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Jaren000

Probationary Member
13
3
Aug 4, 2020
Rexburg, Idaho
This might be a little lengthy so I apologize in advance, but I want to give everything I know so far. I've been through about 20 idle threads by now but I just wanted to see what advice I might benefit from as well that wasn't already included in most threads.

For starters, almost everything on my 1990 GST is stock, and for better or for worse, most of it is original too, including a lot of hoses.

Starting on 1/16/21 as I was driving home late at night I noticed my 1990 GST started idling pretty high leveling out consistently at 2450rpm. When the car is first started it immediately starts slowly rising to 2450 as the engine gets warmer. If the engine is warm it will drop or rise to 2450 like it's normal.

I know it's still injecting fuel because the boost gauge (which isn't completely accurate, I know, but it is consistent) reads higher than it would when it used to idle normally. If I engine brake from above 2450 the gauge falls to full vacuum, but as it gets to about 2500RPM it starts injecting fuel again causing the vehicle to buck a little bit and for it to stop reading full vacuum.

The exhaust smells slightly rich at idle, but not by a lot. There's no check engine light for anything and I can verify that my light does work, it comes on when the key is turned before turning off like normal.

I do have a slight exhaust leak between the catalytic converter and the rest of the exhaust back, but it's been there for just over half a year so I don't think it's related.

Here's what I have and haven't done in trying to diagnose the issue.
As far as I can tell the throttle plate is closing all the way. There's slack in the throttle cable when it's closed and it's making contact with the idle switch.
I have checked the resistance on the ISC when the engine was cold. They all read between 25 and 26 ohms which is low, but given the relationship between electrical resistance and temperature (it was around 20F when I checked) it seems to be in spec.
I'm pretty sure I can verify the idle switch works. When I disconnect it the engine stops trying to fuel cut to keep RPMs at 2450 and instead will hold steady at 2450 with no fuel cut on a warm engine.
I've checked the idle with both the ISC and idle switch disconnected on a warm engine with no change in idle. I've also adjusted my BISS with both disconnected, but it had a minimal effect on idle, didn't seem to bring it much lower than 2200.
I have visually inspected all of the vacuum hoses around the throttle body and intake manifold. I've also run my hand along all of them in that vicinity with the engine running to see if I could feel any of them pulling in vacuum through a cut or something. I have also done my best to feel for leaks around the throttle body gaskets, but from what I can feel, there's no leaks.
I have disconnected the battery and drained the electricity from the vehicle (by trying to start it) in the hopes of resetting the ECU and maybe fixing the issue. I then left it for about 11 hours before reconnecting the battery.

I have not taken off the elbow to inspect the throttle body (Mostly because I really don't want to lug my toolbox down the stairs into my apartment parking lot and work in the cold)
I have not tested the oxygen sensor at all due to the lack of a CEL.
I have not removed any of the vacuum hoses as I'm a bit worried as to how they'll stand up to being removed.
I have not pulled and visually inspected the ECU at all.
I have not pulled the ISC to verify plunger operation.

I'm kind of at my wits end before I started checking each hose individually and removing the throttle body elbow, but I was wondering if there was anything else worth checking that I haven't noted or if there's some obvious issue I might be overlooking. From my understanding if there was air leaking in past the MAF the engine would run lean due to the presence of unmetered air, so I've been operating under the assumption it's some sort of electrical issue, but if anyone could shed some light on that as well I'd be glad to hear it.
 

Vegas Smith

Proven Member
4,266
2,106
Dec 2, 2002
Houston, Texas
Sorry, I'm not going to read all that. An idle that high is likely due to a mechanical problem such as an improperly setup throttle cable. You couldn't adjust a normal throttle that high with just the biss screw. Either that or it's a giant vacuum leak.
 

pauleyman

DSM Wiseman
6,396
1,647
Nov 19, 2011
oklahoma city, Oklahoma
Its also not possible to idle that high with a properly functioning idle position switch unless its not actually closed (throttle and or cable problem) or the ground isn't functioning. What behavior it would exhibit is anytime idle goes above 1500 ecu cuts fuel and it surges back and forth. Up down up down every few seconds. What changed leading up to this? Has the car been running fine? Any recent work?
 

Jaren000

Probationary Member
13
3
Aug 4, 2020
Rexburg, Idaho
Its also not possible to idle that high with a properly functioning idle position switch unless its not actually closed (throttle and or cable problem) or the ground isn't functioning. What behavior it would exhibit is anytime idle goes above 1500 ecu cuts fuel and it surges back and forth. Up down up down every few seconds. What changed leading up to this? Has the car been running fine? Any recent work?
I think the most recent work I've done is replacing one of the FIAV coolant hoses, probably around 4 months ago. Up until the 16th the car was perfectly fine, no idling problems or anything. I'll check to see if my idle switch is actually grounding or not.
 

