Weird idle issue

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Jan 7, 2021 | 10:12 AM

johnmallen78

Thread Starter

3rd Gear

Joined: Jan 2021

Posts: 3

Likes: 1

Weird idle issue

So I have a 1987 F150 with a 4.9l. It has a weird issue that just recently popped up where after I have driven it for a bit and come to a stop (stop light or stop sign) it will idle normally for about 10 seconds and then it drops to almost 500 rpm then back up to about 750 and cycles like that for about a minute then finally stabilizes at 750. Things I have already done:

Cat delete with Y-Pipe and O2 sensor in place
New IAC
New EGR
New MAP
Just had the engine de-sludged because the previous owner apparently didn't believe in changing his oil...but this was happening prior to that.
Checked for vacuum leaks and replaced any leaky hoses

It acts like an IAC issue but having a new one on there I'm confused...

Jan 7, 2021 | 11:05 AM

R&RFord

Fleet Mechanic

Joined: Aug 2015

Posts: 1,838

Likes: 367

From: Candy Mountain

Is it just responding to your AC compressor?

Run codes.

Feb 27, 2021 | 11:36 PM

andywould90

New User

Joined: Nov 2020

Posts: 16

Likes: 0

i would test the tps does it idle up when cold and drop down once it reaches warmer temp

Mar 5, 2021 | 06:23 PM

OBSADDICT

Trailering

Joined: Mar 2020

Posts: 24

Likes: 0

Had a similar issue

Hey,

I had a similar issue with my truck, a 93 5.0. The tps and cleaned EGR that solved it. Imho, these trucks have too much crap on em to diagnose easily. Have you pulled codes yet?.