Idle not dropping when I clutch

[JeTexas]

I ran the Actual Values on my commute home from work. Surprisingly, the car cooperated by hanging idle for the first 20 minutes and then returning to normal. Here's a sample from each section. When the throttle percentage is 0, I was clutching and letting the car coast.

When it hangs, I get triple the air mass kg/h along with generally lower Idle Air Temp Sensor and Engine Coolant Temp Sensor values (although in the complete Excel file, the hanging/regular temp values overlap some, so I don't know if that's a factor).

Anybody know what this means?

Are there other values I should monitor?

Edited January 13, 2009 by JeTexas

[JeTexas]

I logged more data yesterday evening, but I haven't had time to go over it yet. A few more pieces of info ...

When the idle is hanging and I'm at speed, but take my foot off the gas without clutching, instead of slowing the engine the car continues at speed like it's on cruise control, but with some surging. The surging is much more pronounced when the temperature is under 50 degrees F outside. I really hadn't noticed it when the weather was warm.

The problem starts and ends only when the car comes to a stop. (Well, basically a stop. It has to drop under 10 mph, which seems to reset the computer to the idle circuit and drops the rpms to 800.) If I'm on the freeway, and the car isn't doing it, it won't randomly start doing it unless I either come to a stop in heavy traffic or get off the freeway and come to a stop. Then if it does start, it will keep doing it until the next time I come to a stop. No variance in speed (aside from dropping below 10 mph) or amount of clutching will get it to start doing this or to stop doing this once it starts.

The only consistent factor is that everytime I come to a stop sign or stoplight, it's a crapshoot whether or not the engine will do this until I stop at the next stop sign or stoplight.

This problem started about a week after I had my spark plugs and oil changed back in July. I didn't think it was related at the time, but going back over my receipt I saw that after "Spark plugs" they mention "Slight tuning." I'm wondering if this isn't completely computer related due to Firestone's attempt at "Slight tuning" and if I should go have a Porsche specialty shop re-tune it.

Anyone know a good shop in the Houston area and how much a tuning would cost?

[whall]

JeTexas,

Ref. my Dec 5 post, your problem sounds even more like the problem I had with my MY77 280Z.

I thought there was a "flat spot" in the engine's acceleration at about 3000 RPM. So I took the car to Bob Sharp's Datsun dealership in Connecticut. Due to his success in racing, I figured his technicians could "slightly tune" my engine to perfection. A couple of teenagers with a screw driver - and no instrumentation - adjusted a couple potentiometers (as I watched in disbelief) and.....................

I lived with the problem for a while. Later on, I bought a Bentley-type service manual and a multimeter and reset the fuel injection system back to normal operation. I also replaced the front brake pads which had worn more quickly than normal.

I don't know much about the Porsche fuel injection system's capability to be tuned without changing the computer programing. But, maybe you could ask the fellow who worked on your car what he did so you can brief a proper Porsche technician on what to look for. That might keep the trouble shooting (and possible repair) costs down.

Bill

[JeTexas]

The chart above is from last night's drive home with the last time the rpms hung and then the first time the car started running normally. Notice the trigger to the change was my speed dropping to 5km/h.

Looking at the Actual Values logs from the past few days it looks like when the problem is there, the voltage on the 02 sensors ahead of the cats fails to drop to zero when I clutch. Once I hit less than 10 mph, the voltage drops to zero and the idle returns to 800 rpm or when the car is behaving correctly and I clutch at any speed with my foot off the gas, the O2 voltage will go to zero and the engine returns to 800 rpm.

Along with the O2 sensors dropping to zero, I've also noticed that when the car is behaving correctly the MAF will drop into the 1.3 - 1.5 range, but I'm guessing that reading is dependent on the air flow, not the cause.

So my question is, what sensor or switch creates the fuel cut off or new programming loop that leans the mix and makes the 02 sensors blip to zero when I clutch, so I can fix it?

