Speed density idle question

[5.0aintslow]

Hey everyone. New to Stangnet, and I’ve got a question. I’ve got my ‘87 5.0 hatch, runs great, pulls hard etc, no serious issues but I’ve got a rather higher idle. I’ve heard speed density’s are notorious for this, so I’ve narrowed it down to 4 things; IAC (just replaced, old one was filthy but didn’t fix much), TPS (coming in the mail) MAP sensor could be it, or the vacuum hose feeding the map, looks like it’s seen better days lol.

The idle isn’t up and down or erratic, it usually hangs around 1000-900 rpm and will never die. First thought was vacuum leak, but thinking it’s more sensor because it steadily hangs around 1000.
I’ve also seen there’s a technique for replacing the TPS, you’ve gotta get like 1v across right?

Any ideas?

 

[Mustang5L5]

Idle reset procedure. Basically unplug the IAC and then back the idle down using the idle stop screw on the TB. This sets minimum idle. You should be able to get it to idle around 700RPM give or take.

Then, reset the ECU by unplugging the battery for 30 mins or so. Plug the IAC back in, hook up the battery and start the car and just let it idle for a few mins.

IAC acceptable range is anything from 0.5 to 1.25V at idle. No need to chase 1.0Volts

 

[5.0aintslow]

So I must start it up, find the idle stop screw on the throttle body with the iac disconnected, shut it off for 30 min with battery disconnected, reconnect iac, start it up and let it run?

 

[Mustang5L5]

Correct. Start it up, and unplug the IAC. It should stay running if the idle is set too high. Back the screw down to where it should idle at, and then reset the ECU.

If it stalls when you unplug the IAC, then that idle screw is probably set to low. Open it up a few turns and try to get the motor to idle, and then back it down.

It also wouldn't hurt to pull the IAC off and clean it. SOmetimes they gum up and stick

 

[5.0aintslow]

This new iac I reckon should be good, I put it on yesterday and it didn’t change much. I’ll have a poke under the tb later, you wouldn’t happen to have an idea or photo of the screw I should be looking for do you? Is it an Allan screw?

 

[Mustang5L5]

It will look something like this. Located at the 6 o clock position

 

[5.0aintslow]

Beautiful, I’ll take a look later tonight, wrapped up at work for the next while. Will let you know how it goes. Appreciate it!

 

[5.0aintslow]

Is this a common fix?

 

[KZGUNS]

Is this a common fix?

its the standard idle setting procedure

 

[Rdub6]

Be careful when you first turn that screw. Steel screw and aluminum throttle body are a recipe for a broken head (ask me how I know).

 

[5.0aintslow]

Was gonna say, took a look at mine and it looked a bit crusty. Will tweak it this weekend, not a pressing issue.

 

[stykthyn]

Do yourself a favor and replace your vacuum lines and rubber fittings before you start, including the rubber grommet going to the brake booster. These are notorious for cracking and you'll be chasing a hanging idle forever.

 

[5.0aintslow]

I’ll definitely take a good look, there’s the map sensor hose that could get replaced, but this hanging idle isn’t really “erratic” or “lopey” or even rough, it stays around 1000 but isles as if it’s set to idle there it’s consistent. Real head scratcher lol

 

[Mustang5L5]

Not really. Pretty common as many don't understand the idle reset procedure.

But a consistent, higher idle is also a sign of a vacuum leak

 

[stykthyn]

Even a high idle is a sign of a leak or improper adjustment. Go through the base idle reset procedure and go from there. There's a lengthy idle sticky in the tech forum that will go into much more detail if your problem persists.

 

[5.0aintslow]

A vacuum leak usually brings a rough idle along with a hanging idle. I’ve just got a higher idle. I’ll replace the Map vacuum line when I can find suitable hose, I’ll send a pic of what it looks like tmr. But yeah your not wrong I’ll definitely see if I can smoke the lines and find any leaks

 

[General karthief]

I’ll definitely see if I can smoke the lines

 

[5.0aintslow]

Didn’t realize the hose was this bad lol. Tps came in the mail yesterday too. Will do idle reset procedure tonight.

 

[5.0aintslow]

How would you go about it? Replace everything you see?

 

[General karthief]

How would you go about it? Replace everything you see?

i think you may be too young to catch the attempted humor.
but to answer your question is to remember how old those rubber lines are, some get hard and brittle and some rot and collapse so inspect and replace. Rubber grommets like the one for the pcv harden up and leak vacuum as well.
I find pulling the upper intake give you access to most everything you need to check.