I have a 1985 Fiero with an iron duke 2.5L and the Isuzu manual trans. I’ve been trying to deal with a high idle issue and was looking through my vacuum lines and got a little confused when I got to the vacuum canister lines.
I’ll attach an image bellow
I have an extra vacuum lines attached to the hard line of the vacuum canister. The vacuum diagram says that it should connect onto the TBI unit but I found it just pointed down at the ground. The is also already a vacuum like going into said vacuum port.
There should be a vacuum diagram on the engine info sticker but if not heres what I found
That hose may go to the cruise control
shem
According to the vacuum diagram the hose I found hanging should go to the throttle body but on mine there’s a hard line going from the vacuum manifold “tree” on the intake manifold to said throttle body port. I’ll attach an image below. Is this wrong? It’s been like this since I bought it.
With that mini-filter sticking out of your valve cover, what air filter canister are you using on your TB?
Hi Patrick!
So when I first got the car I decided to do a “hot air intake” and cut up the stock air cleaner and put in that mini air filter on the valve cover where the hole was that the stock air cleaner went into. I saw a bunch of people do it with no problems so I assumed it was ok.
So when I first got the car I decided to do a “hot air intake” and cut up the stock air cleaner and put in that mini air filter on the valve cover where the hole was that the stock air cleaner went into. I saw a bunch of people do it with no problems so I assumed it was ok.
Hi Steve, personally, I'd recommend against that. You're actually losing horsepower by doing this. The air cleaner / system on the Fiero is actually REALLY, REALLY good. When you re-install the factory air cleaner, you're effectively pulling cool air directly from the side air intake, which is much better than the hot air that you're pulling from the engine bay.
Another thing is... with the factory air cleaner hooked up to the crankcase ventilation, you're sucking back into the intake any "blow-by" ... but the way you have it now... the blow by isn't being sucked out of the crankcase, and therefore your exhaust will have smoke coming out during shifts and things like that. If you want to improve the air flow, the thing that people often do is break out the water separator on the air intake, and replace the factory air filter with a K&N air filter.
quote
Originally posted by Scuba_steve45:
I have a 1985 Fiero with an iron duke 2.5L and the Isuzu manual trans. I’ve been trying to deal with a high idle issue and was looking through my vacuum lines and got a little confused when I got to the vacuum canister lines.
My daughter has the exact same car (85 2m4 w/ 5-Speed Manual). I'll try to take some pictures tonight if you can hold out. I'll take pictures of the TBI unit with all hoses attached, the intake with the vacuum tree, and I'll also take pictures of the air canister (which is ONLY for Cruise Control, by the way). I'll respond to this thread with the images.
My daughter has the exact same car (85 2m4 w/ 5-Speed Manual). I'll try to take some pictures tonight if you can hold out. I'll take pictures of the TBI unit with all hoses attached, the intake with the vacuum tree, and I'll also take pictures of the air canister (which is ONLY for Cruise Control, by the way). I'll respond to this thread with the images.
Hi! Thank you so much for your reply you’ve been very helpful with my Fiero issues in the past (mostly idle stuff lol)
I understand the downsides of the hot air intake although I did not know smoke would come out the exhaust during shifts because of it. But I am still young and just out of highschool so I do dumb things haha.
I’m at work currently so no rush on the pics. Thanks again!
I saw a bunch of people do it with no problems so I assumed it was ok.
Where'd you see it being done? It certainly wasn't here.
I won't give you a bad time over it, but you'd be much better off to return the air filter system to its original condition. Nothing has been gained by doing what you did. If anything, it's been detrimental to engine performance. Buy hey, you're learning!
Where'd you see it being done? It certainly wasn't here.
I won't give you a bad time over it, but you'd be much better off to return the air filter system to its original condition. Nothing has been gained by doing what you did. If anything, it's been detrimental to engine performance. Buy hey, you're learning!
Haha I saw one or 2 forum posts on here about it… full of people saying it’s bad for performance. I just like the way it sounds and looks. I might eventually change it back to stock if I ever get a stock air cleaner lol.
