Good Day to Y'all,

I have a 4.1L Coupe DV which has suddenly (today) started to idle either too fast or too slow.
When too slow the engine cuts out in gear.
It ran very nicely

Periodically the Engine Check Light comes on and then goes off up to a minute later.
The Engine Check Light indicates catalytic converter problems.
There is a great deal of bad muffler noise emitted from around the converter so it is probably time to replace it.

Is a new catalytic converter going to cure the idle speed problems or should I look elsewhere for that in the meantime.
Air filter is brand new and the engine is in good tune.

Any assistance will be appreciated.

 

A clogged cat can cause drivabilty issues. I doubt thats it, but don't rule it out altogether.
Did the light come on the same time the idle started acting up?
What fault codes are stored in the ECM?
Lots of things could cause this.
Steps i would take:
Check all trouble codes.
Check for va***n leaks.
Check linkage and throttle for binding.

1981..... Thats still got a carb on it, right?
If it does, check the choke and linkage.
Its been awhile.

 

Thanks FLCaddytech for the good tips.

The Engine Check Light has been flashing on and off periodically for about the past year.
But we have probably only driven about 500 miles in the past year.

I will check ECM codes - good idea as I did not figure a car this old had such!
I have the GM workshop manuals so that should be fun...

Throttle and linkages feel/look fine.

After start up the engine idles fine for a few minutes and then when changing into gear the idle just dies and one has to give gas to keep it alive.
Almost like a choke problem, so that will be another avenue to pursue.

I am getting back into this stuff after a ten year layoff having purchased new vehicles in the interim.
Now my teenage son and I share quality time in, and beneath the hood of the old Caddie.
He has learned how to change oil without going to Jiffy Lube.......

 

FLCADDYTECH steered me off the Cat Converter and back to the most likely culprits.
The ICM (idle control motor?) looks/feels like it has not worked in a while, but nearby was a disconnected vacuum pipe beneath the air filter.
We will tackle the ICU later....
I have not yet figured out whether connecting the vacuum pipe or messing with the idle stop solved the problem, but she idles sweeter than ever before.
The kid is now forever out in the Caddie, so over to the other teenagers Alfa Romeo, a nightmare but fun nevertheless.