Fast Idle - Vacuum Leak?

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Fast Idle - Vacuum Leak?

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Corvette >> Corvette Models >> Corvette C3 Forum >> Fast Idle - Vacuum Leak?

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Fast Idle - Vacuum Leak? - 4/4/2008 1:07:45 AM

pshiflet

Posts: 11
Joined: 3/23/2008
Status: offline

Greetings all....my '80 L48 idles very fast and i'm almost thinking it's a vacuum leak. It idles about 1500 when it's cold and closer to 2000 when it's hot; in park. When i shift to drive it drops to around 800. Once in a blue moon the car will all of a sudden change idle and it will drop to around 800 in park and 400 - 500 in drive. It sounds like it's just on the verge of dying, but it never has died when in this mode. It actually sounds like it has a hot cam in it and just listening to it i feel that it is completely normal and that's the way it should run. If I adjust the idle on the carb, then when it drops into this mode, I couldnt keep it running. I have a problem with my right headlight being very slow to pop up, most of the time it comes up halfway and i have to assist it the rest of the way. This is obviously a vacuum leak of some sort and i've yet to track it down, however, the engine idle changes very little (50rpm maybe) with the headlights up or down. This fast idle problem could be carb related, stuck needle or something? This is where i start to have a lack of carb knowledge. Any help would be appreciated. If you're a headlight expert check my next post on headlight problems. Thanks for reading!

_____________________________

Paul
'80 Black/Black C3 "The Batmobile"
Matching Numbers
93k Original
Customer Side Exhaust w/ Electric
Dumps off the Headers

Post #: 1

RE: Fast Idle - Vacuum Leak? - 4/4/2008 1:27:13 AM

extanker

Posts: 236
Joined: 1/7/2007
Status: offline

when its hot and at that high idle check the the fast idle cam [part of the choke] its on the right side of carb [pass. side] they have been known to stick.this is a Q jet right with an elect. solenoid ?make sure the choke plate is open when hot

(in reply to pshiflet)

Post #: 2

RE: Fast Idle - Vacuum Leak? - 4/4/2008 1:38:18 AM

pshiflet

Posts: 11
Joined: 3/23/2008
Status: offline

Yes this is the Rochester with e/ choke. I will check that, but it idles way too high whether hot or cold, it's just the the idle is a little higher when hot. When it's cold it hesitates really bad when i get on it right away. If i start it up cold and punch it, it starts to go and then nearly dies; it finally kicks in after a second and takes off. That's the only time it will hesitate like that until the car becomes cold again.

_____________________________

Paul
'80 Black/Black C3 "The Batmobile"
Matching Numbers
93k Original
Customer Side Exhaust w/ Electric
Dumps off the Headers

(in reply to extanker)

Post #: 3

RE: Fast Idle - Vacuum Leak? - 4/4/2008 2:16:30 AM

extanker

Posts: 236
Joined: 1/7/2007
Status: offline

the fast idle cam is between the choke body and the carb body.the idle solenoid is on driver side of carb,its used to set the idle running,base idle is set with wire off solenoid and using throtle plate stop screw,mixture screws can be base set by gently turning in till seated then 2/3 turns out.does the choke plate close when cold and open within 2 mins. after starting ?

(in reply to pshiflet)

Post #: 4

RE: Fast Idle - Vacuum Leak? - 4/4/2008 5:49:05 PM

SCHOON

Posts: 344
Joined: 9/8/2006
Status: offline

Before you start adjusting things spray some choke and carb cleaner on the linkage.
Also make sure your return spring is not shot.

(in reply to pshiflet)

Post #: 5

RE: Fast Idle - Vacuum Leak? - 4/5/2008 9:29:24 AM

batesy80

Posts: 61
Joined: 7/15/2007
Status: offline

what should the high idle rpm be on a cold start? I have the same carb on my 80. the rpm is at 2000 on a cold start and takes a while to warm up before I can kick it down.

(in reply to SCHOON)

Post #: 6

RE: Fast Idle - Vacuum Leak? - 4/5/2008 5:51:25 PM

73shark

Posts: 1129
Joined: 4/18/2006
Status: online

The fast idle is set after the engine is completely warmed up. When the engine is cold, it won't run this fast due to thick oil, etc.

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So many toys, so little time

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Post #: 7

RE: Fast Idle - Vacuum Leak? - 4/5/2008 8:40:39 PM

Posts: 1066
Joined: 8/7/2006
Status: offline

Hi pshiflet, does you 80 Vette carburetor have Computer Command Control? (CCC) If so have you checked the oxygen sensor in the exhaust system? And is the throttle position sensor in the carb good?

As for the headlights, most times the problem is the boot seal in the actuator www.mamotorworks.com part # 601-493.
After I changed those parts one headlight still came up slower that the other, I shot a little penetrating oil into each of the 4 holes in each relay and now they come up together.

Hope this helps, PG.

(in reply to 73shark)

Post #: 8

RE: Fast Idle - Vacuum Leak? - 4/13/2008 12:00:52 AM

Dj Envy

Posts: 38
Joined: 1/24/2008
Status: offline

I had a similar problem with mine.. I've never found out why and would like to know if you found the solution.
I adjusted my car to a great idle. Sounded like it had a mean cam, it was great. Then, one night before going to get my brother some McDonald's, my idle went up to 1200some rpm.

(in reply to pg)

Post #: 9

RE: Fast Idle - Vacuum Leak? - 4/13/2008 8:23:31 PM

73shark

Posts: 1129
Joined: 4/18/2006
Status: online

Did the idle increase by itself or just not return to the normal idle after driving and then letting engine return to idle? One thing to check is to make sure the butterflys aren't binding in the bores when closed.

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So many toys, so little time

(in reply to Dj Envy)

Post #: 10