1G ISC On Its Way Out?

[aspekt9]

I think I'm having some issues with the ISC, I just want to confirm that it does indeed sound like an ISC issue. When coming to a stop, letting off the gas, braking, and then finally pushing the clutch in, the car drops down to 400 RPMs before slowly coming back up to 750 RPMs. It has also stalled once. When I first start up the car in neutral, there is no problem with idle at all, 750 constant. If I push the clutch in and move into a gear, again the idle stays constant at 750. If I rev the engine up high and let it fall back down, it goes right back to 750 constant, no lower. The problem only occurs when I'm coming to a stop. Here's an MMcd log so you can see what's happening when coming to a stop. I'm not sure what the ISC values mean, I'm hoping someone can make something of them. Thanks.

[B]Time:[/B]  [B]Timing:[/B] [B]ISC:[/B]      [B]TPS:[/B]     [B]RPM:[/B]
20:51	 10.00º	7	12.55%	1468.75rpm
20:51	 11.00º	1	10.59%	1437.50rpm
20:51	 6.00º	0	10.59%	1093.75rpm
20:51	 14.00º	0	10.59%	718.75rpm
20:51	 14.00º	9	10.59%	406.25rpm
20:52	 5.00º	34	10.59%	406.25rpm
20:52	 5.00º	12	10.59%	656.25rpm
20:52	 12.00º	0	10.59%	937.50rpm
20:52	 7.00º	0	10.59%	843.75rpm
20:52	 9.00º	1	10.59%	625.00rpm
20:52	 14.00º	1	10.59%	468.75rpm
20:52	 8.00º	11	10.59%	437.50rpm
20:52	 8.00º	11	10.59%	562.50rpm
20:52	 8.00º	11	10.59%	687.50rpm
20:52	 11.00º	13	10.59%	656.25rpm
20:52	 12.00º	13	10.59%	625.00rpm
20:52	 11.00º	17	10.59%	687.50rpm
20:52	 11.00º	17	10.59%	718.75rpm
20:52	 8.00º	17	10.59%	750.00rpm

 

[RedTurboEclipse]

I test ISCs by measuring resistance between the coils and giving it some juice to make sure it actually functions. If in spec Ive had no problems. Also consider the many other possibilities when it comes to idle issues..

 

[My1GLaser]

It looks like your TPS needs adjusted. It should be at 0% when off the throttle. If it isn't below 5% then the ECU doesn't know to idle the car. Or maybe the throttle cable is too tight and not letting the throttle come back and click the idle switch on the TB.

The ISC appears to be functioning, it jumped up to 34 when your idle went to 406. Ideally, it should be near 32 at idle. If you screw in your BISS 1/4-1/2 a turn, the ISC will open up a little closer to 32 instead of 11-17ish.

 

[aspekt9]

It looks like your TPS needs adjusted. It should be at 0% when off the throttle. If it isn't below 5% then the ECU doesn't know to idle the car. Or maybe the throttle cable is too tight and not letting the throttle come back and click the idle switch on the TB.

The ISC appears to be functioning, it jumped up to 34 when your idle went to 406. Ideally, it should be near 32 at idle. If you screw in your BISS 1/4-1/2 a turn, the ISC will open up a little closer to 32 instead of 11-17ish.

Wow, thank you for noticing I can't believe I didn't catch that... As far as the FAIV, it is NOT blocked off. I'll see if the throttle cable is snagged on something and go from there. If not I'll have to pull the TB out and adjust the TPS, it'll give me an excuse to clean it up and maybe purchase a block off plate. I'll also adjust the BISS screw like you said and see if that helps some, but it sounds like the TPS is what's causing this so maybe I should deal with that first before touching the BISS. Thanks for the help

 

[steve]

It looks like your TPS needs adjusted. It should be at 0% when off the throttle. If it isn't below 5% then the ECU doesn't know to idle the car.

1G logging with MMCd, the throttle closed should read about 10%. That's about 0.5v output and the spec is 0.48v to 0.52v

I'm going to assume you were thinking about a 2G and got confused.

I'd check the BISS, replace the o-ring, and reset it. I run about 30 steps at idle but the factory adjustment grounding the pin puts it at about 12 steps IIRC.

 

[My1GLaser]

1G logging with MMCd, the throttle closed should read about 10%. That's about 0.5v output and the spec is 0.48v to 0.52v

I'm going to assume you were thinking about a 2G and got confused.

No, I was going off my DSMLink readings. I would have never thought that 10% was fully closed. Glad to know that b/c I have to find a logger to tune a friends car and that would have driven me crazy.

 

[steve]

DSMLink code is different from a stock ECU and has it's own set of rules.
You can look at TPSraw and compare with what you have been logging.

Setting the TPS on a 1G much below 9% will cause a CEL.

 

[aspekt9]

So I took out the BISS and the bottom third of it was covered in carbon so I cleaned it up and replaced the oring.. started the car up went for a drive and the same thing happened, when coming to a stop the idle just falls to 500 before correcting itself back to 750. Should I start testing the resistance of the coils to rule out the ISC now?

 

[steve]

Sure, and here are a few more things to check.

The Idle Position Switch is located on the back of the Throttle Body and should have a green wire running to it.
The IPS connector should have 0v when the throttle is closed and at least 5v when the throttle is slightly opened. There is a ground strap, that runs from one of the bolts/studs attaching the TB to the Intake Manifold, to a screw on top of the TB. This is the ground for the TB and IPS.

