj217

it happens on park, neutral and on D in heavy traffic.

this should throw a CEL,, shouldn't?

redridge

Ok.... try eliminating the ac compressor, does the rpm fluctuate with the ac off?


throw a cell?, not if the O2 sensor is in range.... but you mentioned that it also does it in D so.... its unlikely that all 4 sensor are bad (eg:reading constant)

kj217

it happens with a/c off and on,,
but i guess the chance of it happening is more likely the moment a/c goes on.

kj217

where to get that??
how much is it??

Graufuchs

Another possible issue.
Throttle body alignment?????

An unaligned throttle body would cuase a rough idle and poor off the line performance.

Unplug battery.....step on brakes...(making sure there is no current in system)
Reconnect battery turn key to position just before cranking, do not start car...you should hear throttle body cycling...let it go through full cycle. Ussually 30sec to 2min.

Start car. See if this helps.

Macster

You can pay nearly as little or much as you want.

Somewhere in the middle best I think. Course it depends upon what you want to do.

Since you are attempting to trouble-shoot a puzzling problem you might want to get a full feature OBDII code reader/data viewer, scan tool, diagnostics tool which the ulitimate is the Porsche System Tester 2. These are not something you can run right out and buy though.

These are used by Porsche dealerships and seldom come up for sale. Price is when they do come up for sale is near I can tell north of approx. $1100. How much north I can't say. Some have reported though buying one for $1100 IIRC. Recenlty one went unsold at just over $1000.

The next tool down is the Durametric. Search the web and you'll find company/product link.

Below the Durametric you're in generic OBD2 scan tools. Actron makes a good one (several in fact) and there are other brands.

A problem is even if you are able to look at the OBD sensor data and derived data it may not tell you anything. Even when there are error codes the failing sensor or component may not jump out at you.

Sincerely,

Macster.

kj217

can it be a bad alt/battery?
the rpm is more likely to drop when car is hot, fans on and a/c on,,

Macster

An increase in parasitic load from some accessory drive I guess could account for the RPM drop. Alternator accessory drive is as good as any to suspect I guess.

Would have to be a pretty big load though. Something abnormally big. At idle A/C load (and electric fans, cabin fan) ups the load by just a few tenths of a percent. Maybe from 8.8hp to 9.24hp. IOWs, it requires about 1.25hp at idle to turn the alternator to provide the electricity the A/C electrical loads. Then the A/C compressor load also accounts for some of this 1.25hp.

The engine controller knows the A/C is on and will compensate to keep the idle at the desired level and to accomodate the fact the compressor is cycled on and off. (With autoclimate control the compressor is variable and this is adjusted to produce more or less cooling. Much smoother operation. Seamless in fact as near as I can tell.)

If you know how to hook up and use a volt/ohm meter you might be able to spot some voltage change of significance concurrent with the rpm drop and the rise.

Sincerely,

Macster.

kj217

I am afraid that the sound i hear with the symptom is the sound of a fan draining too much electricity due to an impairment.

Also, i don't see the rpm accomodates any. It is always plays between 600-700 in all cases.

Macster

That the rpm stays stable between 600 and 700 (IIRC the rpm variation is +/-40) is the engine controller accomodating load variations as accessories are turned on and off and as for instance A/C loads increase or decrease.

I again listened to the clip you posted and I can't hear any fans or any thing other than the background noise of the engine exhaust.

I would think that if an accessory load was causing the engine rpms to drop by the amount I see in the clip were you outside the car at various places (one place at a time -- no being in more than one place at a time!) you couldn't help but hear the device or whatever react at the same time the engine rpms dropped and then react again as the rpms recovered. Also, I note the rpms never jump up? Which kind of is strange... unless the load goes away slower than it comes on, if it is a load.

I'm leaning away from some exceptional load causing this. Apparently other loads are introduced then removed by your action and the action of various subsystems and the engine controller is able to accomodate them just fine.

Without seeing fuel trims, long and short term fuel trims, ignition advance, and and engine load (just in case it is some exceptional parastitic load) you and everyone else offering help is really shooting in the dark.

It could be the throttle body/E-Gas control module acting up but I'm not about to suggest replacing that. If you could borrow a throttle body from another car... Would to be one very accomodating Porsche owner though. And if you handed him back the throttle body/E-Gas control module and it didn't work right you would have to hand over a check for a new one right then and there. Then call him a cab to run him to nearest Porsche dealer to buy (order) a replacement.

Ouch!

Sometimes a salvage yard that sells parts salvaged from cars might let you try a salvaged part, but I don't know if there are any where you are and if they'd do this with a throttle body/E-Gas control module. You'd still be on the hook if somehow the unit was working ok before you tried it -- though how to know? -- and if not after -- again how is one going to know? -- to buy the now broken unit.

Sincerely,

Macster.

Pac996

The temperature is up there about where the fans should kick in. Any good dark place with a white wall where you could wait for the pressure drop and observe the headlight brightness? If it is an amps available drop due to a connection I'd think it would show up in the headlights having to run off the battery (with a bad connection the batterys full amps wont be available and the alternator wont put out enough amps in the lower rpm). A big amps drop would probably drop the rpms of the fuel pump. Problem sourse in that situation would be bad cells in battery, or bad connection to a ground or positive, or just the battery post connections need a good cleaning. Fan engines going bad can suck a lot of power. Could be the radiators just need cleaning if the fans are sucking against a blockage.

Macster

Well, I ran your symptom by a couple of Porsche techs this AM.

Neither had much to suggest. It is an odd symptom and not one either is familiar with. (And they between them have over 40 years experience on Porsches.)

One tech suggested it might be related to a going bad or bad battery or possibly a bad connection between the battery and car (and car to battery).

Along with this I can think of a marginal connection at any one of a number of connectors that are vital to the proper running of the engine.

Too many to list.

Without any error codes, without any ability to instrument the car and check its vital signs before/during/after the symptom appears...everyone's just shooting in the dark.

I note the CEL doesn't come on, doesn't flash. No misfires then. So it is not spark, not fuel supply (that is I think these can reasonably be ruled out).

Thus the only thing I can think of is it is an air supply problem: Throttle acting up. Or E-Gas pedal acting up.

Can't recommend replacing the throttle body with no more than just some WAG on my part to go by.

(Did you ever do the E-Gas recalibration thing I covered in an earlier post to you?)

Not sure this is doable, but with everything at where it needs to be and the symptom showing up shutting off engine and disconnecting (one at a time) the E-Gas pedal from the wiring harness or the throttle body controller from the engine harness and seeing if the symptom returns is about all I can think of.

And not sure the engine will even start and run with either one of these control systems disconnected. Or another worry is the engine starts then races out of control.

And even if it does start and run (whether the symptom present or absent) one or more error codes likely to be logged which could require using an OBD scan tool to clear them.

Sincerely,

Macster.