Surging Idle

[tmoss]

Always start by checking the basics: vacuum leaks, throttle position sensor voltage, resetting idle, cleaning the IAC, and pulling codes. Once all these ideas are checked out and crossed off the list, check the ten-pin connectors on the main harness. These connectors are notorious for developing loose contacts. Ford even issued a TSB on some model years, and dealers installed an "extension harness", thinking perhaps the original harness was too short causing the large connectors to pull apart.

The "ten pin" connectors are these salt and pepper shaker looking things found at the back of 5.0L engines -between the intake manifold and firewall. These connectors are notorious for developing poor connections and causing strange EEC quirks, like idle fluctuations, check engine lights, and unexplainable diagnostic codes to be triggered. On many Mustangs there may be a dealer installed extension harnesses, making it appear as if there are a total of four of these connectors.

To open the connectors squeeze the four side clips and pull apart. Do not pry the clips apart as with other connectors. The inner workings reveal ten male and female connectors. Using a nail or sharp punch spread the male connectors open just slightly. This will ensure a complete contact. If you open up the ends too much use needle nose pliers to sqeeze the "leafs" back together. Before reassembly, take some spray electrical contact cleaner and clean the male and female connectors.

After you have cleaned out EGR, IAB, TB, checked ECT, cleaned and tightened the 10-pin connectors, reset the idle, played with the TPS, on & on, and still no relief...idled smooth but now it takes way too long to DECELERATE, and hangs up around 1200-1300 rpms and then finally drop...real PITA to drive. Sound familiar? Without further embellishment. Here's a fix.

Start engine, turn idle adjustment screw counterclockwise reducing rpms to around 550-600. Blip throttle a few times and you should notice that the hanging high idle behavior is improving...i.e. it's actually decelerating the way it should and doing it's dashpot thingy at around 900 rpm. Fine, but now the idle is too low and almost gonna die. AH HAH! Not to worry. When you're satisfied with the rpms at which the IAB is doing its thing, shut off the motor. Set TPS to a pretty low setting like .87 (it's probably already at a low point because you've turned down the idle so much.) Disconnect the battery for 20+ minutes and do the reset idle drill according to the corral tech instructions except DO NOT touch the idle adjustment screw. Now the engine will still be loping along at 600 rpm. Don't sweat it. The goal is to teach the IAB some manners with a new memory by which it will do its thing. So now you've done 2 minute idle drill with accessories off, shut off the motor waited 2 minutes, started it with all electrical accessories on for 2 minutes etc., & shut off engine. Now the engine decelerates fine with the IAB doing its thing at around 950 or so but now you're stuck with a way too low of an idle. NOT!!! Start motor, turn idle adjustment screw clockwise to increase rpm’s. Adjust to the desired rpm’s. Blip throttle a few times and now you've got smooth idle at around 775+/- and you're not hanging high anymore. If you're a TPS freak you can measure it and discover it's around .93-.94 (at least mine was) which will still give you some room to make further tweaks.