RPM issues at idle and in gear

[Psybion]

Alright, I've done a lot of searching but I can't seem to find any threads that are discussing my issue.

Stock 2g gst auto is having problems with RPM. I grabbed a video of it because I feel that word descriptions are often inaccurate or hard to understand.

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Let me explain what is happening in the video: the video starts with the car in drive. When I switch it to neutral, the car jerks (you can see the camera shake) and the RPM jumps to around 1400. I put it back in drive (car jerks again) and the RPMs drop down, but a little higher than they were before. Then back in neutral and the RPMs go nuts.

Some other things associated with this that are not in the video. BEFORE I drive it around for a while, the RPM is around 900 in park/neutral but plummets to 600 or less in reverse/drive. Also when I'm stopped with my foot on the brake: if I let of the brake and tap the gas, the RPM will jump (from giving it has) but then immediately drop so far that it will sometimes die. If it doesn't die, it will bounce back up to normal.
Also my idle screw is backed out A LOT, or the car will not start.

New alternator
New battery ( terminals kinda suck a little though)
New plugs and wires
New ignition coil packs
New CAS
New IAC (changed nothing)
New MAF

Thank you for your time.

 

[eSpyder407]

Sounds like something with throttle cable or TPS... check the throttle cable behind your intake manifold and see if it's loose/tight.

You can run some tests on your TPS, but it'll require you to remove the throttle body; here's the link: http://www.plymouthlaser.com/tps.htm

Outside of that the new MAF sensor issues maybe?

BTW... I'm jealous of your mileage and working oil pressure gauge, have yet to fix mine and I don't trust my crappy aftermarket one.

 

[Psybion]

Thank you so much for the suggestions.

I have cruise control in the car and it doesn't work, lights turn on when I set it and everything but it just doesn't hold the speed. I intend on removing it and replacing it with the non-cruise cable this weekend if I have the chance. While I'm at it I'll check up on the tps too. Is there anything else I should check on while I have all of that apart?

I'm super happy about the mileage, I actually managed to snatch up the car for about 3k when it only had 68,000 on it!

 

[pauleyman]

Looks like idle surge. Idle circuit isn't working, car idles up, exceeds 1500rpm but throttle is closed, ECU cuts fuel and rpms drop. Repeat.
Possible bad ISC. BISS out of adjustment or ECU ISC drivers bad. Given the age even if you find a bad isc pull the ecu anyway and have a look. Bad ISC can kill the drivers.
Also possible manifold leak. Throttle cable wouldn't cause this as the throttle would no longer be closed so rpms won't drop as the idle position switch isn't closed (or TPS isn't at lowest value).

 

[pauleyman]

Also when you changed ISC did you correctly set the BISS for idle control?

 

[Psybion]

Please forgive my ignorance, but when we say ISC and IAC we are talking about the same thing right? Just air vs speed in the name?
I'm gonna say no. My idle was previously very low and I backed out the biss a lot to keep the car from dying, when I changed the idle controller I didn't really touch the screw, and judging by the say you worded that I'm gonna assume that's NOT correctly set. I wasn't aware there was a correct way to set it, I thought it was just "keep it at a point where the car idles at 750 or so"

 

[pauleyman]

Please forgive my ignorance, but when we say ISC and IAC we are talking about the same thing right? Just air vs speed in the name?
I'm gonna say no. My idle was previously very low and I backed out the biss a lot to keep the car from dying, when I changed the idle controller I didn't really touch the screw, and judging by the say you worded that I'm gonna assume that's NOT correctly set. I wasn't aware there was a correct way to set it, I thought it was just "keep it at a point where the car idles at 750 or so"

Yes they are the same thing and no that is not correct.

 

[pauleyman]

www.dsmtuners.com/threads/top-10-most-popular-dsm-q-as.55410/

 

[Psybion]

I really appreciate the information Paul.
For some reason the idle surge link in that thread is broken for me, it takes me to the dsmtuners home page.
I'm gonna do some research and adjust the biss correctly.
Now that I have a name for what's probably happening I should be able to find more information on fixing it.

Thanks again for the info, I'll be sure to update this when I fix the problem for any future searchers.

 

[pauleyman]

Go to the faq page and you'll find it. Biss adjustnent

 

[Vegas Smith]

Never heard of it. All I know how to adjust is boost.

 

[1990TSIAWDTALON]

Don't you do that with the RIGHT FOOT???

 

[1990TSIAWDTALON]

Biss screw help:
https://www.ecmtuning.com/wiki/idlesurge
http://www.dsmtuners.com/threads/how-to-properly-adjust-biss-screw.396890/

 

[Vegas Smith]

Don't you do that with the RIGHT FOOT???

Haha, indeed I do.

 

[Psybion]

Alright, turns out that at least 95% of the issue was indeed the biss.
Not having access to a scan tool I decided to do what someone suggested in another thread, which is unplug the IAC, start the car, turn off accessories, adjust the biss til idle is correct, kill the engine, plug IAC back in, start car back up, let sit for 10 mins. For future readers, I can't promise this is correct or will always work.

After doing this the car idles at a perfect 800, even when the fans kick on. That being said, if I turn on the AC the idle drops to 600. Then if I put it in drive, it drops to 500. I would expect a drop, but should it be so extreme? If so, should I be setting the idle to be 750 while in drive with AC on, or while in park with AC off?

 

[1990TSIAWDTALON]

I would bump it up 50-100 rpm at idle and see if it likes it better. Not too much because you are an auto and its hard on the trans if your engagement rpms are TOO high. Bump it 50 and see how you engage. If it is harsh back it back off. If you feel it could take a bit more, give it to it. Not more than 100 over recommended idle tho. You don't want to be on "cruise control" all the time if it's too high. Speaking of which, I just aquired another 1g that has working cruise and I sure wish my car I built had it now. It's a nice luxury if you could get it to operate. Let us know how it goes!

 

[Psybion]

Now that I understand the system better I started experimenting to see what I could figure out, and I ended up getting it solved.

I performed the same procedure I noted in an above post, but this time around: instead of having the AC off when I started the car after plugging the IAC back in, I decided to have it on. The car now idles at a smooth 700 with AC on in drive, 800 with AC on in park and 900 with AC off in park. Since it's Florida and I pretty much never turn the AC off, this seems to be near perfect.

Now, I'm just hypothesizing here: I think if I had used the scan tool or link to properly hold the IAC at a middle point, this would not have happened. I think with the way I did it, the IAC was actually still hitting it's limit, causing it not to be able to raise the idle back up when. The AC bogged it down.

1990TSI: I don't even use cruise control in cars that have it. Traffic changes so much, so fast, where I drive that it just becomes annoying to constantly fiddle with it.