Here's a breakdown of the likely causes and how to troubleshoot:
1. Transmission Kickdown Linkage/Cable:
* Problem: The kickdown linkage (or cable, depending on your model) connects the throttle to the transmission. If this linkage is improperly adjusted, it can hold the throttle slightly open, resulting in a high idle when in neutral or park. When you shift into gear, the transmission's load overcomes this slight opening.
* Solution: Carefully inspect the kickdown linkage. It might be binding, out of adjustment, or disconnected. A shop manual will show you the correct adjustment procedure; it usually involves an adjustment screw or linkage position. Adjust it according to the specifications in your shop manual. Improper adjustment can damage the transmission.
2. Transmission Vacuum Modulator:
* Problem: The transmission vacuum modulator controls the shift points and can also influence idle speed by sending vacuum signals to the carburetor. A malfunctioning modulator can cause a high idle.
* Solution: Inspect the vacuum modulator for leaks, cracks, or damage. If there's any doubt, it's a relatively inexpensive part to replace.
3. Throttle Valve Adjustment:
* Problem: While less likely to be the *sole* cause of this specific symptom, a slightly sticky or improperly adjusted throttle valve in the carburetor itself could contribute.
* Solution: Carefully inspect the throttle valve for smooth operation. A light spray of carburetor cleaner (following manufacturer's instructions) *might* help resolve stickiness, but avoid over-spraying into the carburetor. Improper adjustment here often requires carb rebuild/calibration knowledge and tools.
4. Idle Air Control (IAC) Valve (Less Likely but Possible):
* Problem: Though less common on carburetors, some later-model carbs had early forms of an IAC. If your carburetor has such a system, a faulty valve could contribute to high idle.
* Solution: Cleaning this valve might help; replacement is also an option.
5. Vacuum Leaks:
* Problem: Vacuum leaks anywhere in the system can affect the carburetor's operation, leading to an unstable idle. This is a broad issue, but it is worth checking.
* Solution: Carefully inspect all vacuum lines for cracks, loose connections, or damage. Pay special attention to lines connected to the carburetor and the transmission.
Important Considerations:
* Shop Manual: A factory service manual for your specific 1990 Jeep Wrangler is crucial for proper adjustment procedures. Improper adjustments can cause damage.
* Professional Help: If you're not comfortable working on the carburetor and transmission linkage, it's best to take your Jeep to a qualified mechanic specializing in older vehicles. Improper carburetor adjustments can cause engine damage.
* Safety: Always disconnect the negative battery terminal before working on any electrical or fuel system components.
Start with the kickdown linkage/cable adjustment and inspection of the vacuum lines. These are the most likely culprits for this specific problem. If those checks don't solve it, you may need professional assistance to diagnose the more complex issues.
