Probably a dumb question, but I could not find the answer in a search.

Has anyone tried increasing the idle speed the old fashioned way of simply adjusting the stop on the throttle valve designed for that?

It looks to me as though the electronic idle stabilization valve works against a completely closed throttle valve by bleeding in just enough air to maintain and stabilize the idle and it seems it would take the valve a bit of time to "catch up" allowing the engine to starve for air and die before the idle stabilization valve opens.

Why not open the throttle valve a crack so the default setting is not completely closed and have the throttle valve work with that - maybe the engine would not be so quick to stall?

Any thoughts or experience?

 

In the short term yes, but the 993 Motronic has an adaptive idle control that will soon return it to whatever the chip calls for

As you inferred this is tied into the starting procedure and is why you do not touch the gas when starting the car

 

Having the throttle open a crack is one trick tuners use to set Idle on ITB cars, as there is no other way of bypassing the throttles.

As Bill says, the ecu has its parameters and will adjust both timing and fuel to get the idle back down. THe ISV runs at a frequency of approx 150 hz or so (from memory). It can adjust very quickly to the needs of the engine.

 

If you start the car w the accelerator floored does it shut off the injectors?

 

No it confuses the DME as to the angle that the throttle plate is when it is off throttle, it will eventually rectify itself.

The problem w/ the twin winding ISV is that it will eventually develop a worn spot at the idle position. You can extend it's life by removing the epoxy seals on the 3 screws that hold the case and the winding's together then rotating them relative to each other, then restart and let idle w/o touching the throttle during the start, it would probably speed things up if the battery is disconnected first too, at least that's how I do it on my OBD1 cars, I dunno about the OBD2 versions

 

When/why/how is the car stalling? Are you running a LWF?

 

Plan is to run a GT3 RS light flywheel in my latest engine. It weighs around 3# less than the usual 964/993 RS light flywheel. So, I am looking for solutions should stalling problem arise.

I won't have power steering and will use a RS fan pulley, so parasitic drag should be less than usual 993 and thus I am hoping stalling will not be much of an issue. I will see.

 

I remember reading that somebody had readjusted the ISV mechanically to allow for more air flow as a new basic setting. I think it was a OBD I mod.
I have the LWF and OBDII and don't have that stalling issue even if I try. Been running this setup for 2 years now. So even 3 lbs less weight wouldn't matter I guess.
Ed

 

You'll be buying a new chip, yes?

The engine in my '76 is the non v-ram which was originally in my 993, l/w 915 flywheel, no air etc. It always had a low idle problem in the 993 but not since the t/p into the '76. The things that changed are, all new sealed vacuum lines and a Steve Wong chip.

 

I was one of the experimenters, it works until the DME reestablishes it's programmed idle and closed throttle positio. In other words it's just a matter of time till it goes back to where it wants to.

 

In the olden days with mechanical FI and carbs, the throttle butterfly "idle stop" was the way idle was adjusted. Same too as Trophy points out with ITB's and ECU management when no ISV.

I am just learning about this Bosch DME stuff. It appears to me that the ISV is nothing but a computer moderated throttle that bypasses air around a closed throttle butterfly and has a basic setting that is approximately where it should be with a steady idle at the ECU's programmed RPM. The ISV can adjust the air it passes above and below its basic setting to adjust for temperature and atmospheric pressure to create a steady idle RPM. Undoing the screws on the ISV and tweaking it merely shifts its basic adjustment and the ECU will figure this out and return the idle to program.

It seems to me that if one warmed the engine up and then opened the throttle valve (not ISV) by turning the idle screw on it, the idle would increase initially, then decrease to specification as the ECU commands the ISV to close and decrease the air it bypasses and slow the idle speed. Continuing to open the throttle will finally arrive at the point where the ISV is completely closed (no air bypass) and thus cannot slow the idle to specification - correct?

It seems to me then that opening the throttle valve to raise its idle air to a point just below where the ISV loses idle speed control would allow a touch of default air to always pass and perhaps help "catch" an engine that does not have enough flywheel inertia to maintain RPM wile the ECU figures things out. Even at the 150 HZ ECU ISV cycling speed that Trophy points out.

The idle would be normal speed, but with air both from ISV and throttle valve, not just ISV.

Am I nuts?

 

Has anyone tried this? It could be the magic solution....

 

W/ your cams you will want to bump the idle anyway, RS which doesn't have really lumpy cam profiles has it's idle bumped 160rpm from a normal 993.

Have you made a decision about the chip you will use?

 

You're lucky. Mine wants to stall with the stock dual mass FW. I can't even think about switching to a LWFW.

 

No decision on chip as yet. Maybe leave stock? Here in CA with 91 octane fuel, if anything the chip should be down rated to minimize knock sensor triggering.

I would like to change the idle speed and rev limit to RS specs though.

I always wonder about chips, until I understand exactly what is reprogrammed as far as fuel and timing curves or other features such as idle stabilization, I will be a bit skeptical about them.

The factory spends a lot of engine dyno time coming up with complex ECU mapping and I wonder how it can really be improved on by some guy in his garage - particularly with a closed loop system.