I have a 1985 Fiero with the 2.5L iron duke and whenever I disconnect the battery and reconnect it, or when the battery dies, it idles at around 3k rpm and eventually hunts for a little bit around 700-3k rpm. I rebuilt the TBI unit and got a new IAC. If I plug the top IAC hole on the TBI unit it will go down under 1k and if I keep it mostly plugged for a while the idle eventually stays there. But when the battery dies or is disconnected I have to start all over again.
I have a 1985 Fiero with the 2.5L iron duke and whenever I disconnect the battery and reconnect it, or when the battery dies, it idles at around 3k rpm and eventually hunts for a little bit around 700-3k rpm. I rebuilt the TBI unit and got a new IAC. If I plug the top IAC hole on the TBI unit it will go down under 1k and if I keep it mostly plugged for a while the idle eventually stays there. But when the battery dies or is disconnected I have to start all over again.
Does anyone know what could be causing this???
I don't really know what you mean but the "top IAC hole," but it sounds like you have a vacuum leak.
This below video is what you can expect for a normal "cold start" on an Iron Duke:
This is a nearly-stock 1985 Fiero 2m4 with Iron Duke and manual transmission. The engine was rebuilt (it still only has like 40 miles on it), and all the emissions stuff is working. It may help you figure out what the issue is.
~2,000 rpms is normal for the initial start-up. GM intentionally programmed the ECM to do this so that it warms the car up as quickly as possible... specifically for emissions. There's several ports on the throttle body that are engaged at various points, though if any of them were open, it would be causing a vacuum leak and a higher idle. The one I think you may be having an issue with is the ThermAC, but not sure. This is controlled by the ThermAC thermostat valve in the air cleaner. One goes from the TBI to the valve, and another from the valve to the ThermAC flap at the mouth of the air cleaner.
But definitely check all your vacuum lines on the TBI unit (passenger side). The other place you may want to look is also the "tree" that sits on top of the intake on the left side (closer to the drivers side). There's like three ports on it. When I end up in situations like this, I usually just plug all of the ports and start slowly connecting them back to see where there's a vacuum leak.
Originally posted by 82-T/A [At Work]: I don't really know what you mean but the "top IAC hole," but it sounds like you have a vacuum leak.
This below video is what you can expect for a normal "cold start" on an Iron Duke:
This is a nearly-stock 1985 Fiero 2m4 with Iron Duke and manual transmission. The engine was rebuilt (it still only has like 40 miles on it), and all the emissions stuff is working. It may help you figure out what the issue is.
~2,000 rpms is normal for the initial start-up. GM intentionally programmed the ECM to do this so that it warms the car up as quickly as possible... specifically for emissions. There's several ports on the throttle body that are engaged at various points, though if any of them were open, it would be causing a vacuum leak and a higher idle. The one I think you may be having an issue with is the ThermAC, but not sure. This is controlled by the ThermAC thermostat valve in the air cleaner. One goes from the TBI to the valve, and another from the valve to the ThermAC flap at the mouth of the air cleaner.
But definitely check all your vacuum lines on the TBI unit (passenger side). The other place you may want to look is also the "tree" that sits on top of the intake on the left side (closer to the drivers side). There's like three ports on it. When I end up in situations like this, I usually just plug all of the ports and start slowly connecting them back to see where there's a vacuum leak.
Thank you. I will post a picture of the hole I am talking about on the TBI. I deleted my thermac system and plugged the vacuum port as well. I will try messing with the “tree” that you mentioned as well.
Do you have a functioning 195°F thermostat installed? And is the CTS (Coolant Temperature Sensor) sending the correct temperature reading to the ECU?
If the coolant is actually cold, and/or is reported to be cold (to the ECU), the idle speed will remain high.
Yes I have a new 195F thermostat that I tested. I thought it could be the CTS but would it really make it idle that high? And after I plug the IAC hole for a bit it idles “normal”.
And after I plug the IAC hole for a bit it idles “normal”.
Well, if the ECU is instructing the IAC to open wide due to a CTS reading of say... -40°F... then sure, blocking the hole that provides air for the IAC will bring the idle speed down to "normal".
