Need help with 77 L-82 idle problem

[Michael C.]

I recently bought a very nice Top Flighted 77 L-82 with 29000 original miles on it. The car still has its original distributor cap, wires, and rotor. It hesitates a bit but I consider that normal for the age of the ignition components. Also a bit jerky at light load, or "chugs" as Lars Grimsrud would say. But, all in all, considered it livable until the weather warmed a bit and I could get to a tune-up.

The following began or appears to have started after refueling for the very first time: When I release the accelerator as when stopping or approaching a stop sign the idle will fall from its normal 800-900 rpm to 500 or less. It proceeds to stumble and will eventually stall unless I respond quickly with feathering and lightly pumping the accelerator. This does not occur everytime the accelerator is released but happens about 30 or 40% of the time. Otherwise it runs pretty well.

Is there something I should look at immediately like float level and or float condition? Fuel filter? Did I just get bad gas? Or do you think the carb needs an overhaul? The car has only been driven about 500 miles per year for the last 20 years. I have all the appropriate GM tech manuals... I'm looking for common experience to give me some direction.

[Mike McKown]

Re: Need help with 77 L-82 idle problem

Before I went any further, I would start with checking all the vacuum hoses, switches and connections on top of the engine for vacuum leaks. It's very common for the hoses and switch nipples to get brittle and crack. Look at your headlight vacuum circuit next. I'm betting you will find your problem here.

[Duke W.]

Re: Need help with 77 L-82 idle problem

Your year engine has literally a ton of add-on emission control equipment. It's full of vacuum hoses and vacuum diaphragms.

Get a 1977 CSM and go over everything in each and every one of those systems. All the hoses and vacuum diaphragms need to be checked for proper operation paying particular attention to leaks.

It's probably a minor problem, but the number and complexity of all those system s is going to take some time to systematically check out.

Duke

[Michael C.]

Re: Need help with 77 L-82 idle problem

Duke, could a vacuum leak explain the intermittent nature of the stumbling? Normally I think of a vacuum leak as a continuous event rather than coming and going as this occasional stumbling is. She idles rather well when this is not happening. Thanks to you and Mike McKown for your comments.

Mike

[Duke W.]

Re: Need help with 77 L-82 idle problem

Yes, it could be a vacuum leak - an intermittent vacuum leak, maybe temperature related. You just have to go through all those systems - CCS, TCS, EFE, XYZ, choke vacuum break, etc.and inspect all the vacuum tubing and vacuum diaphragms. There's probably an intermittent leak somewhere. On a C3 you also need to check the headlight vacuum system.

Even though the car only has 29K miles it's nearly 30 years old, and some of those vacuum hoses or diaphragms may be marginal and leaking. Organic materials like hoses and vacuum diaphragms deteteriorate over time, even if the car has low mileage accumulation.

Duke

[Clem Z.]

Re: Need help with 77 L-82 idle problem

my guess it would be a sticking EGR valve. try plugging the vaccum line to the EGR valve so it does not open

[Michael C.]

Re: Need help with 77 L-82 idle problem

Clem, I isolated the EGR valve as you suggested and found it much more driveable. The jerking under light load (M-21 trans) virtually went away. The stumbling when pulling up to a stop sign is better but not gone. It does not stall the engine but is still there and it doesn't do it every time. I thought it was brake booster related but doesn't do it every time I use the brakes. Any other obvious places to look? Thanks.

[Clem Z.]

Re: Need help with 77 L-82 idle problem

did you check to make sure the distributor vacuum advance is working? i also would disconnect all the vacuum lines from the intake,cap off the ports on the intake and try that but check the vacuum advance first.