Edited January 15, 2009 by JeTexas

[wvicary]

Just a shot in the dark.... but is it possible that the throttle plate or TPS switch base setting got altered? There is a small threaded screw on some throttle bodies that old school mechanics like to fiddle with to set idle speed. The screw is there as a factory setting so the throttle plate doesn't bind in the housing as they expand at different rates. Like I say, just a guess as I'm not sure if Porsche uses this style or not.

[Loren]

... or even a bad/intermittent TPS.

[JeTexas]

... or even a bad/intermittent TPS.

I changed the TPS sensor & IACV with absolutely no change in symptoms.

[JeTexas]

I've been poring through service manuals for days now, and I've finally come across this.

2452 • Idle Speed Air Control Valve

A compact twin-coil control valve (A) is used. It is directly

mounted onto the throttle housing without additional air ducting

hoses.

In conjunction with the DME control unit, the basic quantity of

air required for the corresponding operating condition of the

engine is supplied via the control valve bypass. Deviation

from a programmed basic quantity of air when the engine is

at idle speed is adaptively corrected by the DME control unit

under the following conditions:

• Idle speed

• Zero vehicle speed

• Engine temperature> 79,5° e

In conjunction with the DME control unit, the idle speed air

control valve also regulates the idle speeds required for the

different operating conditions of the engine.

Idle Speed

The idle speed for all transmission variants (manual transmission

and Tiptronic) is 700 rpm (warm engine/coolant temperature

> 60° e), with air-conditioning system switched on:

820 rpm.

However, if the engine oil temperature exceeds 143° e or

less than 11.03 V is applied to terminal 15 (power supply) of

the control unit, the idle speed is increased to 850 rpm by

the control unit.

Downward adjustment of the increased speed after

undervoltage is performed after the next transition from partial

load to idle speed if the supply voltage was higher than

11.03 V.

So upon further inspection of my pages upon pages of Durametric Excel charts, the problem, while intermittent, only occurs at temperatures below 80 degrees C. It also never happens when the car is at "zero speed." This tells me that something crazy is going on with either the DME or the IACV when it's running the Idle Speed program, and I've replaced the IACV and gotten the same problem. So is the DME throwing some wacky voltage? The cable cable between the DME and the IACV shorting out? Is the DME just not running the Idle loop half the time? I'm not quite sure how to test the voltage from the aforementioned terminal 15 since the DME runs the "zero speed" program, not the "idle" program when sitting in the driveway.

Another thought is could the resonance flap be sticking open?

This is driving me crazy.

[Foster]

I haven't fully digested that yet. :huh:

I'll look at it closer later.

But could it be as simple as the IACV wires are mixed up?

Maybe double check those.

[1999Porsche911]

Sure indicates the coolant temperature sensor to me.

[wvicary]

This is very rare but check the ripple output for the alternator diodes (and alt/battery general condition) and also for poor ground connections at engine to chassis and battery to chassis.

I thought I read before that you checked the coolant temp sensor. Just wonder in what way did you check it and also look closely for coolant migration from the sensor into the wiring harness. Another rare fault but it does happen.

[1999Porsche911]

I've been poring through service manuals for days now, and I've finally come across this.

2452 • Idle Speed Air Control Valve

A compact twin-coil control valve (A) is used. It is directly

mounted onto the throttle housing without additional air ducting

hoses.

In conjunction with the DME control unit, the basic quantity of

air required for the corresponding operating condition of the

engine is supplied via the control valve bypass. Deviation

from a programmed basic quantity of air when the engine is

at idle speed is adaptively corrected by the DME control unit

under the following conditions:

• Idle speed

• Zero vehicle speed

• Engine temperature> 79,5° e

In conjunction with the DME control unit, the idle speed air

control valve also regulates the idle speeds required for the

different operating conditions of the engine.

