sefeing

Hi all - having a major problem with my 944S after getting it back from the mechanic. Of course it's the weekend, so I'm troubleshooting myself now.

Brought the car in to a majorly reputable shop here in the NY area to have a few things looked at that I couldn't figure myself. Car idled fine when I brought it in, with the occasional stumble/surge that always seemed to work itself out.
At 115k miles, I had a feeling the ICV should be cleaned / replaced at some point but never wanted to touch the intake manifold myself.

Shop removed, cleaned and replaced the ICV, stating it was totally frozen when they took it off the car. They freed it up, put everything back together and "adjusted the idle" so the car ran at 1100RPM at idle.
On my drive home I noticed a few things - the car was a little odd to drive, it never really seemed to actually be "throttle off", and had a few stumbles where it'd idle at 500 - VERY low.

Once home, I checked the throttle to see whether the TPS was being contacted at idle. It wasn't.
I can hear this audibly, meaning they adjusted the screw that forces the throttle open/closed. Not something you're supposed to do on the 16V cars, and would explain why the car was driving odd.

I tried adjusting the screw so that the TPS was contacted when idle, but when seemingly correctly adjusted (just BARELY making the switch, but hearing the click when the throttle is closed, and open just off idle throttle), the car won't idle right. It drops down to 500, and dies.

I'd LOVE to get the car on the road this weekend for Father's Day. It will be going back to the shop Monday, but it's a huge disappointment.

My guesses are either that there's a big vac leak somewhere after pulling the manifold, or the ICV has finally bit the dust.

This was a notable shop that other members use, I guess they're just not as knowledgeable about the 16V cars.

Any insight is appreciated.

dillon410021

It may be that they bumped or adjusted the tps and not the idle screw. Is that adjustable like the 951's? I would raise the idle and then adjust the tps so it clicks. Not sure if the 88 has where you can use the round plug on driver side and jumper isv like the 951's.

sefeing

Sorry, they adjusted the small screw that basically sets the throttle cam to its idle limit - they adjusted it so that it was never actually clicking the TPS closed (pressing the switch indicating to the DME that "hey, this idle, zero throttle"). I re-adjusted it so that I can audibly hear the click of the switch.

That small screw is not supposed to be adjusted as the S sets its own idle from the DME, and the idle is not supposed to be manually adjusted.

Adjusting that to where it was supposed to be causes the idle to drop to 500, and then stall.

sefeing

update: unplugging the ICV, the car runs great. plugging it in kills the car

when warmed up, it idles high (1100-1200) without the ICV connected.

dillon410021

I dont think having the isv on backwards would do any harm but you could look at that. idle should be 840

sefeing

hadn't thought about that - I'm definitely getting 12V from the connection, and can hear the ICV move when 12V is applied, that said whether it's moving enough I don't know .

dillon410021

and the connector looks good as in no way the wires got switched? The tps sets the idle circuit with the throttle closed switch but it could still be acting up and maybe telling the isv something wrong.

sefeing

I don't think so - I just tested the TPS circuit at the DME connector and it checked out fine resistance wise. I'd imagine the indication of the car dying once the IAC is plugged in would point towards the IAC? Is the IAC normally open, or normally closed?

walfreyydo

The ISV on the 16V cars is what regulates idle speed. I also did not think the 16V cars have an idle adjustment screw (my S2 doesnt) as its completely controlled by the ISV. The ISV is variable on these cars and opens/closes as a way to increase/decrease idle. More air = higher idle speed, more voltage = more open = more air.

Are you 100% sure you have an idle adjustment screw on the throttle body? This would be surprising if so (but maybe the S does?). Per Clarks:

Unfortunately there isnt a lot of documentation on this and clarks only covers the 8V cars, so would refer you to the FSM (downloadable in PDF format)
http://p914-6info.net/944%20Manuals.html

To me, it sounds like your ISV still has some issues, or you have a vacuum leak or wiring issue.

Note: The TPS should click as soon as the throttle plate opens. Clarks Garage has diagnostic steps for the TPS if you would like to test it. Its binary and not variable like the 951/968 TPS. Simply tells the DME when the throttle opens/closes (ie: when the car should be idling). The TPS has adjustment to set the "click" point by loosening the screws on the throttle body and turning the TPS. The click point of the TPS is not adjusted by changing the idle adjustment screw, assuming you have one.

sefeing

Idle adjustment was the wrong term - moreso a throttle stopper. Easy to force the engine to idle higher, but the screw just seems to really just adjust the slack on the throttle when at idle.