I didn't get a chance to take pictures... but I will do it today. I'm cleaning the house since my wife has guests coming over later... but I'll be in the garage so I don't have to hang out with the girls... so I'll take pictures then.
I didn't get a chance to take pictures... but I will do it today. I'm cleaning the house since my wife has guests coming over later... but I'll be in the garage so I don't have to hang out with the girls... so I'll take pictures then.
Ok, sorry... it's been storming... but I ran out there and got some pictures. They aren't the best, but you'll have to look at a couple of them. I'll do a few at a time...
Fuel Evaporative Canister... which connects to the two rear body vacuum hard lines and the vent line on the Fuel Lines...
Ok, hard to see... but look at the vacuum diaphragm. There are two vacuum lines coming off the forward facing part of it. The TOP one connects to the vacuum valve (with the red connector).
The lower vacuum line from the diaphragm connects to the SMALLER port on the vacuum canister.
The LARGER port on the vacuum canister connects to the port on the hard line that's mounted to the back wall, right there next to the trunk switch.
Random-ass picture of my daughter's Iron Duke, since it looks nice...
MAP Sensor should be obvious since you can see it... left most port.
The port directly to the right of that connects to the EGR Valve, using the hard factory vacuum line.
The larger port just above and to the right of that is plugged off... we couldn't figure out where it goes, and nothing is missing.
The smaller port which is to the right (and below, and kind of out of sight) is the one that connects to the underside of the air cleaner that wraps around to the ThermAC temperature sensor, which then goes on to the ThermAC valve. If you are not using the ThermAC, then plug this.
And yet, another gratuitous picture of the 85 Iron Duke in all it's glory... she needs to strip the decklid vents and repaint them, because they look really bad.
Hopefully all that is correct. My daughter checked and double-checked the service manual and pictures on the internet. The engine runs really well, and all the "things" do what they're supposed to do according to the programming, etc.
MAP Sensor should be obvious since you can see it... left most port.
The port directly to the right of that connects to the EGR Valve, using the hard factory vacuum line.
The larger port just above and to the right of that is plugged off... we couldn't figure out where it goes, and nothing is missing.
The smaller port which is to the right (and below, and kind of out of sight) is the one that connects to the underside of the air cleaner that wraps around to the ThermAC temperature sensor, which then goes on to the ThermAC valve. If you are not using the ThermAC, then plug this.
And yet, another gratuitous picture of the 85 Iron Duke in all it's glory... she needs to strip the decklid vents and repaint them, because they look really bad.
Hopefully all that is correct. My daughter checked and double-checked the service manual and pictures on the internet. The engine runs really well, and all the "things" do what they're supposed to do according to the programming, etc.
Thank you so much for sending all of those pictures and for all of your help with this. I appreciate it very much. I hate to keep asking you questions and stuff but I have a few more lol.
For the vacuum canister lines; I have 3 vacuum lines going into hard lines coming off of mine. 1 to the fuel tank, 1 to the vacuum manifold fitting (I assume that’s the vacuum tree thing on the intake manifold) and the other to the one vacuum port on the TBI that you have capped off. On all vacuum diagrams I see it calls it a “purge hose”. Do I have this hose in the right spot?? What could be different?
Oh and I should add that when I got the car I had a hard lines going from the vacuum tree to said vacuum port on the TBI. I changed it to what the vacuum diagrams said.
Long story short I’m just very confused and want to make sure I have everything right.
[This message has been edited by Scuba_steve45 (edited 03-16-2025).]
Thank you so much for sending all of those pictures and for all of your help with this. I appreciate it very much. I hate to keep asking you questions and stuff but I have a few more lol.
For the vacuum canister lines; I have 3 vacuum lines going into hard lines coming off of mine. 1 to the fuel tank, 1 to the vacuum manifold fitting (I assume that’s the vacuum tree thing on the intake manifold) and the other to the one vacuum port on the TBI that you have capped off. On all vacuum diagrams I see it calls it a “purge hose”. Do I have this hose in the right spot?? What could be different?