Idle Speed

The idle speed for all transmission variants (manual transmission

and Tiptronic) is 700 rpm (warm engine/coolant temperature

> 60° e), with air-conditioning system switched on:

820 rpm.

However, if the engine oil temperature exceeds 143° e or

less than 11.03 V is applied to terminal 15 (power supply) of

the control unit, the idle speed is increased to 850 rpm by

the control unit.

Downward adjustment of the increased speed after

undervoltage is performed after the next transition from partial

load to idle speed if the supply voltage was higher than

11.03 V.

So upon further inspection of my pages upon pages of Durametric Excel charts, the problem, while intermittent, only occurs at temperatures below 80 degrees C. It also never happens when the car is at "zero speed." This tells me that something crazy is going on with either the DME or the IACV when it's running the Idle Speed program, and I've replaced the IACV and gotten the same problem. So is the DME throwing some wacky voltage? The cable cable between the DME and the IACV shorting out? Is the DME just not running the Idle loop half the time? I'm not quite sure how to test the voltage from the aforementioned terminal 15 since the DME runs the "zero speed" program, not the "idle" program when sitting in the driveway.

Another thought is could the resonance flap be sticking open?

This is driving me crazy.

If you already spent money guessing it was the TPS and ICV by buying new parts, why won't you go spend another $30 for a temperature sensor? Seems to me that would be the cheapest and easiest thing to do to either correct the problem or eliminate it as the cause. This sensor, if bad, is a common cause of idle problems, not only on Porsches but on VW, BMW, Honda, Lexus, Land Rover and almost every car made in the last 20 years.

Part number: 996 606 405 01

Jim

[JeTexas]

I'll try to find a temp sensor today to swap that out -- just to be sure. It seems to be sending the correct temps to the Durametric though and the DME kicks into the correct program at 80 C.

[RFM]

It is possible with the PIWIS tester, to read out the received temp.value, coming from the sensor in the DME. Perhaps Durametric can do the same thing?

[1999Porsche911]

I'll try to find a temp sensor today to swap that out -- just to be sure. It seems to be sending the correct temps to the Durametric though and the DME kicks into the correct program at 80 C.

It is a very complex and involved job to find a fault in the temp sensor. Accurate gauge reading and scanner reading will not conclude it is working properly. Ressitance to the 5 volt voltage is fed to many controls, including injectors. Kinda like a bad battery. Easier and cheaper to spend the money for a new one than to remove and have it tested.

Edited January 16, 2009 by 1999Porsche911

[JeTexas]

Nothing's ever easy. I think we've got 5 Porsche dealerships in the greater Houston area, but the closest dealership with a temp sensor in stock is in Plano. So it's on order, but I'm not going to be able to test the theory this weekend. (Not keeping parts in stock is probably a great business model, but sucks for the customers and for your service techs.)

However, to make things worse, I was messing with the car again after visiting the dealership, and when I went to pull the plug on the coolant system the little tab on the bottom just fell off in my hand. Now I need a new plug for sure. The question is, can I buy just the plug or am I stuck ordering new wiring?

[1999Porsche911]

Nothing's ever easy. I think we've got 5 Porsche dealerships in the greater Houston area, but the closest dealership with a temp sensor in stock is in Plano. So it's on order, but I'm not going to be able to test the theory this weekend. (Not keeping parts in stock is probably a great business model, but sucks for the customers and for your service techs.)

However, to make things worse, I was messing with the car again after visiting the dealership, and when I went to pull the plug on the coolant system the little tab on the bottom just fell off in my hand. Now I need a new plug for sure. The question is, can I buy just the plug or am I stuck ordering new wiring?

I broke mine too, many years ago, (almost no plastic left) but if I remember correctly, the sensor is the female side of the connection. Anyway, just installed it, connect the broken connector, tape it if you need too, and you're good to go. The connection is very tight so it may be secure without tape.

[CarMan356]

Any chance the clutch could be slipping?

