Foxbody high rpms and surging rpms

[86foxseth]

Hello, I have a 1986 mustang and one day while driving my rpms wouldnt go down past 1250 rpms. After letting it sit for a few hours and starting it back up it would surge between 500 and 750 rpms. After a week or two the rpms would occasionally get stuck above 1000 rpms, so I swapped out the IAC. It fixed the rpms getting stuck and now its surging between 500 and 750 rpms when idling. Any ideas what the problem can be? Im at a lose right now.

 

[X-cam34]

https://www.oreillyauto.com/shop/b/ignition---tune-up/ignition-module-control-unit-ignitor/9331d0e7f826/v/a/5473/automotive-car-1986-ford-mustang

 

[AeroCoupe]

Start with this thread. Read and work through it step by step. Solve the vast majority of the siege issues.

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...

• jrichker
• 1986 1993 86-93 check engine light code 67 cranks ok dump codes eec-iv mustang no start
• Replies: 575
• Forum: Fox 5.0 Mustang Tech

• 

 

[CAMTWO1070]

Hello, I have a 1986 mustang and one day while driving my rpms wouldnt go down past 1250 rpms. After letting it sit for a few hours and starting it back up it would surge between 500 and 750 rpms. After a week or two the rpms would occasionally get stuck above 1000 rpms, so I swapped out the IAC. It fixed the rpms getting stuck and now its surging between 500 and 750 rpms when idling. Any ideas what the problem can be? Im at a lose right now.

When idle issues raise their head in a speed density system its usually the map sensor thats the issue or even maybe a broken vacuum line..

Id look into that new IAC and see if its actually working because all EEC-IV ECUs are designed to rev up during startup to fight the startup friction for roughly 60-80 seconds and they rev near 1050rpms...

The other potential problem inducer could be the coolant temp sensor...Good Luck

 

[Noobz347]

I also did not see any mention of a Base Idle Reset after changing the IAC.

Of course, that would be identified in the checklist that @AeroCoupe linked.