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Wayne at Pelican Parts's Avatar
Still stumped - 959 Fuel Injection problem...

Here's the original thread:

Frustrating 959 Problem -> Need suggestions...

I now have more info. I'm pretty convinced that this has nothing to do with the fuel, that the gas station thing was just a coincidence. The problem is that the computer is sending a signal to close the Idle Control Valve (ICV) when it shouldn't be.

I unplugged the ICV from it's electrical connector and the car starts and idles pretty good (a little erratic and high, but it definitely starts right up). Plug in the ICV and it chokes and dies. So, I took one ICV and hooked up the plumbing to it, but disconnected the electrical connection. Then I plugged in the electrical connection to another ICV so that I could watch it as the car was running. It slammed itself completely closed when I started the engine. It's not supposed to do this, obviously, it's supposed to let air in at idle.

So, it's thinking that the car is not at idle. I checked the throttle position switch, both at the switch itself, and at the harness connector to the main Motronic brain. The open / close switch operates correctly, and the POT that changes resistance as the throttle opens registers the proper values. I also checked the two temp sensors that the manual suggested checking. Tomorrow, I will probably hook up the osciliscope feature of my multimeter to the unit and test the engine speed sensor. That is a likely culprit too, I suppose, since the car may think that the throttle is closed, but the engine speed is higher than idle.

Still no codes output by the computer, which is confusing to me. The car is running okay, but this idle problem is causing problems with stalling and rough running when cold and when coming off of full throttle. Despite searching, I have not been able to locate the O2 sensor. I don't have the car up on the lift, so it's difficult to see under there, but I still can't seem to locate it. I did locate what seems like an area where it should be (complete with bung), but there's nothing but a plug there. That I find odd - perhaps the conversion wasn't 100% completed. I'll have to call down to GIAC tomorrow and ask them if they have any more details.

There are some other details, I will post about them later on...

-Wayne
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Wayne R. Dempsey, Founder, Pelican Parts Inc., and Author of:
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Old 11-27-2007, 06:31 PM
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Reasons for ECU to close your idle motor shut might be following:

- ECU believes engine is idling too high.
- Step-motor in idle valve is faulty.
- Idle valve is wired incorrectly.

If you let the engine run with disconnected valve and it runs with somewhat high idle RPM, ECU will try to lower that RPM by shutting the valve closed. That might be the reason that secondary valve connected to ECU (but not in the loop) is getting shut by ECU. I would recomend swaping the valve and checking if idle stabilizes.
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Old 11-28-2007, 12:51 AM
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Quote:
Originally Posted by beepbeep View Post
Reasons for ECU to close your idle motor shut might be following:

- ECU believes engine is idling too high.
- Step-motor in idle valve is faulty.
- Idle valve is wired incorrectly.

If you let the engine run with disconnected valve and it runs with somewhat high idle RPM, ECU will try to lower that RPM by shutting the valve closed. That might be the reason that secondary valve connected to ECU (but not in the loop) is getting shut by ECU. I would recomend swaping the valve and checking if idle stabilizes.
Swapped the valve out with four different ones - they all behave the same way. The engine thinks the engine speed is too high, which makes me think the rpm sensor is faulty. I have dug out my oscilliscope, and I will test it tomorrow. I have also ordered a new one, but I have no idea how to get it installed in this car. I guess I will find out tomorrow - I will probably have to put it on the lift.

-Wayne
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Old 11-28-2007, 01:49 AM
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Here's a photo of the idle control valve, hanging loose on the engine. The open hose is just a breather that connects to the air filter...



Gratuitous engine shot with the air cleaner off:

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Old 11-28-2007, 01:50 AM
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Here I am testing the throttle position switch:




This is somewhat disturbing - the tabs on the connector to the Motronic harness are broken off. Bad previous mechanics!

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Old 11-28-2007, 01:52 AM
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Wayne
Isn't the tachometer driven from the "brain"? This be the case, if there was a faulty sensor giving higher than actual RPM this would display a high tacho reading.

The following is assuming that this idle valve is a three wire two coil unit.
My understanding of the operation of the idle control valve is a "Push/Pull" PWM (Pulse Width Modulated) control operating the two internal coils at a fixed frequency (say 200Hz). Both coils are being driven with the time on for each coil changing (the PWM signal). That is say to hold the valve at a 40% open position the open coil is energised for about 40% duration and the close coil energised for about 60% of the duration (these percentages will change subject to the work load of the internal arnature). Hence if there is a problem with the circuit to the open coil the 60% duration of the close coil will drive the valve shut. - this aligns with your discription above of the valve slamming shut.
My knowlegde of idle valve control improved when getting the MoTeC ECU to drive the 3.2L idle valve, which after understanding both pieces of equipment now works a treat.
This PWM driver principal we also use on industrial actuators operating at 20kHz, and vary the time on/off for each direction.
Get that multimeter (or scope) and check the two output signals from the ECU at the valve plug. The fluke should have a % duration selection, use it. Don't bother with volts reading. I would be interested as to the operating frequency of the valve while you are there with the meter.

Paul

Last edited by CruiseControl; 11-28-2007 at 03:02 AM..
Old 11-28-2007, 02:40 AM
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I forgot to mention that the valve opens almost completely when the throttle is revved up. Dropping down, it closes. Weird, but I would think that the operation of the valve should be the opposite. I'm going to bring in my oscilliscope tomorrow and check the engine speed sensor. If the engine thinks that it's at idle, then it will close the valve.

As for the tach, I'm not sure where it gets that signal from - probably from the distributor...