[Foster]

This problem started about a week after I had my spark plugs and oil changed back in July. I didn't think it was related at the time, but going back over my receipt I saw that after "Spark plugs" they mention "Slight tuning." I'm wondering if this isn't completely computer related due to Firestone's attempt at "Slight tuning" and if I should go have a Porsche specialty shop re-tune it.

Well there's your problem. :rolleyes:

I'm not sure how the IACV is set up because I don't have one.

Some cars use a 3 position soleniod. So there's your 3 idle speeds.

Sometimes one or all of the adjustemnts are on the throttle body.

So changing the valve won't change the adjustments.

If the mechanic "slightly tuned" (sheesh sounds like took the screw out and

threw it away) one to 2000rpm he wouldn't have known because didn't drive

it enough.

That's my last guess. If that ain't it, give it up and go to the dealership. :lol:

Edited January 16, 2009 by Foster

[JeTexas]

The guys at Porsche North Houston took pity on me. Apparently the service techs have these briefcases full of all the plugs on the wiring harnesses, so they can replace them if they accidentally break one. They hooked me up with both a new Idle Speed Sensor plug and a new Temperature Sensor plug along with some pigtail repair leads in case I can't get the wiring out of the old plug.

I still have to wait a week to see if changing the temp sensor makes any difference though.

[JeTexas]

Some cars use a 3 position soleniod. So there's your 3 idle speeds.

Sometimes one or all of the adjustemnts are on the throttle body.

So changing the valve won't change the adjustments.

If the mechanic "slightly tuned" (sheesh sounds like took the screw out and

threw it away) one to 2000rpm he wouldn't have known because didn't drive

it enough.

That's what I started thinking too -- that somehow the cold idle speed, which is supposed to be 850 rpm got adjusted to 2200 rpm. However, it's intermittent, so some days it does it, some days it works correctly. Also, I can't find any physical way to adjust the idle speed. I think it's all computerized. All I can figure is that some compenent is throwing wacky voltage to the DME and it's locking the Idle Speed Sensor wide open and not shutting off the fuel injection when I clutch instead of keeping everything where it should be.

[Foster]

Well there's one way to find out.

Unplug the IACV and see if the problem goes away. :drive:

[whall]

For anyone who needs to replace a plastic connector, have your Porsche technician (not parts guy) check his “Porsche Electrical Connector Repair Kit” for the replacement connectors. The entire repair kit (for sports cars) is known as CARTOOL – Nr 96 0 000 or PNA 721 043 600. It is full of connectors the parts guy would like to sell you the entire wiring harness for. There is a sparate repair kit - different CARTOOL/PNA number - for Cayennes as well.

Bill

[Foster]

Not to get off on a tangent, but if break a connector and can't find it anywhere you can sometimes find

something that will work if you search around on digikey or mouser or somewhere similar.

I looked for the connectors for the cluster in case I completely broke one but the part number is only available

over seas and in bulk. $50 min order was the best I could find.

I found something which would probably be compatible at digikey. Although you'd have use some tape or something to snug it up.

Molex Connector

There are links to crimp sockets if you break one off. Couple dollars for roll of a hundred.

Or if you're a real cheapskate notice the "request sample" link. ;)

Edited January 18, 2009 by Foster

[JeTexas]

Well, continuing to monitor and track the problem for the last few days has blown my theory that it only happens when the coolant temp is below 90 degrees C. Last night it continued to idle high long after the car warmed up even after several "zero speed" stops, which drop the idle to 800 and usually make it quit running that way.

One interesting thing I noticed is that after slowing down for a toll booth, the idle began dropping, and I thought the problem was over for the evening, but then as the idle was dropping, it felt like something kicked back on and the idle shot back up. It was like the cruise control kicked on or something suddenly started injecting more air/fuel into the engine. I don't know. It was strange.

Of course, it didn't do it at all this morning.

Still waiting on the temperature sensor.