Reaper19

Okay, I have fought this rough idle for several months now on my 1981 C3. I have checked every sensor and item on the car, changed out vapor can, all vacuum lines, M/C, TPS, new Magnaflow High Flow Cat, temperature sensor, EGR valve, spark plugs, spark plug wires, cap, rotor, timing at 12 BTDC and checked for vacuum leaks.

I have not changed out the distributor coil or MAP sensor, but no codes for these or issues noted that I can see. I bought a new ECM but have not installed it yet since I am not getting any codes for issues or no engine light. I know the engine light works and the CCC is controlling all sensors from troubleshooting and following the shop manual troubleshooting sections.

The engine and carb have been rebuilt, had carb completely rebuilt and brought back to carb re-builder 10 times to change and check it out, then re-tune each time. Each time they found something else wrong and fixed the carb. The car runs fantastic at WOT and cruising, plenty of power, but I cannot get the erratic idle under control. When the car goes to open-loop runs like a champ, goes into closed-loop and erratic idle up/down RPM's and like the motor is missing or running lean.

Not sure if the computer is trying to over compensate for the newer parts installed with the engine build or something else is wrong. I have dialed in the carb to factory settings, idle mixture screws 3 1/3 out, dwell at 30 degrees, TPS 0.56 volts and even tried running it richer with no luck. cannot bring the idle down further than 700 RPM in drive or the symptoms get worse.

These are my specs for the new engine build, nothing too over the top.

1981 Corvette Specifications
Carburetor – Model 17081228 with Black M/C solenoid and electric choke

Motor: Chevrolet 350
- Original block
- Bored over 0.30
- Pistons EngineTech P15334 Flat Top 9:5:1
- Cam EngineTech ES1014R – These are the numbers that are close. The builder did not provide exact specs and these were pulled from the internet for the Cam with make and model that was installed
o Stage 1
o Hydraulic flat followers
o 0.50 Duration
 204 Dur. Int.
 214 Dur. Exh.
o Cam Lift .280/.295
o Lob center
 C/L Int 107
 C/L Exh 117
- Heads - Chevy 333882
- Intake – Original factory Aluminum 14033058
- Exhaust – Original factory design 2-1-2 with Magnaflow High Flow Catalytic Converter
- Transmission TH350C
- Rear Differential gear ratio 2.87

Any advice or if someone had a similar issue and corrected it, I would appreciate any assistance. My next thought is to tear out the CCC stuff and go with a vacuum advanced distributor and non-CCC carburetor, then install something to control the torque converter locking up.

Thanks

Jebbysan

Does that carb have an Idle Mixture Control solenoid? Top of carb...with a plug going to it.

http://quadrajetparts.com/mixture-co...1990-p-52.html

Sometimes they short out..or do not seat.....had an 81' Monte Carlo that did this.

Jebby

Reaper19

Yes, it has a black one now, we tried two different blue ones and the carb was running rich throwing rich codes while at cruising speeds. Supposedly there is no difference between the black or blue models, but this one finally corrected the rich condition. The funny part, I can set the dwell anywhere I want low to high so the M/C is working as it should, but still rough idles. Thanks for the suggestion, just not sure what else to look at or try before scrapping the CCC, which I really do not want to do unless absolutely necessary.

Jebbysan

Try running a unlit propane torch around the intake area with the gas on of course.
Run it all around the intake face and carb gasket area....
See if idle changes....

Jebby

toobroketoretire

I'd start by reducing the initial timing back to it's original 6 degrees and see how it idles. Are you sure your EGR valve is clean with no carbon buildup? I also suggest running your mixture screws in to 2 to 2-1/2 turns open to make the mixture a bit richer and see how it runs. I have also found reducing the spark plug gaps to .035" often smooths the idle so you could try that.

Jebbysan

This is not a bad idea as I believe the CCC controls timing to a certain degree.
Look at the tag on the firewall (emissions) what does it say for base timing and is there a special procedure? Later cars wanted a beige color wire unplugged to set base timing.
The EGR suggestion is solid too....

Jebby

Big2Bird

10 times? The carb builder has not a clue.

gungatim

I've always had trouble with the electronic idle carbs like that, 2bbl and 4bbl. seems once that idle solenoid goes out, it's never the same...almost like the replacements do not fit right or something...sorry, wish I had more help...

David Mc

Sounds like he forgot the o-ring on the idle air bleed. How's the fit of the throttle shafts in the base plate? What's the dwell number at idle?

Reaper19

Thanks everyone, I am still trying to work on the idle issue and will take some of the pointers everyone suggested and try them next. I'll try changing the timing back to 6 degrees factory setting on hood label. The engine was rebuilt recently, new cam, pistons etc. along with new EGR. The engine builder had it at 8 degrees BTDC and I pumped it up as another suggestion.

I have tried several combinations of setting the mixture screws at 1 1/2, 2 1/2, 3 1/3 up to 4 1/2 and then adjusting the IAB and lean screw, still not idling smooth and RPM's continue to go up down if not idling at 700-750 in drive. Dwell is at 30 degrees, new bushings on base plate shafts, not loose anymore.

zee66

LOL, Im going through the same thing with my 1981 coupe with 15,000 original miles on it. Bought the car knowing it and thought something minor was taking place, that was 4 months ago still no luck. You can call or text me at 716-628-1363 and maybe we can find this problem. Thanks Phil p.s. My wife is ready to burn this car so she can have her garage space back!

Reaper19

Hey ZEE66 sounds like we need to get the wives together with a bottle of wine while we work on this issue. My wife sometimes feels the same way about losing here space in the garage, but just wants to start going on fall Vette cruises when the weather breaks here in Florida, so I want to make sure the car is running great.

Anyway, I'll try to catch up with you this morning when I get a break at work.

Thanks

Randy