Strange Fluctuating High Idle Problem, Eventually Works Itself Out
11-28-21, 09:46 PM
#1
Junior Member
Thread Starter
Strange Fluctuating High Idle Problem, Eventually Works Itself Out
Hello, I have a problem with my '86 GXL, the idle seems to bounce around the 1500RPM mark after the fast idle warms up, however after a minute or two, it stabilizes around 1250RPM and it's rock solid, once it stays there then it stays and doesn't move a hair.
I'm trying to get the idle to ~750 as per the FSM, however when I play with the idle air screw on the top of the intake manifold, it begins fluctuating again until I adjust it so that the idle stays still. (All idle modification attempts have been performed with the Initial Set Connector jumpered) On a side note, the adhesive on the Idle Fuel Mixture screw is gone and the VR has been clearly played around with, I re-adjusted it until the O2 mixture light was blinking as per the FSM
The fluctuating is very controlled and symmetrical, the RPM drops to ~1000 and then climbs back up to ~1500RPM until it repeats, it feels like the engine is compensating for something and adding more fuel to keep it around that ~1250RPM range as the Mixture light that is beside the two diagnostic connector pins goes to the On (Rich) as the fluctuating idle bounces up, and turns Off (Lean) as the idle drops back down. It feels like the engine is trying to get it to that ~1250 mark and once it's there, it doesn't move.
I have adjusted the TPS sensor so that it is within spec the car works well all around and pulls hard except for this idle problem, all help is appreciated and thank you for your time
I'm trying to get the idle to ~750 as per the FSM, however when I play with the idle air screw on the top of the intake manifold, it begins fluctuating again until I adjust it so that the idle stays still. (All idle modification attempts have been performed with the Initial Set Connector jumpered) On a side note, the adhesive on the Idle Fuel Mixture screw is gone and the VR has been clearly played around with, I re-adjusted it until the O2 mixture light was blinking as per the FSM
The fluctuating is very controlled and symmetrical, the RPM drops to ~1000 and then climbs back up to ~1500RPM until it repeats, it feels like the engine is compensating for something and adding more fuel to keep it around that ~1250RPM range as the Mixture light that is beside the two diagnostic connector pins goes to the On (Rich) as the fluctuating idle bounces up, and turns Off (Lean) as the idle drops back down. It feels like the engine is trying to get it to that ~1250 mark and once it's there, it doesn't move.
I have adjusted the TPS sensor so that it is within spec the car works well all around and pulls hard except for this idle problem, all help is appreciated and thank you for your time
11-29-21, 06:22 AM
#2
Hello, I have a problem with my '86 GXL, the idle seems to bounce around the 1500RPM mark after the fast idle warms up, however after a minute or two, it stabilizes around 1250RPM and it's rock solid, once it stays there then it stays and doesn't move a hair.
I'm trying to get the idle to ~750 as per the FSM, however when I play with the idle air screw on the top of the intake manifold, it begins fluctuating again until I adjust it so that the idle stays still. (All idle modification attempts have been performed with the Initial Set Connector jumpered) On a side note, the adhesive on the Idle Fuel Mixture screw is gone and the VR has been clearly played around with, I re-adjusted it until the O2 mixture light was blinking as per the FSM
The fluctuating is very controlled and symmetrical, the RPM drops to ~1000 and then climbs back up to ~1500RPM until it repeats, it feels like the engine is compensating for something and adding more fuel to keep it around that ~1250RPM range as the Mixture light that is beside the two diagnostic connector pins goes to the On (Rich) as the fluctuating idle bounces up, and turns Off (Lean) as the idle drops back down. It feels like the engine is trying to get it to that ~1250 mark and once it's there, it doesn't move.
I have adjusted the TPS sensor so that it is within spec the car works well all around and pulls hard except for this idle problem, all help is appreciated and thank you for your time
I'm trying to get the idle to ~750 as per the FSM, however when I play with the idle air screw on the top of the intake manifold, it begins fluctuating again until I adjust it so that the idle stays still. (All idle modification attempts have been performed with the Initial Set Connector jumpered) On a side note, the adhesive on the Idle Fuel Mixture screw is gone and the VR has been clearly played around with, I re-adjusted it until the O2 mixture light was blinking as per the FSM
The fluctuating is very controlled and symmetrical, the RPM drops to ~1000 and then climbs back up to ~1500RPM until it repeats, it feels like the engine is compensating for something and adding more fuel to keep it around that ~1250RPM range as the Mixture light that is beside the two diagnostic connector pins goes to the On (Rich) as the fluctuating idle bounces up, and turns Off (Lean) as the idle drops back down. It feels like the engine is trying to get it to that ~1250 mark and once it's there, it doesn't move.
