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.
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.