1G 91 Gst weird idle

Turshin · Dec 16, 2018

Just got my 1g GST home from the shop after getting the head rebuilt and instainstalled. Turn the key, no start. Turn it again nothing. Turn it for a third time and it cuts on and shoots to 2k rpm. My first thought was vacume leak. The throttle modulates between 1500 and 2000 rpm. Gradually as the car warmed up and the rpm settled down. Fumbling and stumbling around 750 rpm. Did some research while sitting in the car about the symptoms my car was having. FIAV, ISC, and vacuum/boost leak kept coming up in my searches. Let the car for 20 mins and noticed the idle was stumbling around 600 rpm but didnt stall. Eventhough i know the motor mounts are good, the engine was still shaking a bit. I reved it up to 3k. It shook a bit and stumbled and acted like it wanted to stall out initially but when it got past 1500 rpm it smoothed out. So i took it for a drive. Same thing happened when i pulled out in 1st. Initial stumble and fumble and then smooth. Shifted to 2nd and felt the boost kick in pretty good at 3k rpm. As i slow down and come to a stop the idle still stumbles around 600 rpm. I get home and order up a new ISC. I noticed i had the brown one and swapped in a black one. Problem remained. I only have any type of idle surge during a cold start. So i ruled out the FIAV. Ran the smoke machine on it and no vacuum leaks were found. What could be causing my terrible idle?

Vegas Smith · Dec 16, 2018

Poorly adjusted BISS screw, isc o-ring leak (wont show up on leak test sometimes). Check idle position switch and cam timing.

Turshin · Dec 16, 2018

Vegas smith said:
Poorly adjusted BISS screw, isc o-ring leak (wont show up on leak test sometimes). Check idle position switch and cam timing.

The isc o-ring is new and installed correctly. I will check the biss when i get home today and upload a video link to help identify the problem. Thank you for your reply.

motomattx · Dec 16, 2018

Check cam timing first after knowing it was messed with.

Turshin · Dec 16, 2018

Here is the first video of the car after the cold start

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If the revs come down as the car warms up does that mean the ISC is functioning correctly? The engine doesnt surge when warm either. I also turned the biss screw to see if it wouldve affected the idle. I turned it both ways as nd i didnt notice any change. Thanks in advance for any help.

Turshin · Dec 16, 2018

Smh i posted the same video twice. Just got from working a 12 hour shift so im a little tired. This is the second video

Watch "December 16, 2018" on YouTube

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motomattx · Dec 17, 2018

Its normal on a cold start to go from 2000 rpm or so down to 750.

Turshin · Dec 18, 2018

Hello everyone. After doing more research ive found out that im missing something altogether. I dont have an idle switch at all. And of course when it rains it pours the pig is straight chopped of with just a bare wire. SMH after spending almost 4 grand the tuner shop i took it to could've at least adressed the issue. I dont know if its the issue thats causing my part throttle shake and stumbling idle but i would think it needs to be there more than not. Ill keep you guys posted.

1cleangsx · Dec 18, 2018

Have you personally verified base timing with a timing light? Obviously your mechanical timing is good, but I had similar symptoms after my engine rebuild and it was due to a bad leak at the brake booster and my base timing was at like 15btdc instead of 5, the car drove great, just had the erratic idle, after setting it straight it was like flipping a switch. Also in regards to your idle switch was it there before the shop had the car? I don’t think your car would stay running, I disconnected mine once and the car either wouldn’t start or stay running can’t remember. Do you have ecmlink? I adjusted my idleswitch in quarter turns until throttle position read 0 in ecmlink. It seems like there’s so many different things that can effect idle on a dsm, from tps, biss, isc and abc to xyz LOL

. Honestly tho I would boost leak test again, personally verify your base timing, don’t assume it’s right, and also assume your throttle body shaft seals are bad unless it was rebuilt in the last decade. And for what it’s worth that idle doesn’t look that bad in the video for a dsm I’ve seen a lot worse LOL

. But I salute your efforts and hope you figure it out man :thumb:

Turshin · Dec 18, 2018

Ok. I cant find an Idle position switch. So i want to kmow is there anyway to wire up the 2g 4pin TPS which has the internal IPS, so that the 1g ECU can read it?

