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It appears that one of the 993 common ailments is a lower than normal idle. As a result, the lower idle has a tendency to be a little bit rough. And, in some cases, the car will then stumble or stall.
Since the engine idle speed is controlled by the ISV which takes its signal from the ECU, the question posed is whether there is a way to "adjust" the ISV. There is the ability to break loose the three screws that hold the assembly in place within the barrel of the ISV. Once these screws can be loosened, you are able to rotate the mechanism within the ISV housing. This apparently results in an increase in idle speed. However, the adaptive nature of the ECU leads to the ISV being given a signal to lower the speed after several run cycles. Some say this may or may not happen.
I just recently heard of someone on the west coast who came up with a way to drill one or more holes either in the vanes or the inlet to the ISV (not clear to me where the holes are placed). This apparently creates a bypass around the valve that the ECU can't overcome. And, the ECU then has to richen up the mixture to compensate for the extra air. This then leads to an increase in idle speed. Or, so the story goes.
Has anyone had experience with this or other mods that can be done to a 993 that allows us to compensate for the too low speed programming of a 993's ECU?
Indeed, the ISV can be slightly modified to make it "adjustable" for idle speed by allowing one to rotate the housing to admit more or less bypass air.
Now,......anything that enrichens the fuel mixture will be counterproductive since the ECU (and its O2 sensors) will read that and change short-term fuel trims to compensate. Further, this will NOT affiect idle speed.
Has anyone had experience with this or other mods that can be done to a 993 that allows us to compensate for the too low speed programming of a 993's ECU?
I adjusted mine. It does not raise the idle so much as stops the stalling when snapping the throttle closed with LWFW.
Take the valve out and flush it good with carb cleaner and brake clean. Once the gunk is out clean the tamper proof glue (red IIRC) out of the adjustment screws. Mark the position with a scribe so you have a place to go back to if you get too far out of whack.
It appears that one of the 993 common ailments is a lower than normal idle. As a result, the lower idle has a tendency to be a little bit rough. And, in some cases, the car will then stumble or stall.
Since the engine idle speed is controlled by the ISV which takes its signal from the ECU, the question posed is whether there is a way to "adjust" the ISV. There is the ability to break loose the three screws that hold the assembly in place within the barrel of the ISV. Once these screws can be loosened, you are able to rotate the mechanism within the ISV housing. This apparently results in an increase in idle speed. However, the adaptive nature of the ECU leads to the ISV being given a signal to lower the speed after several run cycles. Some say this may or may not happen.
I just recently heard of someone on the west coast who came up with a way to drill one or more holes either in the vanes or the inlet to the ISV (not clear to me where the holes are placed). This apparently creates a bypass around the valve that the ECU can't overcome. And, the ECU then has to richen up the mixture to compensate for the extra air. This then leads to an increase in idle speed. Or, so the story goes.
Has anyone had experience with this or other mods that can be done to a 993 that allows us to compensate for the too low speed programming of a 993's ECU?
One of the factors that are touted on the Motronic 5.2 systems is the adaptive nature of the operating system
If it's not properly adapting there is an issue somewhere
Idle speed is controlled by the twin coil ISV and the DME tells it to push/pull to a value specified by the DME software
w/ manual trans w/o AC on 800+/-40 is the oe spec
w/ manual trans w/ AC on 880+/-40 is the oe spec
cold ~1100
There is also an adaptive throttle potentiometer
the 0 or idle position is determined in an adaptive process by the DME, IF the battery has been disconnected or the DME separated from the system the engine must be restarted w/ the throttle fully closed, The systems iis supposed to select the smallest opening angle of the throttle as the idle position
If the throttle is opened the engine idle settings are switched off by the DME as soon as an opening angle of 1° is reached
a system tester 9288 can be used to measure the throttle opening as a %, 0 at idle 100% at WOT
Looking at the main throttle butterfly in the throttle body I note that it closes 100% at idle allowing no air flow.
Thus the only airflow to operate the engine at idle must come via the ISV and if the ISV doesn't open quickly enough to "catch" the engine as the RPM drop to idle with a fully closed throttle body butterfly, the engine stalls.
How quickly the engine drops to idle is variable and a function of peripheral drag on the engine from power steering, air conditioner compressor and alternator loading.
If the ISV is sticky with deposits it makes sense that cleaning it may speed up operative function.
The other strategies of drilling bypass holes in the ISV or adjusting the ISV so there is bypassed air can be accomplished by adjusting the idle stop screw on the main butterfly of the throttle body (pictured below with a yellow dab of paint on it).
If just a bit of air were allowed at the main butterfly rather than 100% closure, then the ISV could perhaps better catch the engine more easily. The trick would be to allow enough air for say a 600 rpm idle and let the ISV add the rest of the air needed for the 800 RPM idle spec.
Ernie,
If in your shoes, I'd have to say an adjustment is worth a try~
If it does nothing, what have you lost?
P.S. I would not use brake cleaner on the ISV, but there are those who would...Carb cleaner contains the chemicals necessary to deal with the burned fuel soot buildup.
I've always had success dealing with old fuel in throttle bodies or pilot jets in carbs, with a quality carb cleaner. Brake cleaner has consistently fallen short in breaking down and rinsing old fuel residues.