Oh and I should add that when I got the car I had a hard lines going from the vacuum tree to said vacuum port on the TBI. I changed it to what the vacuum diagrams said.
Long story short I’m just very confused and want to make sure I have everything right.
Let me take a look... at first thought, it wouldn't make sense why the TBI should be connected to the vacuum tree on the intake manifold... since both the throttle body and the vacuum tree get vacuum from the same place. It would be like putting a straw in your nose and sticking the other side in your mouth.
As for having that hard line behind the TBI hooked up to the open port on the TBI... you could also do it that way since they both have vaccuum. The port on the TBI isn't controlled by anything, so it's basically just an open port into the intake as well. If it does consider it there, it's kind of a waste in my opinion... it would be a longer vacuum routing with no real benefit. That is what's frustrating though... there are no good examples of original cars that people have taken pictures of...
I'll try to take a look in the service manual... I don't think I've ever seen anything like that before, but of course I could be wrong.
Originally posted by 82-T/A [At Work]: Let me take a look... at first thought, it wouldn't make sense why the TBI should be connected to the vacuum tree on the intake manifold... since both the throttle body and the vacuum tree get vacuum from the same place. It would be like putting a straw in your nose and sticking the other side in your mouth.
As for having that hard line behind the TBI hooked up to the open port on the TBI... you could also do it that way since they both have vaccuum. The port on the TBI isn't controlled by anything, so it's basically just an open port into the intake as well. If it does consider it there, it's kind of a waste in my opinion... it would be a longer vacuum routing with no real benefit. That is what's frustrating though... there are no good examples of original cars that people have taken pictures of...
I'll try to take a look in the service manual... I don't think I've ever seen anything like that before, but of course I could be wrong.
I was pretty confused about the like going from the intake manifold tree to the TBI as well. It looked like it’s been like that for a long time as well. I changed it to how you had it in your pictures. Still a super high idle.
I was looking at other forum threads regarding high idles and I saw a few that mentioned the ecm might be commanding the IAC to give me a high idle. Do you think that’s what could be happening here? Should I just try to drive it over 35 for a bit and see what happens?
Originally posted by Scuba_steve45: I was pretty confused about the like going from the intake manifold tree to the TBI as well. It looked like it’s been like that for a long time as well. I changed it to how you had it in your pictures. Still a super high idle.
I was looking at other forum threads regarding high idles and I saw a few that mentioned the ecm might be commanding the IAC to give me a high idle. Do you think that’s what could be happening here? Should I just try to drive it over 35 for a bit and see what happens?
Generally, when I get a car that's totally jacked up or that's been messed with. I try to block off all the vacuum lines as much as possible. The high idle problem is usually a Fiero V6 problem (almost always the EGR tube).
Couple of things first I'd like to ask: - How are the brakes? Does it require a lot of effort to brake at pretty much any speed? - Does the idle drop down to normal after the car is warmed up? I think I asked this already, so I apologize.
My two things here are, first one... a high idle, particularly when applying the brakes, is usually as a result of a failed brake booster (diaphragm inside has failed). For the second one, the Iron Duke has a naturally high idle when cold. It's not uncommon for it to be 2,000 rpms or a little higher on initial start-up. It seems crazy to me, but this is programmed in to get it to warm up as quickly as possible to reduce wear initial wear for someone who's going to just get in and go. For me... I usually blip the throttle and the engine RPMs should drop a little bit.
Beyond that, the best thing to do is to block literally everything off to eliminate vacuum issues. I mean, everything.
1 - Get vacuum plugs for the entire vacuum tree. Remove the brake booster and the other two connectors and plug them completely. Obviously, you won't want to drive it like this, because you won't have brake assist, and it could be dangerous, but do this while the car is parked in your driveway or in the garage with the garage open.
2 - Disconnect and plug ALL of the vacuum ports on the throttle body (passenger side), EXCEPT... for the Manifold Air Pressure / MAP sensor. EVERYTHING else should be plugged.
