This is my first post so I hope it's in the right place.
My 1986 has recently developed an issue when it comes to starting it up when hot or cold. 50% of the time when I go to start the car it fires almost instantly and idles normally. Great!
Now, the other 50% of the time it starts right up but stalls almost immediately despite struggling to to keep itself running. If I go restart it after that has happened, it starts and idles fine..
I believe I have ruled out possibility of a fuel issue because even if I cycle the key multiple times before starting the car (and hear the pump run) the problem still appears (Also, many major parts of the fuel system are less than 5 months old, such as: Injectors, pump/sock, filter, pressure regulator).
If I get my foot on the gas pedal in time before it stalls I can keep it running but as soon as I let off it idles uncomfortably low/stalls when putting into gear.
Much of my ignition system is new: wires, plugs, cap, rotor.
All of the parts that are new/recent were on the car before it started acting like this.
I put in a new IAC a couple months ago (as well as cleaned all the TB passages) and performed the minimum idle/relearn procedure according to spec.
I also suspected that I could have a vacuum leak. However I went around the engine bay several times listening for hissing as well as spraying carb cleaner with no results.
When it idles normally I feel a bit of a vibration occasionally and my fuel economy seems a little low (10-11mpg city) but the car drives great and I don't have any codes.
I wouldn't have made this post, but nothing else I've seen on the forum has related to this issue, so I hope someone might be able to point me in the right direction.
Thank you

 

any evidence of fuel in the fpr vacuum line ?
fuel pressure hold after the pump prime ?

 

The fpr is quite new and the hose going to it is bone dry. It doesn’t run pig rich either so i know that’s not it. As for the fuel pressure, It holds to spec and doesn’t bleed down after priming the pump.

 

have you watched fuel trims and iac steps ?

 

Yes. On a cold start I get about ~35 counts at idle and when warm I'm getting ~15-20 counts.
My BLM values remain stable at ~128

 

Is it entertaining closed loop ? Seems odd cold start should be 160 steps .

 

The IAC is likely out of synch with the ecm or it is not moving equally in both directions as commanded resulting in unpredictable behavior.

Prior to starting the engine, the IAC should be at 144 counts. This should provide enough airflow for cold starts and allow it to idle around 1200 rpm. As PID control takes over, the IAC position should drop in an attempt to control the idle to the desired idle rpm. This is the normal case.

Now if the IAC is not really at 144 counts on startup, the engine may over-rev, or be starved for air. The IAC (now offset high or low) should be commanded in the appropriate direction, but it may or may not not follow.

You have to find a way to confirm that the engine is actually responding to the IAC both up and down. This is easy if you have an emulator, not so easy otherwise.

If you're scanning an 1986, you're probably scanning at 160 baud. When scanning at 160 baud, the idle rpm should be 1,000 and the IAC counts to maintain 1,000 rpm should be around 100 or so. Your posted values don't add up.

 

Interesting...
I'll hook up my scanner when I have the time and do some more testing/get more data.
I'm starting to think my IAC is the problem despite the fact that it is new.

 

Here's something you can do with your 160 baud scan tool (w-10k resistor) to force the IAC into action in both directions (assuming the coolant temperature is already warm/hot):

With the cable plugged in (w-10k resistor) it should command 1,000 rpm idle. If you unplug the cable and/or open/switch off the 10k resistor, it should then command the normal 550-575 rpm idle (warm/hot in gear). Try alternating between both several times at say 30 second intervals to see if the idle follows the commanded rpm both up to 1,000 rpm and back down to 550-575 rpm and if it can reach and stabilize at both target rpm values.

 

From looking at your post, you have checked the obvious and done a base idle setup including checking fuel trim and iac steps.
I would check the ignition module and coil next, these might show up when cold or hot if anything is not right there.

I have found with my 20+ years of experience, sometimes rough running can result from the ecm trying to compensate a perceived issue.
Be it from a bad connection at one time or knock signals in error many things can go awry, then correct themselves but the ecm gets itself into knots trying to correct something.

A simple reset of the ECM by disconnecting the battery a few minutes and reconnecting it, let it relearn the engine management solved most rough running issues that pop up from time to time.

I have been amazed how often that is all it needed, even when there were no fault codes. just one stall or few knocks from poor gas grades, or hot engine high ambient temps low rpm high loads the ecm retarding the ignition all get compensated for by the ecm. Disconnecting the battery, clearing the ecm history letting it relearn start fresh solves a lot of weird issues.

Give it a try it only takes a minute, might save you a lot of time chasing a momentary glitch that the ecm is trying to compensate for.

Good luck.

 

Took her for a drive to get some fresh data.
Struggled to start a couple times as usual but eventually got going.
As I was driving, the car suddenly wanted to die at each stop and I had to keep my foot on the gas.
Got home and was struggling at 300-400rpm in park.
popped the hood, heard a distinct hiss when I blipped the throttle, and found that the diaphragm of my EGR valve had completely ruptured (Could see it falling out of the housing).
I'm guessing that the diaphragm has been slowly deteriorating and finally just gave up on me
Kind of embarrassing that I didn't consider that a possibility...
Hopefully that's the issue. New part coming in tomorrow!
Thank you for all of your advice!