Chevy 454 exhaust

Clean Throttle Body (both sides, not just what you can see) and MAF (mass air flow sensor) [spray and shake to dry].

https://www.google.com/search?q=2001+Ford+Focus.+running+rough+and+power+surges
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Rough idle could be caused by many things but being that it came on suddenly, this narrows the field considerably. Assuming that there are no knocking, grinding or banging noises coming from the engine, I would look for a loose spark plug wire(s), loose, cracked, dry rotted, missing or broken vacuum hoses, (with the engine running, sometimes you can hear a hiss if the leak is large enough), loose wires on various sensors around the engine, a loose wire on the oxygen sensor mounted in the exhaust pipe. Unfortunately this can be anywhere from the engine end of the exhaust pipe to the intake end of the catalytic converter. You'll have to look for it. Be sure that the engine is cool. These get VERY hot!!! If it has power brakes, don't forget to check the large hose that runs to the hydrovac unit. it is the round metal canister mounted behind the master brake cylinder on the driver's side firewall. This can be a major vacuum leak if cracked and it can also affect power brake function too. Good luck.

Anrez, your 1991 Chevy trucks O2 sensor could be located in the Y pipe just ahead of converter or in the front portion of the converter.
Check the attached links,instruction and guides, Good luck
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How to change Oxygen Sensor 1995 Chevy Suburban
How to o2 sensor replacement Chevy Silverado GMC Sierra 88 95 Tahoe Yukon...

Usually in the exhaust pipe near the engine.
They don't usually cause a rough idle, and the check engine light should be on if you have a faulty sensor.

I really need to know the mileage.....?..also the question isn't very clear ...Is the whole truck shaking roughly or is the engine shaking roughly or is it both the engine and truck shaking..?..Since I am not sure I will answer what I can....Ok......If the truck is shaking while sitting still at idle the engine/trans mounts could be worn and or loose mounting hardware...If the engine is running roughly at idle causing all to shake that bad it could be a number of things causing it as usual...Quite often a faulty plug wire/and or bad plug will make it have a misfire because it will be running on only 5 cylinders..If that's the case it could be more than one bad...A vacuum leak will cause it to perform in the same manner...however all cylinders will be firing..Normally a vacuum leak is easy to pinpoint..however it could be a faulty IAC valve..(IDLE AIR CONTROL)..The idle air control is designed to hold a high idle at start up then slowly bring idle down to proper Rpm.....A faulty TPS (THROTTLE POSITION SENSOR) will also cause a rough running problem..And let's not forget the O2 sensor..(OXYGEN SENSOR)..The O2 sensor controls air/fuel mixture...and yes an exhaust manifold gasket will make it run rough but not too bad...It has a rough running sound more so than causing it to horribly shake..It will shake but not to bad..
it is located at the exhaust side of engine..The exhaust manifold is mounted to the engine exhaust port..the exhaust pipe is mounted to the exhaust manifold and the muffler/cat converter is mounted to the exhaust pipe...You can check the plug wires quickly..With engine running carefully remove plug wires 1 at a time..If the sound of engine gets worse the wire you removed is good..after verifying it is good or bad plug wire back on then pull the next wire..and so on..If you pull a wire off and there is no change in engine operation that is the bad wire..If that is the case change all plugs and wires...One at a time..(It is a good idea to make sure all wires come off easily before starting engine..old wires will come apart easily..grasp wire tightly at boot and twist left to right while pulling..)as far as the other components I spoke as well as many others,the best,easy way is to have it hooked to a scanner and they can check/test all components for proper operation...Now..If it runs good but is shaking the engine/trans mounts are worn as I stated earlier..any loose engine component will cause an engine shake also but you should notice it at a glance...engine mount bolts/nuts being loose can be tightened without changing mounts if caught in time..Trans could be coming loose.also worn clutch will cause vibration...Send more info and I can guide you to remove and replace any faulty components you may find to save you money.........Brooks/Metalpoet.....

sounds like a vacuum leak check all vacuum ports for cracked , broken or missing hoses

if pieces are come out the tail pipe from the convertor, the convertor my very well be clogged with rust. just beat the hell out of the catalytic convetor with a hammer, then get in and rev the engine to blow the junk out. i personally witnessed this on a vehicle and there was never another problem with it.

Barring any mechanical issues you most likely have a failed injector.

try changing distributor cap & coil

I would check the exhaust pipe a little closewr. White smoke can also mean a blown head gasket. Check the tail pipe to see if moisture is present. Also check coolant level. A blown head gasket can also cause related problems.

Bill