Yep, definitely aware about the DME controlling the idle - I didn't realize the ISV was variable though. I did test the TPS, and the resistance checked out totally good - so more likely it's the ISV than anything. They said when they pulled it out of the car, it was locked up - they freed it up, and re-installed it but more likely it's just shot. If I connect 12V power I can hear the valve move, but that doesn't mean it's moving correctly - and I get 12V from the connector.

Most likely need a new ISV or a vacuum leak. The shop's taking a look but I'm concerned about the owner and chief mechanic's insistence that the idle is controllable on these cars... He ONLY works on Porsches, but mostly 911's.

walfreyydo

Connect the ISV to a 12V (or 9v) source, and it should "snap" very quickly/instantaneously making an audible click when actuated. Variable voltage is what controls how far the ISV opens/closes and this is what controls the idle. If you turn on the AC or headlights, the voltage to the ISV is slightly increased, thereby slightly increasing the idle to compensate. Again, to me it sounds like the ISV may still be gunked up, some issues with wiring, or you may just need a new one. Check the bosch part number on your existing one, as you may find it much cheaper by searching the number as these likely came on other makes/models (porsche markup is real). Or like you mentioned, a vacuum leak (although I think you would have other symptoms if this were the case, like surging idle or poor running)

TPS should click as soon as the throttle plate is opened. This is another test besides just resistance. Make sure that the click happens immediately after the throttle begins to open. This is important as it tells the DME when the throttle is closed/car is idling. Position can be adjusted by loosening the TPS bolts and rotating TPS as needed.

I would not mess with the "stopper" on the throttle plate. Again, idle is 100% controlled by the ISV and changing that stopper is going to compound issues. Its probably there to "catch" the idle from getting too low.

sefeing

All agreed - TPS clicks as soon as the plate is rotated. I CAN hear the ISV move when I apply 12V to it, but yeah... Likely not moving correctly.

The "stopper" unfortunately was messed with by my mechanic first. He had adjusted it to force the throttle higher, off of the idle switch to compensate for the problem with the ISV. When the ISV is unplugged, and the stopper is correctly adjusted, letting the throttle click the idle switch, it runs, but with a high idle at the moment. Car is back with him and I made sure to mention that no, the idle should not be adjusted via that stopper. I've re-adjusted it so that it was as it was before - clicks the second the throttle is touched.

orig944

I've been doing a bit of research on this. It would appear that the ISV is open with no voltage across it. It closes when there is voltage across it. The red/blue wire is supplied with a constant 12V, and the red/black wire is pulsed to ground by the DME. It operates using pulse width modulation, with the higher percentage of time the pin is grounded, the more the valve is closed off. The coil is modulated at 100 Hz, and is pulling against the mechanical spring to close the valve.

Since it apparently shuts off completely when it is plugged in, I would hypothesize that the red/black wire is shorted to ground somewhere, the DME is confused (unlikely) or the DME transistor controlling this signal has blown and is now shorted. My next step would be to measure the red/black wire's resistance to ground, using the positive probe of your meter on the wire.

Edit: With the engine shut off.

sefeing

Will take a shot at this - prior to the car going into the shop, it idled "fine", with the ISV plugged in, albeit a little high at around 950-1000RPM, with the very occasional swing from 850-1000, which would clear itself up.

The "normally open" condition tracks with why the car is idling high with the ISV disconnected, around 1100-1200.
Possible the valve is still bad, and needs replacement if it's not functioning correctly, I'd assume when plugged in.
A few times I plugged the ISV in, the car bogged and ran for 30s or so around 500RPM, so I'm more inclined to wonder if it really is the valve versus the wiring - the issue didn't begin to occur until they removed the valve, cleaned it and put it back in


The car is back at the shop where the issue occurred, so can't quite measure it now, but curious what they'll find too.

What would I be looking for numbers wise? Just a short to ground?

orig944

It should be open when you measure it. There might be some stray current through the DME, but it would read greater than 10K ohms (he says, pulling a number our of thin air.)