will a bad ISC or ISC motor cause this?

[xlMcMlx DsM]

I have idle surge ion my 1990 TSi, but theres this, also. I try to hold the throttle open manually, to hold a steady rpm, and the revs go up and down a bunch, like regular idle surge. Can the ISC being bad cause this?

 

[1999silverGSX]

I hope somebody can correct me if i am wrong, but i believe a bad ISC can cause this. I know when my ISC went out on my car, it would idle at almost 1,500 and sometimes crusieing down the road at a steady speed the rpms would surge a little.

 

[xlMcMlx DsM]

ok, becuase when i give it a constant rev, it just surges up and down and i think thats why its really hard to drive, and why it jerks back and forth really bad.

 

[LordZed]

It's surging under WOT as well?? That may be a boost leak.

 

[xlMcMlx DsM]

thats what i was also thinking. At any constant rev. If you try to hold the throttle open at any position for a while, it trys to rev, but then just keeps trying. It takes a couple quick revs to acctually make it full rev up. And it wil do that while driving also. When trying to drive, you can put it to the floor, and it will also just try to rev, then after a few moments, actually rev. , i used "rev" alot.

 

[xdsmlaser93x]

cant hurt to run a boost leak test. but befor you do that, pull the plate off of the underside of the throttle body, theres i think 5 philips screws, and see if its all gunked up, and idle air control assembly out too. dont use brake or carb cleaner on it! it will melt the plastic on the whole assembly and it will be junk, advanced auto sells a sensor safe cleaner for that. if you cant get the phillips head screws out use vice grips thats what i did. but yeah like i said run the boost leak test after the throttle body has been off cleaned and put back on so you can ferify that the TB gasket is not leaking as well. just to be safe use a bit of RTV silicone for added leak protection, but remember to let it cure for about 10-15 mins befor blting it back on

 

[xlMcMlx DsM]

i actually just finished rebuilding the TB with new seals and what not. I pulled that plate of, and it looked fine in there to me. Nothing was gunked up at all. I tried doing a boost leak test, but i used a bike pump(because i have a 30 gal air compressor, and i didnt want to over do it
) and it just bled off before i could keep pressure. Ill try with the compressor later tonight. keep the help comin guys, its really appreciated.

 

[xdsmlaser93x]

Make sure you turn the pressure down on your compresser!

 

[TC Mueller]

What does the ISC look like because if looks like what I think it looks like Its sitting in my ash tray.

 

[TwoLiterLaser]

Have you recently replaced the ECU with a non-'90 ECU? A non-'90 ECU in a '90 will cause exactly what you are seeing.

 

[xlMcMlx DsM]

i know that the motor was pulled and rebuilt, but i dont think the ECU was changed. But ill look at it. I've heard of this as a cause also, but what should i be looking for? will a number tell me if its a 1990 or not?

 

[xlMcMlx DsM]

so i pulled the ECU, and opened it up and had a good look at all the connections. The ECU is an EPROM(it's got the "E" in front of the numbers on the case) the mother board looks like new. No corrosion or anything, no burns. But when i looked at the cennectors that plug into the ECU, i noticed that #14(the Idle speed one) was cut, but then resoldered. I wrapped it in electrical tape to be safe. And then on the smallest connector #110(ignition switch) was cut and then reconnected. So im thinking the previous owner had an AFC installed, possibly. Otherwise, the ECU looks fine. How could i tell the year of the car by the ECU?

 

[TwoLiterLaser]

Go here: http://www.vfaq.com/mods/ECUs.html

 

[xlMcMlx DsM]

Premium Line
6/90-6/92 - MD166260

That's the one it is. Is that particular ECU ok then for the car? it says its '90 through '92, so it should be ok right?

 

[steve]

Premium Line
6/90-6/92 - MD166260

That's the one it is. Is that particular ECU ok then for the car? it says its '90 through '92, so it should be ok right?