At this point, you should be able to start the car... it may hesitate a little because it's missing the ThermAC (which acts as a choke, etc.), but there should be NO vacuum leaks at all.
NOW... if your idle is still high, here's the next thing it could be... - Failed or leaking EGR Valve, or EGR valve gasket, which means it's sucking in air through the gasket. - Failed or leaking TBI gasket (between it and the intake). - Almost totally impossible, but the intake manifold gasket... though if that happened, you'd likely have a coolant leak as well.
If the idle is no longer high, then you can begin re-installing components until the high idle returns. One of the things I discovered is that many of the old "elbows" that attach to the hard lines will have leaks either between the plastic hard lines, or they just don't seal well. It's also possible that the metal hard lines on the firewall, or even the vacuum canister (which is only for cruise control, by the way) may have rust and is allowing air to be sucked in.
So follow that process and you should be able to diagnose exactly where the problem is occuring.
Originally posted by 82-T/A [At Work]: Generally, when I get a car that's totally jacked up or that's been messed with. I try to block off all the vacuum lines as much as possible. The high idle problem is usually a Fiero V6 problem (almost always the EGR tube).
Couple of things first I'd like to ask: - How are the brakes? Does it require a lot of effort to brake at pretty much any speed? - Does the idle drop down to normal after the car is warmed up? I think I asked this already, so I apologize.
My two things here are, first one... a high idle, particularly when applying the brakes, is usually as a result of a failed brake booster (diaphragm inside has failed). For the second one, the Iron Duke has a naturally high idle when cold. It's not uncommon for it to be 2,000 rpms or a little higher on initial start-up. It seems crazy to me, but this is programmed in to get it to warm up as quickly as possible to reduce wear initial wear for someone who's going to just get in and go. For me... I usually blip the throttle and the engine RPMs should drop a little bit.
Beyond that, the best thing to do is to block literally everything off to eliminate vacuum issues. I mean, everything.
1 - Get vacuum plugs for the entire vacuum tree. Remove the brake booster and the other two connectors and plug them completely. Obviously, you won't want to drive it like this, because you won't have brake assist, and it could be dangerous, but do this while the car is parked in your driveway or in the garage with the garage open.
2 - Disconnect and plug ALL of the vacuum ports on the throttle body (passenger side), EXCEPT... for the Manifold Air Pressure / MAP sensor. EVERYTHING else should be plugged.
At this point, you should be able to start the car... it may hesitate a little because it's missing the ThermAC (which acts as a choke, etc.), but there should be NO vacuum leaks at all.
NOW... if your idle is still high, here's the next thing it could be... - Failed or leaking EGR Valve, or EGR valve gasket, which means it's sucking in air through the gasket. - Failed or leaking TBI gasket (between it and the intake). - Almost totally impossible, but the intake manifold gasket... though if that happened, you'd likely have a coolant leak as well.
If the idle is no longer high, then you can begin re-installing components until the high idle returns. One of the things I discovered is that many of the old "elbows" that attach to the hard lines will have leaks either between the plastic hard lines, or they just don't seal well. It's also possible that the metal hard lines on the firewall, or even the vacuum canister (which is only for cruise control, by the way) may have rust and is allowing air to be sucked in.
So follow that process and you should be able to diagnose exactly where the problem is occuring.
Let us know...
Ok so I plugged everything off and started it up and it idled at 2800. A little lower but still pretty high. I’m gonna try letting it warm up and going for a short drive over 35 to see if that does anything.
Originally posted by 82-T/A [At Work]: Generally, when I get a car that's totally jacked up or that's been messed with. I try to block off all the vacuum lines as much as possible. The high idle problem is usually a Fiero V6 problem (almost always the EGR tube).
Couple of things first I'd like to ask: - How are the brakes? Does it require a lot of effort to brake at pretty much any speed? - Does the idle drop down to normal after the car is warmed up? I think I asked this already, so I apologize.
