87 Chevy TBI IAC Troubleshooting

[87ChevyV20]

so when you started it up, the idle doesn't start higher and idle down as the engine warms? what is the idle speed?

you can jumper AB and force the IAC to close as i described above. then if you unplug the IAC and start the truck, the idle should be lower than normal idle. bc the IAC is all the way closed. if the idle is still very high like that, either somebody messed with the idle set screw (behind a knock out plug) or you have a large vacuum leak somewhere.

It starts at about 2500 rpm.
It does not come down from that.

I don't see the plunger of the IAC at all through the top of the throttle body, which makes me think it isn't moving.

The idle set screw was already uncovered, probably from about 20 years ago, the throttle body was cleaned and new gaskets put in like I did.

 

[87ChevyV20]

Just did an ECM replacement on my '87. It ran super FAT at idle (iraddic) & would not remit any trouble codes. Swapped with another ECM & it strained the issue out.

HS

I don't think the ECM is bad because it is sending IAC counts to the valve and I am getting activity being shown with the power probe.

 

[Strick]

I don't think the ECM is bad because it is sending IAC counts to the valve and I am getting activity being shown with the power probe.

I used my Solus & a paperclip to diagnose mine. It would only post "12" letting me know the PCM had been accessed. I was fortunate enough to have another parts vehicle that had the same ECM calibration number. I swapped the ECM & cured my issue.

HS

 

[87ChevyV20]

I used my Solus & a paperclip to diagnose mine. It would only post "12" letting me know the PCM had been accessed. I was fortunate enough to have another parts vehicle that had the same ECM calibration number. I swapped the ECM & cured my issue.

HS

How did you diagnose yours?

 

[Strick]

How did you diagnose yours?

It ran but doing it horribly. Accessing the DLC with either a "Brick" or a paperclip it would only read "12". That code meant the ECM was accessed. At that point I knew how "fat or rich" it was running that there should be another code (at least 1) posted. Swapped ECM & it will now roast the 35's on it.
Years back we would slap or tap the Atari cartridge to see if they ran (still or different). Old TBI & some MPI GM will run even with a faulty ECM. They perform poorly though. I posted about the ECM issue but this site won't allow a video unless it's YouTube or some other Communist sh@t.



HS

 

[87ChevyV20]

Pretty sure ECM is fine because I can read data with my ALDL cable and TunerPro.

 

[87ChevyV20]

This morning I plugged the old IAC valve in and left it dangling outside the throttle body, jumped the A and B terminals on the ALDL port and turned the key to the on position.

Nothing really happened but if I push on the plunger, it does vibrate a little bit.
It doesn't move, but the tip vibrates a bit.

I have to go to work soon, but I will do the same for the new IAC and see what happens.

 

[Mango]

Disconnect, the negative terminal on the battery, using a jumper wire connected to your negative ground it out on your frame For about 10 minutes that will take all the power out of your EMC. It may take a few miles to get a re-calibrated to your new IAC

 

[87ChevyV20]

Okay, I think I found the issue.

Looking at the wiring diagram I have here, 2 wires are in the wrong spot.

The blue/black and green/white wires are reversed.

You must be registered for see images attach

How do I depin this connector?

 

[87ChevyV20]

Ran and picked up a Lisle depinning tool.

You must be registered for see images attach

I switched the wires, jumped the A and B terminals on the ALDL connector and saw the tip of the plunger in the throttle body.
I was so happy!

Fired it up, let it warm up and the warm idle is about 1050 RPM, so I started doing the base idle procedure and I can't move the idle screw. Actually broke 2 torx bits trying to move it.
I will have to get creative on how to adjust the base idle now.

 

[87ChevyV20]

And I broke 3 torx bits now.

I was gently hammering on the side of the throttle body where the threads are to try and free the idle screw.

Now I get to try Harbor Freights Icon warranty!

 

[Sgt Gus]

And I broke 3 torx bits now.

I was gently hammering on the side of the throttle body where the threads are to try and free the idle screw.

Now I get to try Harbor Freights Icon warranty!

You try some pb blaster for a bit?

 

[87ChevyV20]

Not sure how long that screw is, seems quite long.

I don't think there is anything that can get it loose.

I ordered a new one on Ebay, so should be good.

 

[Ricko1966]

Not sure how long that screw is, seems quite long.

I don't think there is anything that can get it loose.

I ordered a new one on Ebay, so should be good.

Use a soldering iron to apply heat,be patient.

 

[87ChevyV20]

Good call, I am going to try to get it loose after I switch it out with the new one.