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1G 1990 Idle jumps around

punk3rz

Supporting Member
209
68
Jun 1, 2012
Surgoinsville, Tennessee
It’s more than likely a boost leak or bad ISC motor. Start with boost leak testing your car then testing/replacing your ISC motor. Usually when the ISC goes, it takes out the ISC driver on your ECU so you may need to pull the ECU and check for damage if problems persist.
 

1990TSIAWDTALON

Moderator
9,149
4,981
Nov 14, 2013
Independence, Kansas
Throttle cable have sufficient slack in it to not hold the throttle open???
 

SnowmanFD

Proven Member
130
7
Dec 11, 2021
North Seattle, Washington
pulling the ecu, doing a boost leak test, and checking the throttle cable are all things on my to do list for the car i think ill start with the throttle cable do a boost leak test and if that doesn't work ill check the idle speed control thing. also ill be sure to send a photo ina little bit
 

SnowmanFD

Proven Member
130
7
Dec 11, 2021
North Seattle, Washington
Ok so im trying to find my isc motor to test it and its not where it should be. So now im thinking my TB is from a different car.
can anyone identify the TB?
Where is the isc motor?

The photos of it not in the car are from this website https://www.tmzperformance.com/shop...4g63-and-6g72-engine-dsm-evo-1-9-3000gt-gvr4/
But it doesn't help much with identifying it

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Throttle cable have sufficient slack in it to not hold the throttle open???
Idk you tell me? 🤷‍♂️

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19Eclipse90

DSM Wiseman
5,147
984
Sep 29, 2003
OKC, Oklahoma
It's a 1990 turbo throttle body. The ISC is circled below. A quick way to check the throttle cable for too much slack is to unbolt it (the cable) from the intake manifold to see if that changes the idle.

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Also, a note regarding the vacuum lines: 1G - confirm 90 TB ports
 

SnowmanFD

Proven Member
130
7
Dec 11, 2021
North Seattle, Washington
Lol im new to this whole thing sorry

Also are the vacuum lines an issue it was like that when i bought the car

So on the 90 tb where would i find this?
http://web.archive.org/web/20100815101158/http://dmtalon.v8eaters.com/ISC.html

How do i remove this skrew to take the isc off?

Is this plug for the isc?

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19Eclipse90

DSM Wiseman
5,147
984
Sep 29, 2003
OKC, Oklahoma
The vacuum may be creating a leak path so, yes, it could be an issue.

I'm not sure what you want me to see from the link. It's talking about the ISC which I've circled above and you've pointed at the screws for removing already. I was able to get to the screws using a screwdriver bit and some collection of wrenches, simply because I didn't want to remove the fuel rail. Next time, I'll remove the fuel rail.

Yes, that you have circled is the plug for the ISC. The picture shows two connectors, one is a 6-pin and one is a 4-pin. The ISC is 6. The other is for the throttle position sensor and idle switch.
 

steve

DSM Wiseman
14,484
1,408
Feb 3, 2002
St. Charles, Illinois
Yes, that's the ISC connector. Overall your engine bay looks better than I was expecting.

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Here are a few other important parts of the Throttle Body.

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You missing the ground strap that runs from the point marked to the top left bolt that attaches the TB to the manifold.

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You also missing the Throttle Body Stay that helps support the intake manifold. Lastly you should cap the TB ports rather than loop them so you don't create a bypass around the TB butterfly. The two on the side will do that. Also no RTV use around the TB so it doesn't get into places it shouldn't.
 

SnowmanFD

Proven Member
130
7
Dec 11, 2021
North Seattle, Washington
Thanks a million this helps a ton.
I plan on testing it later today, if it is broken. what are my options for fixing it? i cant seem to find one for sale also Ive seen people just delete the whole isc would that be a good idea? What are the pros and cons?

Also here are some photos of the whole engine bay. Let me know if anything else looks wrong 😆 LOL

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pauleyman

DSM Wiseman
8,025
2,622
Nov 19, 2011
oklahoma city, Oklahoma
Thanks a million this helps a ton.
I plan on testing it later today, if it is broken. what are my options for fixing it? i cant seem to find one for sale also Ive seen people just delete the whole isc would that be a good idea? What are the pros and cons?

Also here are some photos of the whole engine bay. Let me know if anything else looks wrong 😆 LOL

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You have no cooling fans.
You must replace the ground on the throttle body. The throttle body does not receive a good ground because it's gasketed. The solution was the factory ran a ground direct from the top to one of the bolts that holds the throttle body to the intake. The intake manifold is itself grounded to the body. Without this ground the ECU does not know if the idle position switch is working because it can't go to ground as it doesn't have one. That is part of the idle control process and you're missing that step.
 

SnowmanFD

Proven Member
130
7
Dec 11, 2021
North Seattle, Washington
So i just tested the isc and got good readings so im thinking its just because its not grounded is there somewhere i can buy the part from or do i need to have one custom made?