What did you adjust your ISC steps to when you reinstalled the BISS?

 

[aspekt9]

I didn't adjust the ISC past what it was originally, do I do this by screwing in the BISS 1/4 - 1/2 a turn? In turn this would also set my idle lower, correct? Also, is it weird that this problem only occurs when the engine is coming out of load i.e. coming to a stop? What about the carbon build up as well, perhaps I should take the whole TB off and give it a good cleaning?

 

[need2boostpsi]

is your BOV recirculated, if not, or there is a boost leak or leak in the return air line of the BOV, it will cause the exact same thing to happen, the mixture becomes to rich to idle and it stalls when the throttle plate closes and air from the turbo is sent out the bov or boost leak.

 

[aspekt9]

is your BOV recirculated, if not, or there is a boost leak or leak in the return air line of the BOV, it will cause the exact same thing to happen, the mixture becomes to rich to idle and it stalls when the throttle plate closes and air from the turbo is sent out the bov or boost leak.

I've actually been unable to get the car to stall anymore, the lowest it hits is 500 now. I'm positive there is no boost leak. I'm running a MAF-T setup in blow through so the BOV is not recirculated. I'm thinking about removing the TB and giving it a good cleaning and then running some seafoam through the brake booster again and seeing if it solves anything. I'll also check the voltages and resistances on those two sensors (ISC & IPS) to see if anything looks odd.

 

[My1GLaser]

My Laser had idle issues until I blocked off the FIAV. I literally tried everything else before I blocked it off.

Also, the other thing that I am leaning towards is a malfunctioning idle switch. I had similar symptoms on my GSX and found that the switch was faulty.

 

[aspekt9]

My Laser had idle issues until I blocked off the FIAV. I literally tried everything else before I blocked it off.

Also, the other thing that I am leaning towards is a malfunctioning idle switch. I had similar symptoms on my GSX and found that the switch was faulty.

What do you do with the coolant lines after you block off the FIAV?

 

[NB4G63T]

What do you do with the coolant lines after you block off the FIAV?

Cap them,,, when the FIAV in bypassed there irrelevant.

 

[steve]

I didn't adjust the ISC past what it was originally, do I do this by screwing in the BISS 1/4 - 1/2 a turn? In turn this would also set my idle lower, correct?

Also, is it weird that this problem only occurs when the engine is coming out of load i.e. coming to a stop? What about the carbon build up as well, perhaps I should take the whole TB off and give it a good cleaning?

If the idle circuit is working it won't lower your idle, the ECU will step the ISC out more letting additional air past to maintain the idle speed. Once you force the ISC open all the way (if you can) then it would drop because the ISC is out of range.

If your drops then you know that the ECU isn't managing the speed and that would explain your problem.

I consider blocking off the ISC and FIAV butchery, my 91 still has both because I really did try everything to fix my idle issues in the past. I was successful, it doesn't take much additional air to cause surging. When the factory says don't touch the IPS adjustment because it's critical they really mean it.

 

[aspekt9]

If the idle circuit is working it won't lower your idle, the ECU will step the ISC out more letting additional air past to maintain the idle speed. Once you force the ISC open all the way (if you can) then it would drop because the ISC is out of range.

If your drops then you know that the ECU isn't managing the speed and that would explain your problem.

I consider blocking off the ISC and FIAV butchery, my 91 still has both because I really did try everything to fix my idle issues in the past. I was successful, it doesn't take much additional air to cause surging. When the factory says don't touch the IPS adjustment because it's critical they really mean it.

Alright so I screwed the BISS in about half a turn and it read 750 constant at idle with no load, then I took it out for a drive, when coming to a stop, it dives even lower then it did before, goes down to high 300s. Does this mean the ISC motor is functioning since it didn't lower my idle from 750? I guess now I'll move on to checking the other things.

Update: It just started doing this today: sometimes the rev arrow will bounce up and down when it gets at like 400 and will oscillate between 400 and 800 trying to correct itself, sometimes it takes several bounces before it recenters on 750.

 

[steve]

Good. What are the ISC steps at idle. MMCd will also log the status of the IPS as the I flag under FLG2. Not all versions do it right so that's what I tell people to measure it.

After a bad ISC and the ECU damage they can cause the next most common fault seems to be a missing throttle body ground from people discarding it.

 

[aspekt9]

Good. What are the ISC steps at idle. MMCd will also log the status of the IPS as the I flag under FLG2. Not all versions do it right so that's what I tell people to measure it.

After a bad ISC and the ECU damage they can cause the next most common fault seems to be a missing throttle body ground from people discarding it.

Ok, update: MMCD FLG2 for the IPS was not giving me any data, am I measuring the connector when opening and closing the throttle or the actual sensor pin? Also, I don't see any type of ground strap at all. Is it only on turbo cars? I removed the ISC and tested the coils. It's a black cover ISC and all the coils were 40ohms exact. I didn't have a 6volt battery, so I suppose it could be mechanically malfunctioning. The idle is getting worse and sometimes when I go to down shift from 3rd to 2nd around a corner the car stalls. For the price of a 6volt battery I can get a new ISC so I think I'll start there.