84 2.5 auto will not idle, sometimes... (Page 1/1)
gb.84.fier0 OCT 12, 06:18 PM
My duke likes to randomly die at idle. Sometimes itll sit there and idle perfect for like 20 seconds, then die. It has been incredibly hard to diagnose, because it seems to do it randomly. Sometimes itll idle in park or neutral, then die if put into gear. Sometimes it will not even idle in park. Ive replaced many sensors and other things, including the IAC and the computer. The TCC is unplugged as well. Any leads would be greatly appreciated.

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84 stock :(

Patrick OCT 12, 06:45 PM

quote
Originally posted by gb.84.fier0:

Ive replaced many sensors and other things...



Instead of us simply guessing, list off everything you've done to troubleshoot and what you've replaced.

[This message has been edited by Patrick (edited 10-12-2024).]

gb.84.fier0 OCT 12, 07:13 PM

quote
Originally posted by Patrick:

Instead of us simply guessing, list off everything you've done to troubleshoot and what you've replaced.




I would have, but its been a while since ive done all that. I can remember some, but its likely not all of them.

IAC, TPS, Oxygen sensor, Computer, (reman) MAP sensor.

No vacuum leaks anywhere.

What I haven't done is pop open the egr to see if its stuck.

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84 stock :(

Patrick OCT 12, 09:50 PM
Check the EGR valve by pushing up on the underside of the diaphragm (or use a vacuum pump). Doing so should stall the engine.

Off the top of my head...

Checked ignition timing with ALDL jumper in place?

Checked spark plug gap and condition?

Checked spark plug wire resistance?

And then there's also the coil and the ignition control module if spark is a problem. A healthy spark will easily jump a 1/4" gap to ground in atmospheric pressure.

Checked fuel pressure?

Checked injector spray pattern?

[This message has been edited by Patrick (edited 10-12-2024).]

armos OCT 14, 01:02 AM
What's the background on this car/engine? Is it a daily driver that was running normally until recently? Did you just revive it after being parked for 10 years? Did you recently acquire it and don't know the history?
Is the problem just with idling, or does it also drive badly? Any relationship with being cold/hot?

Does it run better if you feed it extra fuel through the intake (starting fluid or just some extra gas on the TBI assembly)?

As was mentioned, check the fuel pressure.
Check the ignition timing with the ALDL jumper in place.

You can check the Coolant Temperature Sensor (CTS) with a multimeter. The resistance varies according to temperature.

The sensor that talks to the computer is different from the one that feeds the dashboard. I don't remember where it's installed on the 4cyl.
But this is what they look like:

Obviously the sensor that talks to the computer is the one that will affect how the car runs.
When you unplug the connector that plugs into the CTS, one wire on the connector should be Grounded and the other should read 5.0V (only true when it's unplugged). So if you measure across the 2 terminals you should see 5.0V with key on.

On the MAP sensor connector, one of the wires should have a constant 5.0V on it. Another should be connected to Ground.
You'll see the same on the TPS - one wire is constant 5.0V with respect to another wire which is Grounded.

On the MAP and TPS connectors, the 3rd wire is the signal going back to the ECM. It's voltage will vary from 0-5V depending on the current manifold pressure (MAP) or throttle position (TPS). To see that signal, you'd need to backprobe the wire while it's plugged into the sensor. If you can't get a probe into the back of the connector then you can stick a more suitable sized piece of wire in, using that to carry the signal out where you can get a probe on it.
The signal voltage from the MAP sensor should look like this:

(The sensor on our cars is 1-Bar)
With the key on and engine *not* running, the sensor should see Atmospheric pressure so it should be in the 4-5V range. With engine idling it should drop down around 40kPa I think, so based on that chart the signal wire would be around 1.5V when idling.

Be mindful of broken electrical connectors - sometimes they don't stay in reliable contact and can make the car act flaky.

[This message has been edited by armos (edited 10-14-2024).]