84 Duke Problems (Trying Again) (Page 1/2)
Dukesterpro JAN 31, 09:39 AM
Hello!

I am starting fresh at tackling an issue that my 84 2.5 is having. It otherwise run fines beside this really annoying quirk. In fact I driven it over 5000 miles like this.

The only way I can describe it, is that it is if the high idle is reversed. I know it sounds stupid but its so consistent. When its cold it idles rough and choppy at about 800. It doesn't like to find idle and sometimes shudders and drops down to where you can hear individual exhaust pops out the tail pipe. As the engine warms up, the idle comes up, until at full operating temperature it settles out around 1800-2000.

This happened about a year ago, after I changed a leaky intake gasket. Which made me think vacuum leak. However, it does this with all the vacuum lines plugged with brand new rubber caps. So I am starting to think it may be a coincidence. I have ran the entire intake over with starting fluid while it idling and have not found a leak.

I have changed so far:

Intake Gasket x3
EGR
Vacuum Lines
MAP x4 (I had a bunch laying around and another Fiero to test them on)
Distributor (The whole thing, brand new)
Spark Wires
Spark Plugs
Injector
Fuel Filter

I have also:
Reset the ECM
Reset the IAC
Confirmed the idle screw is untouched (still sealed away)
Confirmed Ignition Timing
Confirmed Fuel Pressure


I have the cable that shows live engine data from the ALDL connector on my Radio, I dont remember seeing anything weird with the MAP, Fuel (BLM etc). But I can check again.

this isn't exactly a rush, I mostly drive my 86GT these days plus the 84 is waiting on new brakes, but I miss my cheap little commuter commuter

Thanks again guys
82-T/A [At Work] JAN 31, 11:27 AM

quote
Originally posted by Dukesterpro:

Hello!

I am starting fresh at tackling an issue that my 84 2.5 is having. It otherwise run fines beside this really annoying quirk. In fact I driven it over 5000 miles like this.

The only way I can describe it, is that it is if the high idle is reversed. I know it sounds stupid but its so consistent. When its cold it idles rough and choppy at about 800. It doesn't like to find idle and sometimes shudders and drops down to where you can hear individual exhaust pops out the tail pipe. As the engine warms up, the idle comes up, until at full operating temperature it settles out around 1800-2000.

This happened about a year ago, after I changed a leaky intake gasket. Which made me think vacuum leak. However, it does this with all the vacuum lines plugged with brand new rubber caps. So I am starting to think it may be a coincidence. I have ran the entire intake over with starting fluid while it idling and have not found a leak.

I have changed so far:

Intake Gasket x3
EGR
Vacuum Lines
MAP x4 (I had a bunch laying around and another Fiero to test them on)
Distributor (The whole thing, brand new)
Spark Wires
Spark Plugs
Injector
Fuel Filter

I have also:
Reset the ECM
Reset the IAC
Confirmed the idle screw is untouched (still sealed away)
Confirmed Ignition Timing
Confirmed Fuel Pressure


I have the cable that shows live engine data from the ALDL connector on my Radio, I dont remember seeing anything weird with the MAP, Fuel (BLM etc). But I can check again.

this isn't exactly a rush, I mostly drive my 86GT these days plus the 84 is waiting on new brakes, but I miss my cheap little commuter commuter

Thanks again guys




The only thing I can figure is... is it possible that you're Idle Air Control valve is bad? It can be bad without giving an error code. This is definitely weird behavior though...

Have you removed it to see if the passage is clean? Typically, there should also be a gasket on the IAC between the throttle body and the valve. If that's missing, it upsets the distance of the IAC and can affect idle...
Dukesterpro JAN 31, 06:52 PM
Im not sure, are the v6 and i4 IACs the same. I have a extra v6 one
Stingray92 JAN 31, 07:16 PM
How about the throttle body gasket? Rough idle cold, higher RPM warm seems to suggest it's fuel/air issue. Don't forget the power brake booster line. Hook a vacuum gauge to the intake and start cold. I forget where mine runs at but it was steady, even after going through putting a different engine in and reinstalling everything.
82-T/A [At Work] JAN 31, 08:07 PM

quote
Originally posted by Dukesterpro:

Im not sure, are the v6 and i4 IACs the same. I have a extra v6 one




I don't think they are actually. You can check Rock Auto and see if the part numbers are the same.
Stingray92 JAN 31, 08:38 PM
Something else that came to mind is the fuel pressure regulator or weak/bad fuel pump. I had good pressure at/before starting but when running it would do odd things. If you have an IR gun what does the exhaust manifold reach?

I don't believe the IAC's are the same from an 84 2.5 to later years.
Dukesterpro FEB 01, 10:03 AM

quote
Originally posted by Stingray92:

How about the throttle body gasket? Rough idle cold, higher RPM warm seems to suggest it's fuel/air issue. Don't forget the power brake booster line. Hook a vacuum gauge to the intake and start cold. I forget where mine runs at but it was steady, even after going through putting a different engine in and reinstalling everything.



Brand new. Plus I have hunted the whole intake manifold with starting fluid, there are no leaks at any temp that I can find
Dukesterpro FEB 01, 10:04 AM

quote
Originally posted by Stingray92:

Something else that came to mind is the fuel pressure regulator or weak/bad fuel pump. I had good pressure at/before starting but when running it would do odd things. If you have an IR gun what does the exhaust manifold reach?

I don't believe the IAC's are the same from an 84 2.5 to later years.



good thought, but I already verified that. Fuel pressure is perfect. Actually the car runs great beyond this weird high idle startup procedure. It pulls as hard as a duke can at wide open. No issue maintaining 90mph
82-T/A [At Work] FEB 01, 10:55 AM

quote
Originally posted by Dukesterpro:

good thought, but I already verified that. Fuel pressure is perfect. Actually the car runs great beyond this weird high idle startup procedure. It pulls as hard as a duke can at wide open. No issue maintaining 90mph




I would definitely check the IAC then. It's the only thing you haven't replaced at this point, haha...

I just checked Rock Auto for you... there is a completely different IAC part number for each of the following years:

L4 / 2.5
1984
1985-1986
1987-1988

V6/60
1985-1988

I confirmed that none of the part numbers interchange between these year groups... that means that none of these would realistically be compatible. They also each come with spacer gaskets to ensure they actually work properly too... which again appear to be unique to the year/engine. I definitely didn't know this... so other than trying to help, I didn't mind looking it up because this is good information. As far as my logic goes, if it looks the same as all the rest, and it has the same connector... it should just work... but apparently it definitely does not.

So I definitely think your problem is the IAC valve. If I remember correctly, you have an 85 engine but an 84 ECM? or something like that? You'll want to go with whatever IAC applies to the ECM that you have. So if everything is GOOD... and you have a 1984 ECM with 1984 stuff... then buy a 1984 IAC.

Honestly... it's almost like your IAC wiring is reversed... which might be as a result of the different years. 2,000 rpms would not be abnormal when the car is stone cold, and 900 rpms is totally normal when it's warm at idle. When you're driving with the throttle body opened... the IAC does absolutely nothing, and therefore would explain why your car runs really well when you're actually driving around.

[This message has been edited by 82-T/A [At Work] (edited 02-01-2024).]

Stingray92 FEB 01, 07:26 PM
Unplug the IAC connector before start see what happens. Also sense the problem is RPM going higher as it gets warmer you could cover/plug the IAC port to rule out the IAC. It's a stepper motor and works on pulses, I don't know if the wiz"s tried to get cute and change the connector orientation from one year to the next. At any rate something to try to rule it out.