My two things here are, first one... a high idle, particularly when applying the brakes, is usually as a result of a failed brake booster (diaphragm inside has failed). For the second one, the Iron Duke has a naturally high idle when cold. It's not uncommon for it to be 2,000 rpms or a little higher on initial start-up. It seems crazy to me, but this is programmed in to get it to warm up as quickly as possible to reduce wear initial wear for someone who's going to just get in and go. For me... I usually blip the throttle and the engine RPMs should drop a little bit.
Beyond that, the best thing to do is to block literally everything off to eliminate vacuum issues. I mean, everything.
1 - Get vacuum plugs for the entire vacuum tree. Remove the brake booster and the other two connectors and plug them completely. Obviously, you won't want to drive it like this, because you won't have brake assist, and it could be dangerous, but do this while the car is parked in your driveway or in the garage with the garage open.
2 - Disconnect and plug ALL of the vacuum ports on the throttle body (passenger side), EXCEPT... for the Manifold Air Pressure / MAP sensor. EVERYTHING else should be plugged.
At this point, you should be able to start the car... it may hesitate a little because it's missing the ThermAC (which acts as a choke, etc.), but there should be NO vacuum leaks at all.
NOW... if your idle is still high, here's the next thing it could be... - Failed or leaking EGR Valve, or EGR valve gasket, which means it's sucking in air through the gasket. - Failed or leaking TBI gasket (between it and the intake). - Almost totally impossible, but the intake manifold gasket... though if that happened, you'd likely have a coolant leak as well.
If the idle is no longer high, then you can begin re-installing components until the high idle returns. One of the things I discovered is that many of the old "elbows" that attach to the hard lines will have leaks either between the plastic hard lines, or they just don't seal well. It's also possible that the metal hard lines on the firewall, or even the vacuum canister (which is only for cruise control, by the way) may have rust and is allowing air to be sucked in.
So follow that process and you should be able to diagnose exactly where the problem is occuring.
Let us know...
Ok so I plugged everything off and started it up and it idled at 2800. A little lower but still pretty high. I’m gonna try letting it warm up and going for a short drive over 35 to see if that does anything.
Ok so I plugged everything off and started it up and it idled at 2800. A little lower but still pretty high. I’m gonna try letting it warm up and going for a short drive over 35 to see if that does anything.
Can you do a video recording with good audio? It's possible your tach is totally incorrect and it's not actually 2,800 rpms.
I'd just like to hear the engine idling if I can... and maybe also show the tachometer too.
Thanks!
EDIT: At 2,800 rpms... if it's really idling at that speed, you wouldn't even need to use the gas pedal, you could literally just casually let up on the clutch pedal and the car would accelerate itself. Does it let you do that?
[This message has been edited by 82-T/A [At Work] (edited 03-17-2025).]
Can you do a video recording with good audio? It's possible your tach is totally incorrect and it's not actually 2,800 rpms.
I'd just like to hear the engine idling if I can... and maybe also show the tachometer too.
Thanks!
EDIT: At 2,800 rpms... if it's really idling at that speed, you wouldn't even need to use the gas pedal, you could literally just casually let up on the clutch pedal and the car would accelerate itself. Does it let you do that?
I’m not home anymore but I will get that video to you probably tomorrow or the following day.
I believe my tach is correct as the engine is literally screaming (I took the cat off so it’s a little louder now) but you never know.
Also I took the IAC completely out and had my buddy start the car while I used my finger to control the amount of air that goes into the TBI. When I put my finger completely over the hole it stalls out almost immediately.
I’m not home anymore but I will get that video to you probably tomorrow or the following day.
I believe my tach is correct as the engine is literally screaming (I took the cat off so it’s a little louder now) but you never know.
Also I took the IAC completely out and had my buddy start the car while I used my finger to control the amount of air that goes into the TBI. When I put my finger completely over the hole it stalls out almost immediately.
Have you tried a different idle air control valve? It could just be that the IAC is either totally wrong for the engine, or seriously messed up.
Originally posted by 82-T/A [At Work]: Have you tried a different idle air control valve? It could just be that the IAC is either totally wrong for the engine, or seriously messed up.
Yes, I’ve used 3 different IAC valves and followed the instructions on how to relearn idle and nothing has worked. I made sure to clean the IAC passage as well. Could this be a bad ecm?