I have adjusted the TPS sensor so that it is within spec the car works well all around and pulls hard except for this idle problem, all help is appreciated and thank you for your time
11-29-21, 07:45 AM
#3
Rotary Freak
This is the first I'm hearing of an idle o2 adjust light or test connector. IIRC the factory service manual procedure uses a tachometer and you simply adjust it leaner until the idle drops, then back rich slightly.
Is it possible you're talking about using a TPS test lamp? Could you take a picture of the parts you're working with?
Is it possible you're talking about using a TPS test lamp? Could you take a picture of the parts you're working with?
12-02-21, 05:38 PM
#4
Junior Member
Thread Starter
This is the first I'm hearing of an idle o2 adjust light or test connector. IIRC the factory service manual procedure uses a tachometer and you simply adjust it leaner until the idle drops, then back rich slightly.
Is it possible you're talking about using a TPS test lamp? Could you take a picture of the parts you're working with?
Is it possible you're talking about using a TPS test lamp? Could you take a picture of the parts you're working with?
I adjusted it leaner and leaner but the idle had never dropped a hair, I've gotten the idle to be stable now but no matter how much I turn the VR, it does not change anything.
I also pulled it off to test continuity and found that it was incredibly fine, one turn would adjust it maybe a hundredth of an ohm and sometimes the change would be visible only after a few turns. Is this normal for this VR? Thanks
12-02-21, 07:05 PM
#5
Rotary Freak
The two lamp light is used to adjust the TPS. It reads the relief solenoid and the something-else solenoid, since one is active at idle and one isn't. That's why one light is active at idle, then when you get on the throttle the other one activates. This way you can adjust the TPS so that only one light is active at idle and you know the ECU is happy with throttle position.
Meanwhile the ECU doesn't even use the O2 sensor except under certain conditions in fifth gear. I don't know what it is you're unplugging but neither the TPS nor O2 sensor should be unplugged. The variable resistor just allows for corrections to the injection amount (at idle only). The ECU doesn't "know" what the O2 sensor is reading or use it for anything at idle anyways.
We are talking about the two lamps connected to the green connector under the airbox, right? The only other lamps I'm aware of are when you're using the diagnostic port by the driver's side headlight, but I've never been able to get it to work properly anyways (unplugged my AFM and it still didn't throw a code) so I usually ignore that port.
I adjusted it leaner and leaner but the idle had never dropped a hair, I've gotten the idle to be stable now but no matter how much I turn the VR, it does not change anything.
I also pulled it off to test continuity and found that it was incredibly fine, one turn would adjust it maybe a hundredth of an ohm and sometimes the change would be visible only after a few turns. Is this normal for this VR? Thanks
I also pulled it off to test continuity and found that it was incredibly fine, one turn would adjust it maybe a hundredth of an ohm and sometimes the change would be visible only after a few turns. Is this normal for this VR? Thanks
Basically the correct sequence to set idle is as follows:
- Warm car up all the way
- Jumper the initial set connector (which you have already done)
- Get idle under 1000rpm using idle adjust screw on top of throttle body. Close to 750 is better but under 1000 is good enough
- Verify ignition timing (if equipped with a timing light). If you haven't messed with it, it's probably fine.
- Shut car off
- Set TPS using adjustment screw and lamp. Otherwise people like the voltage or resistance methods, but it seems like you have the lamps already. Set it so that at idle one light is on (but not both), then press the throttle linkage and let it return a few times to make sure it stays adjusted.
- Start car
- Set idle to 750rpm, or as close as possible
- Turn VR all the way towards rich side, then slowly turn back towards lean until you hear the idle start to drop. Then turn it back a bit. (this step is made easier with a tachometer).
- Set idle to 750rpm again
- Remove jumper.
If this isn't working then you have a vacuum leak, bad sensor, bad ground, or other issue.