Using a scanner and/or WinALDL to determine what temperature the CTS is reporting to the ECU is best, but the CTS can be tested using a multimeter as well.
Of course, it could be that the IAC valve is simply non-functional and stuck wide open... but checking the CTS will help narrow down the possibilities.
[This message has been edited by Patrick (edited 02-10-2025).]
Well, if the ECU is instructing the IAC to open wide due to a CTS reading of say... -40°F... then sure, blocking the hole that provides air for the IAC will bring the idle speed down to "normal".
Using a scanner and/or WinALDL to determine what temperature the CTS is reporting to the ECU is best, but the CTS can be tested using a multimeter as well.
Thank you I will test it as soon as I can.
Of course, it could be that the IAC valve is simply non-functional and stuck wide open... but checking the CTS will help narrow down the possibilities.
Well, if the ECU is instructing the IAC to open wide due to a CTS reading of say... -40°F... then sure, blocking the hole that provides air for the IAC will bring the idle speed down to "normal".
Using a scanner and/or WinALDL to determine what temperature the CTS is reporting to the ECU is best, but the CTS can be tested using a multimeter as well.
Of course, it could be that the IAC valve is simply non-functional and stuck wide open... but checking the CTS will help narrow down the possibilities.
I apologize for the late response. I tested the cts and my iac and they were both functioning normally. The car still idle at 3k rpm. I was able to choke it down to 1200-2k but it kept just bouncing between those 2 rpm’s when I took the rag out of the iac hole. When I shut it down all the way and restart it, it goes back up to 3k rpm. What could this possibly be caused by?
If the IAC and CTS are both functioning properly (and the throttle plate stop screw hasn't been messed with), then the only explanation would be a vacuum leak in the throttle body/intake system. Unwanted air is getting in somewhere.
I apologize for the late response. I tested the cts and my iac and they were both functioning normally. The car still idle at 3k rpm. I was able to choke it down to 1200-2k but it kept just bouncing between those 2 rpm’s when I took the rag out of the iac hole. When I shut it down all the way and restart it, it goes back up to 3k rpm. What could this possibly be caused by?
use a smoke tester at this point to rule out vaccum leaks. it is much faster than checking by hand (from personal experience) our 84 with a built engine exhibited this symptoms because of a leaking throttleshaft
use a smoke tester at this point to rule out vaccum leaks. it is much faster than checking by hand (from personal experience) our 84 with a built engine exhibited this symptoms because of a leaking throttleshaft
I did the smoke test by blowing smoke through the vacuum line that normally connects to the MAP sensor. When I did so all of the smoke just came out of the IAC hole. Should I block that hole and try again? Am I even using the right vacuum line to blow into?
Hello scuba-Steve45. I’m glad you were able to resolve your issue. I was about to post a new thread about a constant code 45 CEL I am having, but I decided to add this question here since it’s somewhat relates. I have an 87SE manual 88K miles and I’ve changed the O2 sensor and I changed the fuel filter. I have the same situation too when re-hooking the battery , the CEL comes on approximately two minutes after start.. before assuming I have the same issue with my ECM , I thought I’d ask you guys what your thoughts are with what I mentioned so far . thanks!
Hello scuba-Steve45. I’m glad you were able to resolve your issue. I was about to post a new thread about a constant code 45 CEL I am having, but I decided to add this question here since it’s somewhat relates. I have an 87SE manual 88K miles and I’ve changed the O2 sensor and I changed the fuel filter. I have the same situation too when re-hooking the battery RPM at 2,5 , the CEL comes on approximately two minutes after start.. before assuming I have the same issue with my ECM , I thought I’d ask you guys what your thoughts are with what I mentioned so far . thanks!
Hello scuba-Steve45. I’m glad you were able to resolve your issue. I was about to post a new thread about a constant code 45 CEL I am having, but I decided to add this question here since it’s somewhat relates. I have an 87SE manual 88K miles and I’ve changed the O2 sensor and I changed the fuel filter. I have the same situation too when re-hooking the battery , the CEL comes on approximately two minutes after start.. before assuming I have the same issue with my ECM , I thought I’d ask you guys what your thoughts are with what I mentioned so far . thanks!
Do you know how to check the CEL codes? Check them and let us know.
