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
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...
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.
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.
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
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
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).]
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.
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.
This is a bit easier to do with the 2.8... but on your duke (with the air filter can removed), completely block off the opening to the throttle-body while the engine's running, and see if the engine will continue to run. If it does, there's gotta be a vacuum leak somewhere.
[EDIT] Just make sure that whatever you use to cover the throttle-body doesn't get sucked into the opening!
[This message has been edited by Patrick (edited 02-01-2024).]
It is normal for an engine to run smoother when warm and normally needs less air and fuel to sustain a smooth low rpm idle, as the combustion process is just more efficient, but the 2000 rpm idle is not normal
If your scanner has a IAC steps shown, then record the IAC steps on a cold engine start at KOEO (Key On, Engine Off) as well as engine coolant and air temp.
Start the engine and record the IAC steps, engine coolant and air temp, commanded ignition timing, and BLM value after 1 minute of running.
Let the engine fully warm up.
Once it is up to temp, turn it off, and repeat the process recording values at KOEO and after 1 minute of running.
In both scenarios, the KOEO IAC counts should be higher than the idle IAC counts. The hot idle IAC counts should be lower than the cold idle IAC counts. If the IAC is causing the hot high idle, the IAC counts will be higher than normal. If the IAC is trying to correct for the high idle, then the IAC counts will be close to 0.
Coolant temp also has a large impact on fueling. On a cold start the air temp and coolant temp should be within 10 degrees of ambient air temps. If the coolant temp is reading too low, the ecm will dump more fuel at startup than what is needed, which will bog the engine down, then on a hot start as soon as the engine goes into closed loop, the ecm will start pulling the excessive fuel. If the engine coolant temp is disconnected, then it will provide fueling for running the engine at approximately -40F. This is all resistance based, with higher resistance indicating a lower air temp (and more fuel) so a bad ground will mess with proper fueling.
The hot BLM value will give you an indication of the engine running rich or lean from the base map
If happens only when engine cold, check ECT w/ a Ω meter AND w/ ECM Scanner to make sure ECM get same info.
OLD style ECT connector is iffy @ best & cause ECM get bad data. If you replace this, no longer made & must update to new style w/ new Metripack plug. See https://web.archive.org/web...cast.net/~fierocave/ ECM Sensor Quick Ref
------------------ Dr. Ian Malcolm: Yeah, but your scientists were so preoccupied with whether or not they could, they didn't stop to think if they should. (Jurassic Park)
You mentioned you checked the timing, but just to confirm you followed the procedure in the manual for checking the timing (where you have to ground the pins in the OBD connector)?
If all that checks out it might be time to try swapping the ECU with a known good one. These things are 40 years old now, and can go bad.
My aforementioned 86GT has a wristpin on the way out so my 84 is back on the top of the priority list.
Responding to as much as I can so far:
Ogre, the ECT is new and checks out great with both an OHM Meter and Datastream through the ECU. For shits and giggles, I replaced the pigtail with the newer better version. It is still reporting back okay with both tests. ECT I'm going to write off as OK.
Seem's like most of you recommend replacing the IAC. I have a new one on order. However, even after unplugging the IAC, the car does the same thing. It just takes longer to transition from slow burpy idle to high idle. Which makes me think Vaccum leak.
but...
After performing Patrick's idea of blocking off the throttle body. It immediately dies. I tried again searching for leaks with starting fluid and then set the effing car on fire. So I have to now clean out the fire retardant from the extinguisher before continuing.
After I am done with the brakes I am going to take a breath and check everything again from the ground up. Nothing to bad happened from the fire. The car still runs. It was only on fire for about 15 seconds.
I finally got it fixed. It was a weird 2 - pronged issue. The O2 sensor and the IAC failed together. My running theory is that the O2 sensor when warm was sending weird rich values that the computer was trying to correct at idle by feeding it more air. The 02 Sensor Spent so long in an extremely retracted position it actually ended up sticking there. I bench-tested the IAC and could not get it to close.
When I replaced just the IAC the new one immediately started opening up just like the old one.
It was only when I replaced both the IAC and the O2 sensor that the car came back around. She runs absolutely perfectly now! (Almost)
