84 Duke Problems (Trying Again) (Page 2/2)
Patrick FEB 01, 10:56 PM

quote
Originally posted by Dukesterpro:

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).]

fieroguru FEB 02, 07:16 AM
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
theogre FEB 02, 11:16 AM
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)

The Ogre's Fiero Cave

edfiero FEB 07, 12:25 PM
any update?

I'd also highly suspect the IAC.

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.
Dukesterpro JUL 22, 10:48 AM
Hello, everyone.

I am back.

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.
Dukesterpro AUG 05, 09:23 AM
Good Morning,

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)