12-02-21, 07:08 PM
#6
Junior Member
Thread Starter
Right, so that's what I'm talking about. As far as I know there is no O2 adjust light, or any other function to do so unless you were backprobing the ECU pin to read the voltage directly.
The two lamp light is used to adjust the TPS. It reads the relief solenoid and the something-else solenoid, since one is active at idle and one isn't. That's why one light is active at idle, then when you get on the throttle the other one activates. This way you can adjust the TPS so that only one light is active at idle and you know the ECU is happy with throttle position.
Meanwhile the ECU doesn't even use the O2 sensor except under certain conditions in fifth gear. I don't know what it is you're unplugging but neither the TPS nor O2 sensor should be unplugged. The variable resistor just allows for corrections to the injection amount (at idle only). The ECU doesn't "know" what the O2 sensor is reading or use it for anything at idle anyways.
We are talking about the two lamps connected to the green connector under the airbox, right? The only other lamps I'm aware of are when you're using the diagnostic port by the driver's side headlight, but I've never been able to get it to work properly anyways (unplugged my AFM and it still didn't throw a code) so I usually ignore that port.
No. I'm not sure the resistance readings of the VR (they're in the FSM) but my VR only has about 180 degrees of movement. It doesn't turn more than 1/2 turn in either direction. I'm guessing a previous owner may have broken yours, which could certainly cause idle issues.
Basically the correct sequence to set idle is as follows:
- Warm car up all the way
- Jumper the initial set connector (which you have already done)
- Get idle under 1000rpm using idle adjust screw on top of throttle body. Close to 750 is better but under 1000 is good enough
- Verify ignition timing (if equipped with a timing light). If you haven't messed with it, it's probably fine.
- Shut car off
- Set TPS using adjustment screw and lamp. Otherwise people like the voltage or resistance methods, but it seems like you have the lamps already. Set it so that at idle one light is on (but not both), then press the throttle linkage and let it return a few times to make sure it stays adjusted.
- Start car
- Set idle to 750rpm, or as close as possible
- Turn VR all the way towards rich side, then slowly turn back towards lean until you hear the idle start to drop. Then turn it back a bit. (this step is made easier with a tachometer).
- Set idle to 750rpm again
- Remove jumper.
If this isn't working then you have a vacuum leak, bad sensor, bad ground, or other issue.
The two lamp light is used to adjust the TPS. It reads the relief solenoid and the something-else solenoid, since one is active at idle and one isn't. That's why one light is active at idle, then when you get on the throttle the other one activates. This way you can adjust the TPS so that only one light is active at idle and you know the ECU is happy with throttle position.
Meanwhile the ECU doesn't even use the O2 sensor except under certain conditions in fifth gear. I don't know what it is you're unplugging but neither the TPS nor O2 sensor should be unplugged. The variable resistor just allows for corrections to the injection amount (at idle only). The ECU doesn't "know" what the O2 sensor is reading or use it for anything at idle anyways.
We are talking about the two lamps connected to the green connector under the airbox, right? The only other lamps I'm aware of are when you're using the diagnostic port by the driver's side headlight, but I've never been able to get it to work properly anyways (unplugged my AFM and it still didn't throw a code) so I usually ignore that port.
No. I'm not sure the resistance readings of the VR (they're in the FSM) but my VR only has about 180 degrees of movement. It doesn't turn more than 1/2 turn in either direction. I'm guessing a previous owner may have broken yours, which could certainly cause idle issues.
Basically the correct sequence to set idle is as follows:
- Warm car up all the way
- Jumper the initial set connector (which you have already done)
- Get idle under 1000rpm using idle adjust screw on top of throttle body. Close to 750 is better but under 1000 is good enough
- Verify ignition timing (if equipped with a timing light). If you haven't messed with it, it's probably fine.
- Shut car off
- Set TPS using adjustment screw and lamp. Otherwise people like the voltage or resistance methods, but it seems like you have the lamps already. Set it so that at idle one light is on (but not both), then press the throttle linkage and let it return a few times to make sure it stays adjusted.
- Start car
- Set idle to 750rpm, or as close as possible
- Turn VR all the way towards rich side, then slowly turn back towards lean until you hear the idle start to drop. Then turn it back a bit. (this step is made easier with a tachometer).
- Set idle to 750rpm again
- Remove jumper.