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Also my fan is on the front for some reason

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pauleyman

DSM Wiseman
8,025
2,622
Nov 19, 2011
oklahoma city, Oklahoma
So i just tested the isc and got good readings so im thinking its just because its not grounded is there somewhere i can buy the part from or do i need to have one custom made?

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A simple wire will do. Factory just made it pretty.
 

SnowmanFD

Proven Member
130
7
Dec 11, 2021
North Seattle, Washington
So im going to ground the TB and plug the vacuum lines with RTV and silicone caps, if that doesn't work the next thing ill try is to test for a vacuum leak, clean the TB, ajust the biss (the cap is no longer there so i think its been messed with) and finally fix the exhaust leak. I took it to a shop for an inspection about a week ago and they said it had a big exhaust leak on the down pipe. but a friend of mine says it wouldn't effect it because the leak is after the o2 sensor
 
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pauleyman

DSM Wiseman
8,025
2,622
Nov 19, 2011
oklahoma city, Oklahoma
Thanks a million this helps a ton.
I plan on testing it later today, if it is broken. what are my options for fixing it? i cant seem to find one for sale also Ive seen people just delete the whole isc would that be a good idea? What are the pros and cons?

Also here are some photos of the whole engine bay. Let me know if anything else looks wrong 😆 LOL

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It looks like your brake and clutch reservoirs are empty.
The dipstick has been shoved into the tube.
 

91Eclipse GSX

Probationary Member
16
7
Jan 6, 2017
ionia, Michigan
Coolant temp sensor looks broken off in pic Can cause hard start high idle. Its by thermostat housing. Blue wire also unhooked by it not sure whats that too. The one the looks broken off should have a plastic connector that plugs into it. Id look the whole engine bay over fill the low fluids find out why low Safety first. look for any other broken sensors loose wires missing parts. Might be multiple issues causing it be I'd work on the cheaper and easier ones first.
 

907talon1G

Probationary Member
3
1
Sep 25, 2018
Chugiak, Alaska
You must replace the ground on the throttle body. The throttle body does not receive a good ground because it's gasketed. The solution was the factory ran a ground direct from the top to one of the bolts that holds the throttle body to the intake. The intake manifold is itself grounded to the body. Without this ground the ECU does not know if the idle position switch is working because it can't go to ground as it doesn't have one. That is part of the idle control process and you're missing that step.
As Pauleyman stated, the ground is key for proper IPS function. Fixing my IPS cured my erratic idle.

I would first create a ground strap out of wire and ring terminals, and then make sure your IPS is properly adjusted to your throttle stop plate. This requires a 14mm wrench to break the jamnut on the bottom of the IPS, and then screwing in the sensor until it bottoms out on the throttle stop plate, making sure you are not actually moving your throttle body shaft with the sensor. You should then recheck your TPS voltage and make sure you are still at 0.63v fully closed.

The top connection of the ISP also tends to varnish/corrode. Make sure your connections are clean at the wire terminal and at the sensor.

You can check that the ISP is making contact internally by using an ohm meter on the top of the ISC spade terminal with the wire unhooked, and the other lead of the ohm meter to your ground strap or any clean connection on the throttle body. With the ISP closed, you should see continuity (will beep if you have a sound setting). If you open the throttle plate, you should see 0 continuity. I would repeat this process a couple times and make sure you have continuity every time you open and close your throttle plate. If you don't, your ISP is not making good connection internally, you do not have the ISP tight enough to the throttle stopper plate, or an out of adjustment throttle cable isn't closing your throttle fully every time.

Also, if your coolant sensor isn't hooked up, you will need to fix that so your ecu can use the temperature (180-190 IIRC) to go into closed loop (used at idle)
 

steve

DSM Wiseman
14,484
1,408
Feb 3, 2002
St. Charles, Illinois
then make sure your IPS is properly adjusted to your throttle stop plate. This requires a 14mm wrench to break the jamnut on the bottom of the IPS, and then screwing in the sensor until it bottoms out on the throttle stop plate, making sure you are not actually moving your throttle body shaft with the sensor. You should then recheck your TPS voltage and make sure you are still at 0.63v fully closed.

Touching the IPS adjustment should be a last resort. The adjustment is critical to the throttle body operating correctly and the FSM strongly recommends you not touch it.

Adjusting the 1G TPS output voltage to 0.63v is something DSMLink started and may be the correct setting for DSMLink. The FSM setting is 0.5v +- 0.05v so anywhere between 0.45v to 0.55v would be acceptable. The absolute voltage isn't what the ECU is looking at but the changes from loop to loop in the software. The IPS opening and closing drives major functions like managing the idle speed, using the ISC to emulate a dashpot, decelerating fuel cut. On a 1G the TPS and IPS are decoupled so you have more control over the TPS voltage with the throttle closed but both 1G's and 2G's are looking for the IPS closure more than any TPS voltage.