No, that ECU is used on 91 and 92 model year Turbo FWD cars. (production of 91 MY cars started 6/90)
On a '90 MY car the pins for 6 and 14 would need to be swapped to start with and modifications made to the car or ECU to fix the problems with the Tach.

Steve

 

[xlMcMlx DsM]

so is that a reason my tach doesnt work..... I looked on the drivers side door, and the production date said October of 1989. Wow. I guess i do have the wrong ECU. well, better start looking for one. Thanks guys, i never wouldve found this out w/o you.

 

[TwoLiterLaser]

Steve is, of course, correct. For that ECU to work correctly in your car you will have to read the FAQ for doing that swap. GO here: http://www.dsmchips.com/1990ecu.html

 

[TwoLiterLaser]

so is that a reason my tach doesnt work..... I looked on the drivers side door, and the production date said October of 1989. Wow. I guess i do have the wrong ECU. well, better start looking for one. Thanks guys, i never wouldve found this out w/o you.

Don't look for a new one, just do the swap procedure and it works just fine. You will save a lot of money just doing the swap. And your welcome...

 

[xlMcMlx DsM]

mmm. i just may have to order that. thanks for link TwoLiterLaser
but would that (Possibly) fix my idle surge or should i just get a new 1990 ecu to be safe? what would you guys reccommend?

 

[TwoLiterLaser]

You do not actually have true idle surge. What you have is an ECU that is looking for a certain type of tach signal from the coil(s) that it does not see. The '90 ECU has a different noise filter from other 1g ECU's. What this means is that the car does not know how many RPM's it is turning since the signal from the coil(s) is incorrect. If you do the swap procedure then your problems SHOULD go away. Putting a non-'90 ECU in a '90 causes crazy stuff like this. I have been in your shoes before, and I was skeptical like you. Just save your cash and do the procedure then get back with us. Good luck.

 

[steve]

You do not actually have true idle surge. What you have is an ECU that is looking for a certain type of tach signal from the coil(s) that it does not see. The '90 ECU has a different noise filter from other 1g ECU's. What this means is that the car does not know how many RPM's it is turning since the signal from the coil(s) is incorrect.

Close but not quite correct.

The surging is because the ECU doesn't see the idle position switch due to the signals on pins 6 and 14 being swapped between the 90 and 91+ cars. The fix for that is to release the two pins from the connector and move them. (assuming it hasn't already been done) There is a tech article in the library on how to release the pins.

The Tach issue is due to the difference in the signal between 90 and 91+. The 91+ ECU will corrupt the signal so that the tachometer doesn't recognize it The ECU actually get's it's RPM input from the CAS.

Steve

 

[xlMcMlx DsM]

Ok, im just going to swap #'s 6 and 14 tomorrow and see if that helps. Ill post with more info tomorrow.

 

[xlMcMlx DsM]

today i swapped the pins 6 and 14, which was a real simple task. I stareted her up and she idled great at startup, but it was still surging when i tried to give it gas. But the idle was smooth and perfect. But when it warmed up fully, it started to surge pretty good. So im pretty sure its the ISC, since ive heard of this is the problem only after the car warms up. Ill go do a boost leak test in a little bit and see what comes up.

 

[scottsee]

Close but not quite correct.

The surging is because the ECU doesn't see the idle position switch due to the signals on pins 6 and 14 being swapped between the 90 and 91+ cars. The fix for that is to release the two pins from the connector and move them. (assuming it hasn't already been done) There is a tech article in the library on how to release the pins.

The Tach issue is due to the difference in the signal between 90 and 91+. The 91+ ECU will corrupt the signal so that the tachometer doesn't recognize it The ECU actually get's it's RPM input from the CAS.

Steve

I actually learned somthing today.. Whoooot

 

[xlMcMlx DsM]

I just tested the ISC with a 12v lantern battery. Im pretty sure its bad because it only vibrated right when i touched the pins to the battery, it would just wiggle for a second, then stop, even with the battery connected. That means it bad, right?