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I would look at the vacuum booster control unit .
This is the power assiest for the brakes it runs off the main engine vacuum,
If its leaking inturnailly you would lose engine vacuum that would make it run rough when you hit the brakes .
Also if the check valve inside of this unit were stuck it could cause a vary high idle
Try hitting the brake pedel when its idleing high in park and see what effict it has on the engine speed .
If it changes this is for sure your problem.
To begin with, is your "check engine (CE)" light on? If it was on and now its not, you need to replace the bulb. If CE lights is on, find out the codes before doing anything. If bulb is good and "CE" does not come on, begin with: (1) gapping spark plugs, (2) replace spark plug wires (if haven't changed them within the last 2), then the coil. If your Cinco is still idles rough, adjust gas pedal until idling smoothly. If this still doesn't help, replace your two oxygen sensors. If still running rough, the converter. After doing all this and problem is still present, take it to a mechanic you trust.
I Have a couple of questions. First question does engine surge when idleing cold? 2. Does engine start quickly without a lot of cranking, Cold or warm? 3. Do you have funny smell at highway speed, or after you stop? With out this info and your imput, I would only be guessing. I am retired mechanic of 25 years. and would like to help if u need me.
My 93 s10 blazer idles really ruff and i erratically for the first few minutes until it warms up. It now sound like its going to stall every morning I start it up and black soot comes out the tail pipe onto the pavement but only while its idling rough. After i changed the cap and rotor it idled good for a couple days then started doing it again.Already changed the fuel filter and pcv valve. whats the problem?