-Wayne
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Wayne R. Dempsey, Founder, Pelican Parts Inc., and Author of:
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Old 11-28-2007, 04:13 AM
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Wayne,

Is there a microswitch that is activated by the accelerator linkage— this switch ordinarily (84-89 911 models) closes when 'off the accelerator', and if you are above some DME set engine speed (e.g. ~1200 rpm) the DME will modulate the injectors accordingly. If you’re below this 'measured' RPM, (i.e., switch is closed @idling) it will override and not shut-down the injectors...

In short, if the 'measured' RPM is incorrect that might give rise to what you described above...


Jascha

Last edited by Jascha; 11-30-2007 at 03:53 AM..
Old 11-28-2007, 04:24 AM
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Silly question, but does the ICV get ground from the body of the valve unit? If so, the part hanging there would be ungrounded. Probably a "duh" question on my part, but willing to throw it out there.

Good luck Wayne.
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Old 11-28-2007, 04:28 AM
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Wayne you should probably just sell me the car. I could trade you my 997 if I have to....
Old 11-28-2007, 04:28 AM
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I'm just glad you got a better multi-meter .
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Old 11-28-2007, 04:46 AM
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3.2L Carrera, 964, 993 all drive tach signal from the ECU. It would be surprising that 959 does not.

Is there head temperature sensor? Maybe it thinks the engine is warmed up? Not sure that would explain the ICV valve opening at speed.

Is there a chance the ICV is wired backwards?
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Old 11-28-2007, 06:55 AM
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HI Wayne
You prob have this info??
the engine should have a O2 sensor from the manual I have? No19





have you try'd this to set the idle?






hope it is of some help.

regards mike
Old 11-28-2007, 07:48 AM
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Quote:
Originally Posted by Jascha View Post
Wayne,

Is there a microswitch that is activated by the accelerator linkage— this switch ordinarily (84-89 911 models) closes when 'off the accelerator', and if you are above some DME set engine speed (e.g. 1600 rpm) the DME will modulate the injectors accordingly. If you’re below this 'measured' RPM, (i.e., switch is closed @idling) it will override and not shut-down the injectors...

In short, if the 'measured' RPM is incorrect that might give rise to what you described above...


Jascha
Yes, there is a switch, that has been checked and confirmed working via the Fluke multimeter at the DME connector...

-Wayne
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Wayne R. Dempsey, Founder, Pelican Parts Inc., and Author of:
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Old 11-28-2007, 11:42 AM
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Mike - I do have that info, but that does give me some interesting thoughts there, reading it again. I may have accidentally reset the idle speed by pressing S3 insted of S2 to enter the diagnostic codes. That procedure is wrong too, there's something incorrect about it - someone else sent me the proper one, I'll have to dig it up...

-Wayne
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Old 11-28-2007, 11:45 AM
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So according to that diagram the connector for your o2 sensor should be on the driver's side of the harness but you have no sensor? Do the cars run better with closed loop operation or are they better with the european open loop operation?
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Last edited by BlueSideUp; 11-28-2007 at 11:51 AM..
Old 11-28-2007, 11:48 AM
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Quote:
Originally Posted by 84_Carrera View Post
Silly question, but does the ICV get ground from the body of the valve unit? If so, the part hanging there would be ungrounded. Probably a "duh" question on my part, but willing to throw it out there.

Good luck Wayne.
That's a thought, but the unit is grounded through the harness...

-Wayne
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Wayne R. Dempsey, Founder, Pelican Parts Inc., and Author of:
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Old 11-28-2007, 12:11 PM
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Quote:
Originally Posted by BlueSideUp View Post
So according to that diagram the connector for your o2 sensor should be on the driver's side of the harness but you have no sensor? Do the cars run better with closed loop operation or are they better with the european open loop operation?
I haven't been able to locate it yet - I don't have the car close to the lift right now, we'll do that later on today...

-Wayne
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Old 11-28-2007, 12:12 PM
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Quote:
Originally Posted by DW SD View Post
3.2L Carrera, 964, 993 all drive tach signal from the ECU. It would be surprising that 959 does not.

Is there head temperature sensor? Maybe it thinks the engine is warmed up? Not sure that would explain the ICV valve opening at speed.

Is there a chance the ICV is wired backwards?
Three very good questions!

- I can check the wiring diagrams for the tach wiring, but I'm guessing that you may be right there, that the signal comes from the ECU. I'll check the wiring diagrams soon for that.

- I tested the temp sensors, they seem fine, but even if they were bad, then the car would run fine when it gets hot. So, I don't think it's the temp sensors

- The car was running perfectly fine previously, so I don't think the wires could have magically reversed themselves? Still, stranger things have happened in the past.

-Wayne
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Wayne R. Dempsey, Founder, Pelican Parts Inc., and Author of:
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Old 11-28-2007, 12:18 PM
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Quote:
Originally Posted by DW SD View Post
3.2L Carrera, 964, 993 all drive tach signal from the ECU. It would be surprising that 959 does not.

Is there head temperature sensor? Maybe it thinks the engine is warmed up? Not sure that would explain the ICV valve opening at speed.

Is there a chance the ICV is wired backwards?
Yes, I have confirmed that the tach does indeed get its signal from the ECU. So, yes, it looks like if the tach is working fine, then the inductive speed sensor on the engine should be working fine. Still, I will hook up the osciliscope today to the car and see if the curves look good. I will post that info / pics when I do that - it's actually pretty cool when you get the scope working right...

-Wayne
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Old 11-28-2007, 12:28 PM
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