You can check that the ISP is making contact internally by using an ohm meter on the top of the ISC spade terminal with the wire unhooked, and the other lead of the ohm meter to your ground strap or any clean connection on the throttle body. With the ISP closed, you should see continuity (will beep if you have a sound setting). If you open the throttle plate, you should see 0 continuity. I would repeat this process a couple times and make sure you have continuity every time you open and close your throttle plate. If you don't, your ISP is not making good connection internally, you do not have the ISP tight enough to the throttle stopper plate, or an out of adjustment throttle cable isn't closing your throttle fully every time.

Measuring the continuity to ground at the IPS terminal is a solid diagnostic suggestion but again I'll caution about touching the IPS adjustment until every other part of the car has been checked because it hard to get it exactly right and easy to create other idle problems.

Also, if your coolant sensor isn't hooked up, you will need to fix that so your ecu can use the temperature (180-190 IIRC) to go into closed loop (used at idle)

You can see that the thermovalve for the EGR is broken off but I couldn't see if the Engine Coolant Temp sensor was disconnected. You can see that new connectors had been crimped on so it should be inspected for the reasons you mention.

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steve

DSM Wiseman
14,484
1,408
Feb 3, 2002
St. Charles, Illinois
Not to distract from fixing the idle issue but you can also see that the exhaust manifold had the typical 1G crack in the collector area.

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That the rocker cover gasket is shot and oil is leaking.
The heat shields around the exhaust and turbo are missing.
There is a Gready boost controller that looks to be tied into the BOV line.
Plastic boost gauge hose tapped into the Manifold.
Looks like the Fuel Pressure Solenoid is gone.
The bad dipstick as mentioned.
Low fluid in the brake res as mentioned.
 

waltah

Proven Member
320
135
Mar 2, 2011
fairfield, Virginia
Touching the IPS adjustment should be a last resort. The adjustment is critical to the throttle body operating correctly and the FSM strongly recommends you not touch it.
I see that a little differently. Definitely don't just wade in and change it to see what happens. But if an ohmmeter measurement or scan software says that the idle position switch isn't closed or reliably closed when the throttle is closed that car will never idle right until the problem is fixed. It can't because this switch tells the ECU to take control of idle speed and as explained, 'idle speed' is a complicated job -- engine warmup, air temp, accessory loads, dashpot (slows the rate at which RPM falls when you come to a stop so the engine doesn't stall) -- all are taken into account and symptoms can be just about anything that can happen when the throttle is closed.

The reason for the FSM caution is that it deals almost entirely with situations that arise in cars still being serviced by dealers and thus relatively low miles. There aren't a lot of us who own such cars now: Wear beyond what a dealer mechanic would have had to worry about in 1998 is quite common and the throttle valve assembly's not an exception. If the switch doesn't close reliably then the first step is to adjust it. If you can't get an adjustment that works then the switch itself is defective -- it can happen -- and the assembly needs to be replaced.

I haven't had very good luck with the FSM procedure using a feeler gauge, I think because there's wear causing the switch to behave a bit irregularly. What has worked better is trial and error until it closed reliably under all situations and opened immediately when the throttle did.
 
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steve

DSM Wiseman
14,484
1,408
Feb 3, 2002
St. Charles, Illinois
I haven't had very good luck with the FSM procedure using a feeler gauge, I think because there's wear causing the switch to behave a bit irregularly. What has worked better is trial and error until it closed reliably under all situations and opened immediately when the throttle did.

You lost me, sounds like your now talking about the adjustment for a 2G IPS.
 
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SnowmanFD

Proven Member
130
7
Dec 11, 2021
North Seattle, Washington
Ok so i just grounded the wire and started the car it was warm but not up to operating temp the idle seemd to be good 1,500 then went down to about 750 i decided to go for a drive and it seemed fine i stoped at the store for 20 mins then i went back outside started it back up and it was quiter and the cel was on. I tested it in 3rd gear it on the way home and hit 18 psi when my limit is set to 11. I just went for another drive and it feels alot smoother, throttle is more responsive, it sounds smoother, just all around better.
Also not that i think its related but this has happend about 4 times now and happened before i even grounded it. my check engine light will go on for about 5 minutes then off
So my questions are
Why is it quiter
Why is my limiter not working
What the hell is going on LOL

Also a few other things i dont wanna be a dick but its really hard to tell whos responding to me and whos trying to xc92
Also my clutch and brake fluid are full its just hard to see from the photo, Manifold isnt cracked just got a scratch

Thanks again for all the help
 

XC92

Proven Member
1,396
287
Jul 22, 2020
Queens, New_York
Sorry about causing you confusion. Please everyone focus on the OP's issues, not mine. I just thought that mentioning mine might help.

Btw if you're still in N Seattle that's where I lived for 10 years and found a decent mechanic on N 128th St., Edwards Automotive Service. They don't specialize in DSMs but they do good general work and were able to help me out with some issues back then.
 
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