If this isn't working then you have a vacuum leak, bad sensor, bad ground, or other issue.
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12-19-21, 06:47 PM
#9
Junior Member
Thread Starter
Hello, I got the new replacement VR and plugged it in to retune it, and I noticed that it makes absolutely no difference on the idle or mixture when I adjust it. Is there anywhere that I could start with diagnosing a problem like that? I did test the replacement VR and it works to spec as per the FSM
Thanks
EDIT: I also wanted to add that the idle is rock solid at 1500 rpm no matter what I do
Thanks
EDIT: I also wanted to add that the idle is rock solid at 1500 rpm no matter what I do
Last edited by Nick7928; 12-19-21 at 07:25 PM.
12-20-21, 01:02 PM
#10
Rotary Freak
Earlier in this thread I explained the basics of setting the idle, so if you work through the list I wrote it should work. Unless there's a vacuum leak or something else wrong.
The reason the resistor isn't making any difference in your idle is because the ECU only uses that resistor when it detects idle.
Since you're at 1500rpm, the ECU assumes you're driving and uses a set map that is not adjustable.
You need to get the idle closer to 750rpm using the process I wrote earlier in this thread. Under about 1000rpm and with the TPS set appropriately, the ECU will consider itself "idling" and it will start to obey the VR adjustments.
EDIT: Also worth noting an over-tightened throttle cable can cause high idle. Not common but it does happen.
The reason the resistor isn't making any difference in your idle is because the ECU only uses that resistor when it detects idle.
Since you're at 1500rpm, the ECU assumes you're driving and uses a set map that is not adjustable.
You need to get the idle closer to 750rpm using the process I wrote earlier in this thread. Under about 1000rpm and with the TPS set appropriately, the ECU will consider itself "idling" and it will start to obey the VR adjustments.
EDIT: Also worth noting an over-tightened throttle cable can cause high idle. Not common but it does happen.
01-14-22, 09:47 PM
#12
Junior Member
Thread Starter
Hello, I've since made a little smoke machine and smoke tested the intake and here's what I found:
The previous owner got rid of the 5th and 6th ports, and the little hole right below the bottom stud where the actuator itself screwed in that supplied vacuum to the actuators was just left open, already there were two massive holes left opened, I made a little rubber piece to block it and also used some gasket maker to seal them up well.
Also, there was a vacuum leak at the right where the intake manifold connected to the block. The coolant from the top had been dripping right on it and I can only assume that it softened the gasket until it leaked through? I just used gasket maker compound to make a temporary seal just to track down any other vacuum leaks.
The strangest thing that made a difference was the OMP rod. The spring that retracted the OMP rod had gotten gummed up from oil and as a result, the OMP rod was not following the throttle as it moved up and down but it would stick at around the 40-50% throttle range and just stay there. Once I realized that and pushed it down, the engine went from floating at around 1200-1500 RPM to around 900 without any fluctuations, where I was able to adjust it with the idle screw on top of the manifold. Is this is normal? Moving the OMP rod up manually without the throttle pushing it made the engine behave super strange and begin acting like it did before. Regardless, the car acts much much better now at low RPMs and the brake booster actually does it's job now, thanks to all for the help!
The previous owner got rid of the 5th and 6th ports, and the little hole right below the bottom stud where the actuator itself screwed in that supplied vacuum to the actuators was just left open, already there were two massive holes left opened, I made a little rubber piece to block it and also used some gasket maker to seal them up well.
Also, there was a vacuum leak at the right where the intake manifold connected to the block. The coolant from the top had been dripping right on it and I can only assume that it softened the gasket until it leaked through? I just used gasket maker compound to make a temporary seal just to track down any other vacuum leaks.
The strangest thing that made a difference was the OMP rod. The spring that retracted the OMP rod had gotten gummed up from oil and as a result, the OMP rod was not following the throttle as it moved up and down but it would stick at around the 40-50% throttle range and just stay there. Once I realized that and pushed it down, the engine went from floating at around 1200-1500 RPM to around 900 without any fluctuations, where I was able to adjust it with the idle screw on top of the manifold. Is this is normal? Moving the OMP rod up manually without the throttle pushing it made the engine behave super strange and begin acting like it did before. Regardless, the car acts much much better now at low RPMs and the brake booster actually does it's job now, thanks to all for the help!
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