Jaren000

Probationary Member
13
3
Aug 4, 2020
Rexburg, Idaho
I think the most recent work I've done is replacing one of the FIAV coolant hoses, probably around 4 months ago. Up until the 16th the car was perfectly fine, no idling problems or anything. I'll check to see if my idle switch is actually grounding or not.
So I just checked and after grounding the switch and giving 12v directly to the connector itself it's still acting the same as if it were just plugged into the switch itself. Still cutting fuel at above 2450rpm.
 

1990TSIAWDTALON

Moderator
7,366
3,475
Nov 14, 2013
Independence, Kansas
You shouldn't put 12v on that switch, it is meant only to show the ECU a ground at the idle position.
 

1990TSIAWDTALON

Moderator
7,366
3,475
Nov 14, 2013
Independence, Kansas
Do you have any experience with a multi-meter? There is a ground strap from a screw on the throttle body that sometimes gets left off or not put onto the stud of the throttle body bolt for it to ground. I would use a multi-meter and see if the switch is actually seeing a ground when it is depressed. Then you can check that issue off.
 

TK's9d2TSi

Supporting Member
4,642
1,900
Sep 11, 2017
Cincinnati, Ohio
Its also not possible to idle that high with a properly functioning idle position switch unless its not actually closed (throttle and or cable problem) or the ground isn't functioning. What behavior it would exhibit is anytime idle goes above 1500 ecu cuts fuel and it surges back and forth. Up down up down every few seconds. What changed leading up to this? Has the car been running fine? Any recent work?

So a non functioning idle switch will only surge to 1500? Good to know

+50 BLT
 

pauleyman

DSM Wiseman
6,396
1,647
Nov 19, 2011
oklahoma city, Oklahoma
No you're right, I phrased that way wrong. I meant to say I just connected the connector directly to ground.
That won't test the switch.
Test the continuity of the switch itself.
Then I would apply ground to the harness and rev the motor by hand. Car should not be able to go above 1500rpm.
Then check and make sure there is ground at the switch spade lug when throttle is closed. If there is not you're missing a ground somewhere. There is a ground wire at the back of the intake manifold to firewall. The switch itself gets its ground through the throttle body BUT ONLY if there is a ground present from one of the throttlle body bolts to the screw at the top of the throttle body. The throttle body itself can't be grounded by itself because its isolated by gaskets so to achieve this the factory put a tiny bracket there. It looks useless but it's purpose is a ground.
 

pauleyman

DSM Wiseman
6,396
1,647
Nov 19, 2011
oklahoma city, Oklahoma
So a non functioning idle switch will only surge to 1500? Good to know

+50 BLT
Yes. This is why cars idle surge. Car tries to rev up because bad isc fiav, biss, leaks etc but at 1500 rpm ecu cuts fuel. Idle drops. Ecu jumps back in and it cycles over and over.
 

Jaren000

Probationary Member
13
3
Aug 4, 2020
Rexburg, Idaho
Well, it's started snowing so that's it for me today. I will be looking at picking up a boost leak tester in the coming days so I can check that. I will also go back and make sure I test the idle switch properly next time as well as check that the intake manifold is properly grounded. I believe it's also worth mentioning I removed the 4 colored vacuum hoses attached to the throttle body and then covered each nipple with no change in idle, so I don't think those are leaking unless my understanding of that is wrong. Thanks for all your help so far guys, I really appreciate it. This vehicle was an important first for me so I'm just trying to keep it running well for now.
 

steve

DSM Wiseman
13,676
774
Feb 3, 2002
Boulder, Colorado
So a non functioning idle switch will only surge to 1500? Good to know

Yes. This is why cars idle surge. Car tries to rev up because bad isc fiav, biss, leaks etc but at 1500 rpm ecu cuts fuel. Idle drops. Ecu jumps back in and it cycles over and over.

I'm pretty sure that the ECU needs to think the throttle is closed for surging to happen. Normally the IPS is what grounds the ECU signal out to indicate that the throttle is closed. If the IPS is bad, disconnected, or not grounded via the TB ground the ECU allows the engine to idle uncontrolled.

I believe you need to have a functioning IPS for the car to surge but I guess a blown pull-up in the ECU would cause it to think the signal was grounded but I've never seen that happen.

To verify my premise (That you have it backward) just unplug the IPS and see if the surging stops.
 

pauleyman

DSM Wiseman
6,396
1,647
Nov 19, 2011
oklahoma city, Oklahoma
I'm pretty sure that the ECU needs to think the throttle is closed for surging to happen. Normally the IPS is what grounds the ECU signal out to indicate that the throttle is closed. If the IPS is bad, disconnected, or not grounded via the TB ground the ECU allows the engine to idle uncontrolled.

I believe you need to have a functioning IPS for the car to surge but I guess a blown pull-up in the ECU would cause it to think the signal was grounded but I've never seen that happen.

To verify my premise (That you have it backward) just unplug the IPS and see if the surging stops.
i misread that. A FUNCTIONING idle switch Will cause surge. Its when it does NOT function you can idle above 1500.
Thanks for the correction Steve.
 
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