Engine Code 13 and high idle

CmdrDerp · Dec 14, 2024

Hey guys, I’ve been chasing a high idle issue on my 91 Mustang GT and could use some advice.

I’m getting a Code 13, which I understand indicates my base idle is too high. Without the electrical harness plugged into the IAC, the idle sits around 750-800 RPM with the throttle stop screw fully loosened to the point where I can hand-turn it, and the throttle plate slightly sticks because it’s fully closed.

The car should stall, but it doesn’t. When warmed up and driven, the idle increases slightly but stays steady - not erratic (with IAC it sits around 1000rpm when warmed up and I drive it around town).

Here’s what I’ve done so far:

Removed and cleaned/replaced everything above the upper intake manifold.
Cleaned the throttle body, EGR spacer, and upper intake (all spotless) with new, high-quality gaskets.
Replaced the IAC (tried multiple), EGR, TAD/TAB solenoids, PCV valve, screen, grommet, and hoses (PCV stuff is all Motorcraft parts).
Replaced all major vacuum hoses and vacuum trees, including under the plenum and on the firewall.
Smoke-tested the upper intake—didn’t see anything obvious.
Capped off vacuum lines to the charcoal canister, A/C, cruise control, brake booster, and EGR/TAD/TAB system (temporarily). The idle was still high.
Checked the throttle cable, and it seems fine.

My idle is slightly high but stable, just enough to trigger Code 13 (but of course I want to tighten my throttle stop a bit which will raise the idle further).

At this point, I’m wondering if I might be chasing the wrong issue.

Any ideas on what I could be missing? Everything seems new, clean, or capped off, yet the issue persists.

What else could be causing my idle to be high?

Thanks!

AeroCoupe · Dec 14, 2024

Have you performed a base idle reset? Post #2 here:

Thread 'Help me create the "Surging Idle Checklist"'

Apr 10, 2005

Updated 05 July 2022 to add rough idle when cold and smooth idle when up to normal operating temperature. See the EGR tech information, section #15 for more help.

Many of you are familiar with the "Cranks OK, but No Start Checklist for Fuel Injected Mustangs" checklist and the No Crank Checklist. They are very effective in finding and eliminating the problems by use of a systematic process that checks and eliminates possible causes. This idle problem checklist will use the same troubleshooting strategy and methodology.

By the term surging idle, I mean an idle that...

CmdrDerp · Dec 14, 2024

Yes, that's what I'm trying to do. I've unplugged my IAC and SPOUT connector and I can't set my idle below 750ish with the throttle stop screw fully loosened. From my understanding my car should stall under these conditions, but it doesn't.

The car is mostly stock, no aftermarket cam.

AeroCoupe · Dec 14, 2024

You 100% have an air leak whether it’s a vacuum leak or literally letting air into the motor at or after the throttle body.

Have you used carburetor cleaner and sprayed where the lower intake bolts to the heads?

CmdrDerp · Dec 14, 2024

Hmm no I haven't. I'll try that tomorrow. Thanks

CmdrDerp · Dec 15, 2024

CmdrDerp said:
Hmm no I haven't. I'll try that tomorrow. Thanks

I checked it quickly and didn't find anything. I'm about to go out and try it again. The lower intake mates with the engine around the top of the valve covers right? So just spray along the top of the valve covers? It's kinda hard to get down in there while it's running.

Is it possible that a bad O2 or MAF sensor would cause it to idle high (without generating codes)?

AeroCoupe · Dec 15, 2024

The ECU generates codes for a variety of items but only emissions related codes will illuminate the check engine light. 02’s and MAF fall into that category.

CmdrDerp · Dec 15, 2024

Thanks AeroCoupe,

I'm not getting any codes except that KOER 13 (no check engine lights or anything).

I went back and tried again with the last of my carb cleaner and then tried with a small propane tank too. I blasted down around the fuel injectors on both sides of the plenum. Didn't notice the rpms going up.

Is there another place I should check? I'm not sure how thorough I was, it's hard to get down in there.

I did clean my MAF sensor but it didn't change anything. If I unplug my MAF it does seem to idle a bit lower (almost stalls), but has some serious surging. Not sure if that is an indicator of anything.

Could bad sensors cause a high idle?

Mcmahst · Dec 15, 2024

You need a vacuum gauge. You might be able to rent one (free) from Autozone.

CmdrDerp · Dec 15, 2024

I have one, when I plugged it into a vacuum line with the car idling it was jumping between about 16 - 20 very quickly. Does that indicate an air leak?

Mcmahst · Dec 15, 2024

This site has one of those vacuum diagnostics charts, see if anything on it looks familiar:

https://www.bigdug.co.uk/downloads/data_sheets/1646052680VS402_V2.pdf

Mustang5L5 · Dec 17, 2024

I agree with you having a vac leak. I would smoke test vs the old carb cleaner method. Really helps find leaks that are not in the obvious locations. For example, a split in the charcoal canister vac line down by the fender. That would cause a higher idle, and would be hard to find unless you see it or smoke test for it

Mcmahst · Dec 17, 2024

In the before times, the old needle jumps was an indicator of a stuck valve or miss adjusted valve. The jumping was caused by the compression stroke bleeding into the intake and that quick insertion of charged air lowered the vacuum for just a moment; was why it would recover quickly. Maybe a stuck lifter or? I’m sure others more knowledgeable will chime in. Maybe make a video of it running with the vacuum gauge so we can watch it while listening to the engine? Good luck.

KRUISR · Dec 17, 2024

Have you checked if your throttle body blade fully closes? Could there be oil residue in and around the throttle blade from the recirculated exhaust gases that prevents it from closing? Have you or someone else drilled a hole in the throttle blade (something some people do thinking it helps with idle control - it doesn't)?

Mcmahst · Dec 17, 2024

He said he cleaned everything to try and resolve the issue. I suspect one of the four vacuum related solenoids or something to do with a stuck intake valve, based on his vacuum gauge outcome.

manicmechanic007 · Dec 21, 2024

Why not start with a 1991 PCED and go over the pinpoint tests for code 13?
At least read through them?
Code 13 sends you to KE15
KE15, after the idle set screw does squat, sends you back around to KE-6
Where you check everything else and then, lastly, replace the IAC solenoid
Probably has an intake air leak past the throttle plate and the processor is sending fuel, upping the idle speed
Then the code 13 would be actually telling you it can't control the idle

manicmechanic007 · Dec 21, 2024

Here is a look at those pinpoint tests for code 13

manicmechanic007 · Dec 21, 2024

Sorry about posting the truck pages, but it's the same for the Mustangs and all the cars

Engine Code 13 and high idle

CmdrDerp

New Member

AeroCoupe

lube between the nut and the face. I know my lubes

Thread 'Help me create the "Surging Idle Checklist"'

CmdrDerp

New Member

AeroCoupe

lube between the nut and the face. I know my lubes

CmdrDerp

New Member

CmdrDerp

New Member

AeroCoupe

lube between the nut and the face. I know my lubes

CmdrDerp

New Member

Mcmahst

CmdrDerp

New Member

Mcmahst

Mustang5L5

That is…until I whipped out my Bissell

Mcmahst

KRUISR

Mcmahst

manicmechanic007

manicmechanic007

manicmechanic007

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Engine Code 13 and high idle

New Member

lube between the nut and the face. I know my lubes

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lube between the nut and the face. I know my lubes

New Member

New Member

lube between the nut and the face. I know my lubes

New Member

New Member

That is…until I whipped out my Bissell

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