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first start it, it idles fine then after 30-40 seconds, the idle speed will drop to 300-400 rpm

I had an identical problem for a long time.

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older cars without all the emission controls.

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all vacuum hoses, I found several hose ends that were cracked and probably leaking.

found a bad hose end to the "backpressure transducer", cut off the cracked end and reattached - solved my problem. it's a little flying saucer looking thing towards the back of the intake plenum. there is a pic of it in the Haynes in the emissions chapter, it's part of the EGR system.

the air regulator RT1 mentions can often be a source of 2.6 idle woes and because of its location can be a real pain to deal with. you can relocate it above the intake manifold for easy access in the future.

2.6 Trooper
2.0 Impulse Turbo