Low Idle and Timing Question

[Dandxj]

- 1990 4.0L with AW4

 

I got the truck running again ( Thanks to assistance from Cruiser 54 )but it idles kinda rough and low at 550-600 RPM. When I open the throttle to achieve 750 RPM it smooths out nicely. I can get it to idle at 750 RPM if I poke down into the throttle body with a flat screwdriver and push the IAC pintle back out ( it returns to 550 RPM when I remove the screwdriver )  or if I use the small set screw and increase the butterfly valve opening. Opening the butterfly valve this way the gap is larger than the recommended .030 in. gap. Is this an acceptable fix ? I did not measure how much of an increase is needed. Am I overlooking something?  Also at 550 RPM the timing is 18* BTDC. Is this what it should be?

 

Thanks

[pizzaman09]

I'll be interested to hear your results as my truck acts the same way.

[89 MJ]

I would head over to @cruiser54's website, linked here: http://cruiser54.com/ and perform tips 8 and 11 right away. And then do all of the tips as time goes on. 

[cruiser54]

Remind us which Tips you have completed on my site. 

[Dandxj]

I have completed 1,3 (except headlight connector- for later), 4,5,7,8,9,11,14,18,28,29,31

[cruiser54]

distributor needs to be indexed. Tip 13

[Dandxj]

Tip 13 done. Took forever to find my distributor wrench. I only moved the distributor < 1/8 inch counterclockwise to align it. The engine still idles low.  Note: the truck did not run when I got it and it has sat for several years. Because I was working to get it running I capped off the vacuum port at the back of the manifold since I do not need cruise/heater/CAD right now. I unplugged it to attach vacuum gauge while running. Unplugged it jumped to 750 RPM and runs fine. Vacuum is steady at 20 in/Hg at idle.

[cruiser54]

Okay, now we're going into forbidden territory.

[cruiser54]

There MAY be a little metal cap over that torx screw. 

If so, remove it by hitting it off center with a punch and hammer. Easy does it. 

Then unplug the IAC at idle. Slowly turn the screw counter clockwise and bring the idle up. 

[89XJ_NC]

On 10/30/2022 at 7:09 PM, cruiser54 said:

There MAY be a little metal cap over that torx screw. 

If so, remove it by hitting it off center with a punch and hammer. Easy does it. 

Then unplug the IAC at idle. Slowly turn the screw counter clockwise and bring the idle up. 

I finally got my cap off, my screw seems loose. Should it be? It didn’t take much to turn it out. I’m having same issues. 500-600 low idle. 

[cruiser54]

Should not be loose. Keep track of how far you're turning it. Try going in also. 

[boxyjeep]

I thought we weren't allowed to touch that????  

[cruiser54]

Only with supervision!!

[Dandxj]

OK- Finally got back to working on the truck and took a trip into "forbidden territory" . I got the truck running, unplugged the IAC and backed the torx screw out until the idle reached 750 rpm. It took almost 2 turns ~1 7/8.  It idled nicely.Plugged the IAC back in. I shut the truck off and said hurrah.  Now- on the next startup and everyone since, it does its 2 second high idle and then drops back to 500 rpm. If I just let it idle, it slowly creeps back up to ~ 750 rpm and idles fine, but this takes almost 5 minutes as it warms up. This is better than constant low idle, but sort of odd. Most engines start high and go down in rpm as they warm up. Could this be a weak temperature sensor ?? Any thoughts ?

 

Thanks

[cruiser54]

drive it a while. 

[Dandxj]

Currently I can not drive it very far (legally) , it does not have any plates.

[cruiser54]

You could also have a bad oxygen sensor. 

 

Have you looked into the REM featured on the first page of my website? 

[Dandxj]

Funny you should mention the REM. I messaged Nick last week and he told me it would be sometime in 2025 before they would be available again. The O2 sensor is a new NTK - 2 mos ago.

[cruiser54]

New means little to me any more.

There's a way to test the O2 sensor with a meter. I'm off to work now. 

[Pete M]

the link in my signature should have some sensor testing threads in it.   

[Dandxj]

Thanks to all for your help.   Pete M - I can not find any links associated with your signature block. What am I missing ?

[Pete M]