I have an 86 IROC with the 305 TPI. On cold starts first start of the day the car will start right up but the RPMS will drop just a little then idle kind of high 1500 RPMS or so then slowly work their way down to 8-900. The issue is the entire time as they are working their way down the idle/RPMS are unsteady. They’re not bouncing all over the place but definitely shaky? Once they settle down and I go for a ride no issues. RPMS are steady at stops etc and no issues for the rest of the day until the next cold starts.
I have ensured that all the usual suspects are in order. Car is timed right 6 degrees before top dead center. Injectors are new and functioning fine. Fuel pressure is good. New EGR valve and sensor/switch and the list goes on. Also I have no codes.
My one area of concern is the evap canister. I notice the entire time my car is running there is a hissing sound like an air/vacuum leak coming from the fuel tank area. Only goes away when I shut the car off. Also I am getting pressure in the gas tank to the point where if I don’t relieve it by unscrewing the gas cap I sometimes hear thumps in the gas tank due to expansion. This doesn’t happen all the time and sometimes there’s no pressure when I unscrew the cap. I was hoping it was the fuel tank vent valve causing the hissing so I replaced it with no change.
I then focused on the evap canister. I ran 12 volts to the solenoid and heard it clicking so I assumed it was good? There are two valves on the system one attached to the canister and one not. I replaced the one that was not attached and still no change?
I know it’s hard to diagnose issues on these cars without live data but I have not got to the point where I can do that yet and was hoping to exhaust all options before I take it to a shop. I really feel it is some kind of gas/fuel recirculation issue but not sure what. I do notice that my exhaust fumes are rich at times and I do sometimes hear a rattle in my muffler. I was thinking about replacing my muffler and more than likely will eventually but was told that a bad muffler would not cause an unsteady idle? Also I believe my fuel pump is good it primes every time and holds pressure. I just wonder where that hissing sound is coming from? Is there a vacuum line that runs to the fuel pump/tank?
Sorry to be long winded just trying to get to the bottom of this issue. Thank you in advance.

 

Yes there is a line that goes from the tank to the evaporator canister. It is this line that captures the fuel vapors, and when called open by the solenoid valve it’s drawn in and combusted.

For easy testing/validation, pull the ported vacuum line going to the evaporator canister and plug it to see if that settles it - sounds like a typical vacuum leak.

For future troubleshooting highly recommend getting tunerpro RT, moates cable and data log. It’ll end up saving you money and time - trust me 😁

also, Amazon sells the replacement 2-way valve that mounts above the rear axle for the tank. Did mine about a year ago (see images below). After decades of use, it was seized open.

as for that rattle in the muffler, mine was due to a self gutted cat 🤣 (see image below)

 

I can’t thank you enough for the reply! I actually replaced that two way vent valve by the rear differential hoping it would solve the issue but it didn’t? I will definitely try pulling that ported vacuum line and plugging it but I’m not sure which one you are talking about? I know that there is the large vacuum line that goes from the solenoid to the circular looking valve on the canister forming a U. Then there is another thinner vacuum line attached to that same circular valve that leads out. I would love to know exactly which one to plug and hopefully get to the bottom of this issue. Also will plugging it cause any issues? I never thought about a gutted cat??? I wonder if that’s my issue? Was it causing a rattling in the rear muffler? Finally I definitely have to get that system you mentioned to stream live data! I just home I can figure out how to use it! Again I can’t thank you enough for the response and hope that whenever you can you can specify which hose to plug

 

Is this the hose that I need to remove and plug?

 

You got it. Follow that line to the intake and where it lands pull it and replace with a rubber cap. If you’re unable to get to it where it connects at the intake, plug the line you’ve circled and cap the valve.

essentially you are isolating the evap system to see if it is in fact that system that’s producing the assumed vacuum leak. Again, data logging live streams will help diagnose and help pin point your starting point on the troubleshooting tree.

Here was my culprit of an annoying rattle in my muffler. After removing the exhaust to gain access for a fuel pump replacement, the honeycomb chunks fell out.

 

I can’t thank you enough. I will try that in a little bit! I’m also assuming that’s the issue with my muffler!

 

You should also see one of these down there. Might check it and see if it looks in good condition. You can take them apart and check the diaphragm / spring and clean it up if needed. https://www.rockauto.com/en/moreinfo.php?pk=1597032&cc=1251650&pt=4936&jsn=663

Or is that what is shown in your photo? maybe your year car is different than a 90. looks like it may be mounted to your canister.

 

I replaced the one that’s not attached to the canister so I know that one is good. As for the one attached to the canister there is no way to replace it without replacing the entire canister? And the canister itself is impossible to find!

 

Yes they are a little different mine has two one attached and one not. My 91GTA is different than my 86IROC

 

I'm a bit confused, are you saying post #4, the thing that is circled you have 2 of them installed?

 

Yes the evap systems on the earlier third gen’s were different than the later years. For example on my 86 IROC which I’m having issues with there are 2 of those things like I have circled. One that is attached to the canister that can’t be replaced and one that is not attached but has a tube leading from the canister. I replaced the one that’s not attached with a new one. Now on my 91 GTA there is only one of those and it’s not attached but also has a tube leading to the canister. That’s why I was a little confused when you said to plug the ported one because there are 2. I will take pictures in a little bit and post them

 

86 IROC

86 IROC

86 IROC

 

91 GTA