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Hi all,

Just completed rebuild of a 2.3L non turbo in an 89. Also switched from auto to a manual T5. Have done all the normal adjustments (timing, etc). Engine starts and runs fine but idles way fast 1500-1800 rpm.

Manual idle adjustment screw is turned all the way in (not moving the throttle).

Tried unhooking the Idle air motor as a test and that dropped it to about 1300 but not low enough.

Tried unhooking the main vacuum from the intake (that feed the vacuum distribution tree) and immediately it dropped to about 600 or 650.

Assumed we had a vacuum leak so with everything hooked back up I started unhooking hoses from the vacuum tree 1 at a time. Only 1 hose made it drop and that is the hose that is hooked to the MAP/BAP controller on the firewall. Unhooking this brought us down to the 650. Thought maybe that could just be plugged but shortly after unhooking that the car began operating extremely rich and smoking.

Does anyone have a thought on the next step? Does it make sense that the MAP system is the culprit of the high idle? Is there an easy way to test the MAP controller? Or do I still need to go down the path of the Idle air motor or tps setting? I'd appreciate your thoughts!

Mustang5L5 said:

Unhooking the MAP would just change parameters in the ECU which caused the drop.

Do you have a smoke machine? or can you build one based on the youtube vids? I would vac test using the smoke machine as it is VERY good at pinpointing tiny leaks which will cause your high idle. From what it sounds, i would suspect vac leak here.

SHort of the smoke machine, you just have to randomly test. The vacuum tree itself could be cracked.

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Thanks Mike, I also suspect vacuum leak and I can investigate those smoke machines. I assumed that unplugging the main vacuum line from the intake and plugging the hole in the intake causing the rpm to drop to the expected 650rpm meant that the vacuum leak had to be isolated to one of the components attached to the main vacuum hose. Are you saying that is not necessarily the case?

Mustang5L5 said:

I think you are on the right track. I just wouldn't definitively point my finger at the MAP yet as removing the vac line and exposing it to atmospheric pressure may have caused the ECU to adjust and as a result idle dropped. Or there could be a crack in the neck where the vac line connects. I wouldn't rule it out.

Did Ford route the MAp to the vac tree on the 2.3L from the factory, or should it connect directly to manifold vac like it does on the 5.0 cars?

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Thanks. The MAP is connected by short hose to the Vac tree. It was the only hose that dropped the RPM when removing it. But I see what you are saying now-if that was simple changing the ECU settings and dropping the RPM it could be that the vac leak is somewhere else entirely. The vac outlet on the intake is a tee with one side going to the fuel pressure reg and the other going to the Vac tree.