Rough idle and alot of carbon out exhaust pipe, 1993 gmc jimmy

You have a vacuum leak somewhere, leaning out the system. Most of the time it can be traced down to either one of two things. 1.) Check your EGR valve (clean/check wires/check hose connections/ect) or 2.) you have leaking intake gaskets. The way your describing the symptoms, my two cents would be on that EGR Valve. Good luck, hope that gives you some direction!

You must have an egr valve for this engine to work.Sounds like you are getting oil blow-by combustion causing carbon in the exhaust which may do due to a faulty crankcase ventilation valve, bad piston rings, etc. Whatever the cause, oil combustion produces the carbon, and you will need to fix that.

If egr valve is stuck open can cause raunchy smell from exhaust, hense high readings. usually this causes rough idle though.

not exactly sure, but i have an 89 chev pickup and a 94 gmc and they are both on the exhaust manifold or even on the exhaust pipe near the manifold. hope this helps

Could be a bad MAP sensor, or perhaps the wiring or vacuum hose connecting to it is faulty.

engine rough at idle only?
is the check engine lamp on?
remove egr valve
start engine to blow out carbon
check egr to see if there is carbon holding the valve open
clean valve
reinstall retest

It's an egr valve problem (exhaust gas return) egr position sensor.
Take off the egr valve and turn it over and see if there is carbon build up in the ports, if there is clean it out with a pipe cleaner and a little carbon cleaner, just spray a little into the ports and clean it out. If that doesn't cure the problem, then either the sensor needs replacing, and or the egr valve.

Purchase new egr valve gasket with screen on it. The motor as it get older produces more carbon it then begins to break off and when the valve opens it get caught in there. i have cleaned mine several times and finally found the gasket at napa for around 22 bucks. All you need it 10mm wrech and 10mm socket with ext. takes around 10 min to do.

this is related to the egr system and makes sense when you describe a rough idle. the egr is probably stck open slightly. remove and attempt to clean with carburretor cleaner. it also helps to have a vacuum pump so you can work the diaphragm up and down. happy holidays

none of that will fix a pluged cat the only way to fix a pluged cat is to replace it