idle rough when hot

[Lift3d1]

Hello,

I am struggling with an issue with my 1983 GMC Sierra 5.7. Its my daily driver. It ran great until i changed the oil from 5w/40 to 10w/40. i did knock the air cleaner out lazily and put it back in. its idled bad since. I shot a butt load of starting fluid in the carb and let it sit for a few hours and then ran it. usually this works for me to remove varnish etc. I do run chevron fuel and seafoam. i thought maybe it was coincedence so here's what i've done.

I recently replaced the fuel filter, spark plugs, plug wires, diz cap, rotor, O2 sensor, HEI module, most vaccuum lines, and i still cant get this thing idling smooth when hot.

thing is, when cold and first started it runs good. then as the choke is off it runs like a dog and wants to shake. if i give it gas it runs better so it seems worst at idle under load like at stop sign.

any ideas? Im thinking of trying the coil next or pcv valve? i'm just throwing money at it now. i jumped the a/b obd1 terminal last night and there was a code 13 so i did the 02 sensor. which was dirty but made no difference.

any ideas?

 

[Charlie]

 

[1987 GMC Jimmy]

I’ve found that the Codes 13, 44, and 45 will come up at idle even with a perfect carburetor. It seems silly that a carburetor would be held to the same operating parameters as TBI/TPI, but it was the 80’s. Is your mixture control solenoid clicking when you first turn the key on? This issue happens even with the air cleaner off, correct, so we can rule the thermal air cleaner out? Do you have an idle vacuum reading, and have you made sure everything’s plugged in? Does it try to stall on you coming to a stop (asking because of the TCC)? Finally, when you put the old coil in the new cap, the spacer, bushing, and coil wires weren’t charred?

 

[Lift3d1]

hello. thanks for replying. i do not hear a clicking when turning on the key. The issue does happen with the air cleaner off. i do not know the idle vacuum reading. i can try to learn how to do that. everything is plugged in. i did not see any noticeable issues with the coil when i transferred it to the new cap. The truck does not stall coming to a stop. just runs rough when stopped. slightly better in neutral. worst in reverse. sometimes will die in reverse when steering cranked.

where is the mixture control solenoid located? is that the same a tps?

thank you

 

[Lift3d1]

thanks for the welcome Charlie

 

[gmbellew]

vaccuum leak? you can spray carb cleaner strategically while it is idling and see if the idle changes. if the idle changes, that is where the leak is.

 

[73c20jim]

vaccuum leak? you can spray carb cleaner strategically while it is idling and see if the idle changes. if the idle changes, that is where the leak is.

On same note, a vacuum port not connected.

 

[Lift3d1]

well good news. the truck is is running smooth now. i'm not sure if there was something in the carb?, the egr valve is functioning correctly now?, or it just took a while for the computer to recognize the new O2 sensor?. its been running smooth last cpl days so i guess i'll stop worrying about it.

Thanks for the help everyone.