High idle (Page 1/6)
Scuba_steve45 SEP 17, 10:20 PM
Okay so I just got an 85’ Fiero with the iron duke and 5 speed manual transmission. I decided to just do a bunch of maintenance at once so I replaced the spark plugs, wires, distributor cap and rotor, pcv valve, EGR, vacuum lines, and then I made my own open air cleaner by putting a breather filter on the valve cover and you get the idea lol

Anyways now I start it up and get a very high idle (it will go up and down from 2000-3500). What did I mess up! It ran fine before!

I HAVE NO CODES!!
Patrick SEP 17, 10:34 PM

You probably have a massive vacuum leak. May or may not be related to whatever you did below...


quote
Originally posted by Scuba_steve45:

...and then I made my own open air cleaner by putting a breather filter on the valve cover and you get the idea lol
Scuba_steve45 SEP 17, 10:44 PM
That’s exactly what I was thinking but I don’t know what I should do. I removed the thermac system thing and blocked off the vacuum port on the throttle body and put a breather filter on the valve cover. Could the thermac system be the issue?

quote
Originally posted by Patrick:


You probably have a massive vacuum leak. May or may not be related to whatever you did below...

[QUOTE]Originally posted by Scuba_steve45:

...and then I made my own open air cleaner by putting a breather filter on the valve cover and you get the idea lol


[/QUOTE]
Patrick SEP 17, 11:43 PM

quote
Originally posted by Scuba_steve45:

Could the thermac system be the issue?



For a way too high idle? No.

[This message has been edited by Patrick (edited 09-17-2024).]

Scuba_steve45 SEP 17, 11:54 PM
Do you have any idea what it could be then?

quote
Originally posted by Patrick:

For a way too high idle? No.
Patrick SEP 18, 12:26 AM

Unwanted air is getting into the intake somewhere. Check for obvious things like making sure there's nothing preventing the throttle body butterfly valve from closing. Is the brake booster hose connected?
82-T/A [At Work] SEP 18, 06:13 AM

quote
Originally posted by Scuba_steve45:

Do you have any idea what it could be then?




Hi, my daughter has the exact same car. There are at least a half-dozen vacuum lines on the intake and TBI unit that may have either been compromised or disconnected. Can you take a few pictures of the engine with the air cleaner off, and show us what's going on? Some close-ups of the throttle body and intake would be helpful.

Generally, an open-element air cleaner in a Fiero isn't going to provide more power because you're just drawing heat from the engine compartment. In the factory configuration, you're pulling cool air from the side-scoop, which does actually give you cold air from the outside.

The THERMAC wouldn't cause a massive vacuum leak, but it could / would cause a higher idle since there is in fact a vacuum line that connects to it. With this open, you'd probably get another couple hundred rpms at idle.

Since we have the same car, and my daughter just restored her engine... I can take some pictures to help give an idea of what it SHOULD look like. She's not completely finished with the vacuum lines to the firewall connections yet... but all the ones from the TBI are done.


One thought... the only vacuum line big enough to cause a massive vacuum leak like that, is probably the brake booster. It's the larger of all the vacuum lines. It connects to the drivers side of the TBI unit. If this is disconnected, you're going to be getting some pretty high idle. It's like having the throttle plate cracked 1/4 of the way at all times.
Scuba_steve45 SEP 18, 11:46 AM
I have not done anything with the brakes or brake booster vacuum line but I’m sure it’s possible it popped off or is just leaking due to age. I did cap off the vacuum port for the thermal valve and replace all of the vacuum lines on the right side of the throttle body because I was thinking it could be a vacuum leak but I will post a picture of all of it when I’m off work.

I also heard that maybe the throttle cable could be bonded up somewhere but it revs out fine and doesn’t get stuck anywhere, it just idles high. Could this be part of the problem?

quote
Originally posted by 82-T/A [At Work]:
Hi, my daughter has the exact same car. There are at least a half-dozen vacuum lines on the intake and TBI unit that may have either been compromised or disconnected. Can you take a few pictures of the engine with the air cleaner off, and show us what's going on? Some close-ups of the throttle body and intake would be helpful.

Generally, an open-element air cleaner in a Fiero isn't going to provide more power because you're just drawing heat from the engine compartment. In the factory configuration, you're pulling cool air from the side-scoop, which does actually give you cold air from the outside.

The THERMAC wouldn't cause a massive vacuum leak, but it could / would cause a higher idle since there is in fact a vacuum line that connects to it. With this open, you'd probably get another couple hundred rpms at idle.

Since we have the same car, and my daughter just restored her engine... I can take some pictures to help give an idea of what it SHOULD look like. She's not completely finished with the vacuum lines to the firewall connections yet... but all the ones from the TBI are done.


One thought... the only vacuum line big enough to cause a massive vacuum leak like that, is probably the brake booster. It's the larger of all the vacuum lines. It connects to the drivers side of the TBI unit. If this is disconnected, you're going to be getting some pretty high idle. It's like having the throttle plate cracked 1/4 of the way at all times.
Yellow-88 SEP 18, 11:53 AM
If your IAC system is OK , most likely you have a vacuum leak. I use a "hiss tube", that's a small diameter steel tube connected to a plastic tube you stick in your ear. With it, even a tiny leak sounds big. I've never seen one for sale, you do have to make it. It's not the same as a stethoscope.

How about the throttle. Is something not letting it close all the way?
82-T/A [At Work] SEP 18, 12:32 PM

quote
Originally posted by Scuba_steve45:
I have not done anything with the brakes or brake booster vacuum line but I’m sure it’s possible it popped off or is just leaking due to age. I did cap off the vacuum port for the thermal valve and replace all of the vacuum lines on the right side of the throttle body because I was thinking it could be a vacuum leak but I will post a picture of all of it when I’m off work.



Just to iterate, the brake booster vacuum line is the LARGE vacuum line that connects to the drivers side of the TBI unit. If it's popped off anywhere, whether at the firewall, at the TBI unit, or at the intermediary tube, you're going to get at least 600+ more rpms.

I'll also take several pictures of the engine either today or tomorrow. The engine is friggin' immaculate, and all the original hoses were used, so it can provide a good indication of where most things should go... as I said, she still hasn't connected the firewall vacuum lines back up (to include cruise control, and all that). But at least on the TBI... everything is connected. So I'll try to get that tonight. 



quote
Originally posted by Scuba_steve45:
I also heard that maybe the throttle cable could be bonded up somewhere but it revs out fine and doesn’t get stuck anywhere, it just idles high. Could this be part of the problem?



If it is... you'd see it immediately... as in, with the air cleaner off, the throttle plate would be pitched open somewhat. When the car is idling, the throttle plate should be completely closed... that's because the IAC (as Yellow-88 mentions) gets its air through a separate passage in the base of the TBI unit, which bypasses the throttle plate. So just to reiterate, when the car is just idling, the throttle plate should be completely closed.


... but yeah, post some good quality pictures tonight, and we'll take a look.