Yes, I’ve used 3 different IAC valves and followed the instructions on how to relearn idle and nothing has worked. I made sure to clean the IAC passage as well. Could this be a bad ecm?
I'm at a loss.
Do you have a Check Engine Light? Have you checked the codes yet? Let me know... I apologize if that was asked first already.
Do you have a Check Engine Light? Have you checked the codes yet? Let me know... I apologize if that was asked first already.
I checked after I drove it for a few minutes. No codes and driving it had no effect on the idle. I feel like the ecm has to be commanding that IAC valve open for some reason. Either a sensor is telling it to or something else is going on.
I checked after I drove it for a few minutes. No codes and driving it had no effect on the idle. I feel like the ecm has to be commanding that IAC valve open for some reason. Either a sensor is telling it to or something else is going on.
have you replaced the gasket for the IAC? i was chasing a high idle on a customers 2.8 and found that they had accidently double gasketed it
have you replaced the gasket for the IAC? i was chasing a high idle on a customers 2.8 and found that they had accidently double gasketed it
lol the original IAC that was on it was double gasketed… it was on there so long like that I had to scrape the gasket off the TBI. Im 1000% sure there’s only one gasket on it now and it doesn’t leak.
lol the original IAC that was on it was double gasketed… it was on there so long like that I had to scrape the gasket off the TBI. Im 1000% sure there’s only one gasket on it now and it doesn’t leak.
there has to be something very obvious we are all missing... could you unplug your coolant temp sensor for the ecu or look at the throttle plate position?
Information from an old post Here on re-setting the IAC on a 2.8 engine. It probably works the same for a duke.
quote
From my factory service manual for my 87 V6: If idle is too high, stop engine. Ignition on. Ground diagnostic terminal. Wait a few seconds for the IAC to seat then disconnect the IAC. Start engine. If idle speed is above 800+/- 50 rpms, locate and correct vacuum leak.
Before doing the whole procedure, maybe have someone with their ear close to the IAC during the re-setting procedure (don't actually start the engine) to see if they can hear the IAC moving.
I had a Hyundai Elantra years ago with IAC issues (surging), and I did a similar IAC re-setting procedure with it and I just left the IAC disconnected. It never gave me any further trouble, and I had no check engine lights. I can't promise that a Fiero won't set a code if the IAC remains disconnected, but it might be worthwhile trying if the re-setting procedure initially lowers the idle speed. However, you may discover that your IAC isn't moving/doesn't seat, and that it's stuck open (and thus always has a high idle).
have you replaced the gasket for the IAC? i was chasing a high idle on a customers 2.8 and found that they had accidently double gasketed it
lol the original IAC that was on it was double gasketed… it was on there so long like that I had to scrape the gasket off the TBI. Im 1000% sure there’s only one gasket on it now and it doesn’t leak.
lol the original IAC that was on it was double gasketed… it was on there so long like that I had to scrape the gasket off the TBI. Im 1000% sure there’s only one gasket on it now and it doesn’t leak.
I'd really like to see some pictures of the IAC too... I'm wondering if someone screwed around with the IAC wiring.
As for the engine codes, when you checked them, did you at least get code 12? The engine should flash Code 12 three times, and then if there are no actual codes... it should flash Code 12 again. If you did not see even Code 12, then either your check engine bulb is burnt out, or there's something else going on.
I'd really like to see some pictures of the IAC too... I'm wondering if someone screwed around with the IAC wiring.
As for the engine codes, when you checked them, did you at least get code 12? The engine should flash Code 12 three times, and then if there are no actual codes... it should flash Code 12 again. If you did not see even Code 12, then either your check engine bulb is burnt out, or there's something else going on.
I’m at work now… I can send pics of the IAC when I get home later tonight or tomorrow at the latest.
I do get code 12 blinking so my cel does work.
One thing I did notice when I was driving it yesterday is that the speedometer stopped working. I saw on a few older forum posts that that could lead to a high idle. Is this possible?