Here are some other possible issues
DME
pin 40 gets a signal from the AC and raises the idle when it is present
pin 41 vacuum signal
pin 45 engine temp
pin 51 transmission signal(I have no idea what this one is all about) as pin 9 is the vehicle speed signal this is used when the caar comes to a stop and if the head temp is >114C and the throttle is in idle position, then the DME runs an idle adjustment
pin 54 varient codoing ground, this is used to select one of the three built in maps
pin 54 open is for RoW, Pin 54 fas 1 kΩ esistor for RoW M150(no cat), Pin 54 grounded for US
ISV twin coil controls
Pin 4 idle open
Pin 22 idle close
Not sure how these function vis a vis Pin 4 & 22
Pin 48 idle connection 1-
Pin 49 idle connection 2+
A "too low" idle speed is probably subjective and affected by other things going on with the engine. To me, a "too low" idle speed would be one where the engine feels on the verge of stalling as evidenced by some roughness. To put a number to it, I would say that idle speeds approaching 700 rpm are probably on the low side.
OK. Thanks for the input Steve W. I got a picture of the bypass that someone is using. I won't divulge his name since I have not gotten his permission to put this out there. But, he claims that he has seen other 3.6 motors brought in for a low idle (down around 700). He believes it is a communication issue between the MAF and the ECU. He "experiments" with the sizing of the slot he cuts into the ISV butterfly, starting small and then opening the slot up until he gets the idle he wants.
I did not question him on whether changing out the MAF would solve the issue. Speculation on my part, but he may feel that his bypass scheme is a more cost effective solution to a low speed induced stalling condition.
I did talk to the owner that experienced the stalling issue; and, he reports that the issue appears to have been resolved (he just did get the car back from his indie). I'll check in with him in a couple of weeks to gauge whether his enthusiasm is still there!
So, attached here is the modification done to the ISV and another picture of the rotary file he uses on a Dremel tool to make the modification.
Maybe Bills stated potential issues should be explored first.
I have never opened an ISV, but the potential of grind shards of butterfly valve. Any possibility of lodging in the valve and coming loose at an inopportune time?
Since the other method of adjustment is reversible, why not scribe a line to mark the origination point, make an adjustment and see where it gets you?
Is balance of the butterfly of any importance to proper operation?
Maybe Bills stated potential issues should be explored first.
I have never opened an ISV, but the potential of grind shards of butterfly valve. Any possibility of lodging in the valve and coming loose at an inopportune time?
Since the other method of adjustment is reversible, why not scribe a line to mark the origination point, make an adjustment and see where it gets you?
Is balance of the butterfly of any importance to proper operation?
I posted the modified ISV for discussion sake. Since it would require experimentation to find the "right" size of slot, that is a lot of work. The positive spin is that you would get very good at quickly removing and replacing the ISV!
Since the ISV is open on both ends, and you can flush the cavity, I doubt that the filings would be an issue.
But, at the least, the modification is clever and shows some good Yankee engineering.
My path forward right now is to switch out some of the old sensors that are less expensive and easy to get to. I'm in the process of replacing the flywheel speed sensor right now. There has been some writing done suggesting that with a LWF you should set the clearance to the tighter side of the spec. So, closer to 0.8 mm rather than 1.2 mm. I guess that the induced voltages change with gap setting; and, that that is the basis for the comments. So, I'll set the new sensor at close to 0.8 mm and see what happens.
That would leave the throttle position sensor, inlet temp sensor, and the MAF as the only other sensors that are original to the car. I've changed out the ISV, all (4) O2 sensors, and the cylinder head temp sensor.
One question. I noticed that Porsche no longer offers the knock sensors as a replacement part. So, what are folks doing for this sensor if they need to ever replace one?
Looking at the main throttle butterfly in the throttle body I note that it closes 100% at idle allowing no air flow.
Thus the only airflow to operate the engine at idle must come via the ISV and if the ISV doesn't open quickly enough to "catch" the engine as the RPM drop to idle with a fully closed throttle body butterfly, the engine stalls.
How quickly the engine drops to idle is variable and a function of peripheral drag on the engine from power steering, air conditioner compressor and alternator loading.
If the ISV is sticky with deposits it makes sense that cleaning it may speed up operative function.
The other strategies of drilling bypass holes in the ISV or adjusting the ISV so there is bypassed air can be accomplished by adjusting the idle stop screw on the main butterfly of the throttle body (pictured below with a yellow dab of paint on it).
If just a bit of air were allowed at the main butterfly rather than 100% closure, then the ISV could perhaps better catch the engine more easily. The trick would be to allow enough air for say a 600 rpm idle and let the ISV add the rest of the air needed for the 800 RPM idle spec.
Anyone tried this?
Cupcar,
Thanks for the response and especially for the picture! I will have to check, but I do not believe my 98 993 has the adjustable throttle stop screw that appears in your attached picture. As I am beginning to understand the engine management system, the throttle position sensor (TPS) would not operate properly if that idle adjustment were used. As I understand it, the ECU looks to the TPS for a signal indicating a fully closed throttle in order to select the idle maps for engine idle.
Perhaps someone with more knowledge of the engine management system could comment on the picture.
Also, two other responses to this thread have noted that they made the ISV adjustment which has affected their stalling (but not necessarily their idle speed). Perhaps these guys, as a step in modifying their ISV's, did an excellent job of cleaning the carbon and sludge from the valve.
I know, for a fact, that cleaning the ISV will allow the valve to operate properly correcting a stalling condition. But, in my case, cleaning of the old ISV did not eliminate the stalling. Nor, did the installation of a brand new ISV. So, clearly, something else is going on with my system.
01-19-2015, 07:50 PM
ISV "Adjustment"
