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Q: Rough Idle on 1991 Mazda Protege

Here is the chain of events:
1. speedometer cable broke
2. "Hold" feature on the transmission seems to be stuck on "Hold"
3. Engine is idling very rough
do I have a vacuum leak? how do I track it down?
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The hold switch is a simple on off switch on side is ground and the other side goes straight back to the computer giving it a ground signal. Black is the ground wire at the switch, Brown/black goes back to the computer. See if continuity across the switch changes as you open and close the switch if it doesn't the switch is defective.
Rough idle can be caused by many things, bad ignition components being the usual cause but I have seen bad intake manifold gaskets causing the engine to idle rough like a bad injector or failed spark plug or plug wire. Check the gasket between the cylinder head and intake manifold fro leaks or pieces of gasket missing or protruding out.
Speedometer cable has probably failed but could be a bad speedometer. head or drive at the transmission. The speedo cable is two pieces about $40 each piece and an hour labor to install both cables together.
Hold light is because you unknowingly pressed a button under the top of the transmission lever (just press it in and the problem is solved). This hold is there for driving in snow and to keep it in a particular gear, etc..
It could be an MAF sensor, which will either need to be cleaned or replaced. My suggestion is to replace it with one that has a minimum of 3 years of warranty. They are not hard to replace (be careful not to break the electrical connection when disconnecting). Also check PCV Valve, spark plugs and their wires. Use a fuel injector cleaning when changing oil.
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