Turshin · Dec 18, 2018

1cleangsx said:
Have you personally verified base timing with a timing light? Obviously your mechanical timing is good, but I had similar symptoms after my engine rebuild and it was due to a bad leak at the brake booster and my base timing was at like 15btdc instead of 5, the car drove great, just had the erratic idle, after setting it straight it was like flipping a switch. Also in regards to your idle switch was it there before the shop had the car? I don’t think your car would stay running, I disconnected mine once and the car either wouldn’t start or stay running can’t remember. Do you have ecmlink? I adjusted my idleswitch in quarter turns until throttle position read 0 in ecmlink. It seems like there’s so many different things that can effect idle on a dsm, from tps, biss, isc and abc to xyz . Honestly tho I would boost leak test again, personally verify your base timing, don’t assume it’s right, and also assume your throttle body shaft seals are bad unless it was rebuilt in the last decade. And for what it’s worth that idle doesn’t look that bad in the video for a dsm I’ve seen a lot worse . But I salute your efforts and hope you figure it out man

To be honest im so new to the dsm game that i wouldnt have known what i was looking at if it was there b4 or not. Im used to fords and neons LOL

. But when i got the car from the shop they told me the idle sensor wasnt there at all. I thought they were talking about the idle air control valve. I see now they were talking about the idle switch. I dont have ECM Link because the day i got the car was conveniently thd day they stopped selling it. The stumbling idle for whatever reason didnt show up very well on video. Ill take your advice and get verification on correct timing and any boost leaks.

Vegas Smith · Dec 19, 2018

Just buy a used throttle body and take the switch off of that.

Turshin · Dec 19, 2018

SMDH

Turshin · Dec 19, 2018

Vegas smith said:
Just buy a used throttle body and take the switch off of that.

In the process of searching now. But, like i asked earlier, if anyone knows how to wire up a 2g TPS so a 1g ECM can read the 2g idle switch, im all ears. Would rather fix it for free than spend money on another throttle body.

ThunderChild · Dec 19, 2018

I have a couple TBs that I'm pretty sure have switches, I could pull a switch and send it to you if you like?

Auto RS T · Dec 19, 2018

Turshin said:
In the process of searching now. But, like i asked earlier, if anyone knows how to wire up a 2g TPS so a 1g ECM can read the 2g idle switch, im all ears. Would rather fix it for free than spend money on another throttle body.

I spliced a 2g TPS plug pigtail with the 4 wires already in it from the factory into my 1g harness.
Another option is to add the wire to the 1g plug if you can find the correct style pin and wire, possibly from another TPS harness.
You would simply tie the open pin into the Green idle switch wire and use the 2g TPS.

007jimmy · Dec 20, 2018

Well you need the switch because it’s your throttle stop. Throttle plate is most likely closed too much causing even more problems at low RPM. If you plan to rewire the TPS you’ll need a bolt in place of the sensor at least.

brian9397gsx · Dec 20, 2018

007jimmy said:
Well you need the switch because it’s your throttle stop. Throttle plate is most likely closed too much causing even more problems at low RPM. If you plan to rewire the TPS you’ll need a bolt in place of the sensor at least.

Yes even if you wire in a different switch your still going to need a properly adjusted throttle stop. I went round trying to get my idle right and it was due to me just setting the idle switch to where it closes not putting the proper preload on it when closed. Then also verify that your throttle cable is adjusted correctly.

Turshin · Dec 26, 2018

tametalon92 said:
I have a couple TBs that I'm pretty sure have switches, I could pull a switch and send it to you if you like?

YES!!! i would most definitely like one of your switches. I apologize to everyone for the late reply. For some reason i haven't been getting notifications for this thread. I really want to drive my car b4 Michigan drops a load of snow on the ground.

ThunderChild · Dec 26, 2018

I'll do some looking, pm me a mailing address.

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