engine idle problem

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Apr 10, 2025 | 09:24 PM

Aus f150

Thread Starter

Trailering

Joined: Feb 2024

Posts: 17

Likes: 5

engine idle problem

Hi all, my 90 F150 5.0 AOD. is now increasing the idle when selecting a gear from park or neutral and at the lights the idle sometimes drops down to 500 rpms and then it will hunt. If i touch the acc pedal a little the engine goes back to a nice smooth idle.
No codes are comming up for this problem and i replaced the tps and that made a huge difference to the engine. Have not removed the computer yet for checking leaking capacitators. The truck runs really nice apart from this problem.
Thanks in advance. Getting electrical parts for this truck in Australia is not easy , i use Rock Auto.

Apr 10, 2025 | 09:55 PM

Truckwardo

More Turbo

Joined: Oct 2023

Posts: 607

Likes: 153

From: Canada

Maybe fire the major tune up at it. Plugs, coil, filters, EGR valve, PCV, Idle air control, ground straps and battery cables, order a 12 pack of oil filters and some wiper blades to make shipping worthwhile. Do a smoke test first to check for air leaks.( The idle air motors used to fail alot back in the day. )And they get slow and sloppy. The EGR valves don't work great and need replacing after a few decades. A smoke machine is around 120 AUS$ and great to have.

Apr 10, 2025 | 10:19 PM

Aus f150

Thread Starter

Trailering

Joined: Feb 2024

Posts: 17

Likes: 5

Thanks but i have already given the engine a full tune, replaced all the egr components, No vac leaks , i use Cummins oil filters and will get a new iac valve.

Apr 10, 2025 | 10:22 PM

Truckwardo

More Turbo

Joined: Oct 2023

Posts: 607

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From: Canada

Ok the only weird ghost it can sometimes be is the flat ground straps. They look good. Buy they're causing excessive resistance on a circuit and can sometimes cause electrical gremlins. Particularly with aluminum intakes. But I'm 90% thinking IAC

Apr 10, 2025 | 10:53 PM

Aus f150

Thread Starter

Trailering

Joined: Feb 2024

Posts: 17

Likes: 5

Did all the ground straps with heavy duty cable. My iac is clean inside but i am going to replace it .
Thanks.

Apr 10, 2025 | 10:58 PM

Truckwardo

More Turbo

Joined: Oct 2023

Posts: 607

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From: Canada

Quote:

Originally Posted by Aus f150

Did all the ground straps with heavy duty cable. My iac is clean inside but i am going to replace it .
Thanks.

nice. You're doing all the right things. Rebuilding the ECU isn't too bad. Just a few capacitors and sometimes resistors.

Apr 11, 2025 | 06:15 AM

JJF20

Hotshot

Joined: Nov 2017

Posts: 17,450

Likes: 2,767

From: Northern Ontario

Quote:

Originally Posted by Aus f150

Thanks but i have already given the engine a full tune, replaced all the egr components, No vac leaks , i use Cummins oil filters and will get a new iac valve.

Quote:

Originally Posted by Aus f150

Did all the ground straps with heavy duty cable. My iac is clean inside but i am going to replace it .
Thanks.

Can you give the details to “full tune”. Takes the guess work out of it.

Also note replacing parts with new aftermarket parts today, is not a guarantee they work better, or at all. If you’re not having an issue with something specific, I’d be hesitant to replace it. Especially multiple things at once - makes trouble shooting more difficult if there is an issue.