74 corvette run-on???
Thread Starter
Drifting
Joined: Oct 2003
Posts: 1,348
Likes: 18
From: Dallas Texas
74 corvette run-on???
Guys
all of a sudden I am getting a bit of run on - when I turn the key off it does not immediately stop...seems to want to continue to run for several seconds and then dies out - any suggestions on what to check
Thanks
all of a sudden I am getting a bit of run on - when I turn the key off it does not immediately stop...seems to want to continue to run for several seconds and then dies out - any suggestions on what to check
Thanks
Melting Slicks
Joined: Mar 2005
Posts: 2,838
Likes: 10
From: Austin TX
Agree,..your primary throttle blades are too open at idle and not idling only on the idle circuit as it should. If your initial timing is too low, you're likely compensating with an idle setting that's too high and causing the dieseling.
Good luck!
Good luck!
Racer
Joined: Feb 2007
Posts: 342
Likes: 0
From: Onley VA
The reason run on is also called dieseling is because it is runing without ignition. Idle speed doesn't matter, you could be running WOT, if the ignition is turned off you have no spark and the engine will stop as fast as friction will allow under normal operating conditions. Dieseling is caused by hot spots in the combustion chamber along with compression igniting the fuel/air mix without spark. Detonation is basicly the same thing but occures while the engine is running, it is defined as uncontrolled ignition. Common causes are any sharp areas on the piston such as around valve reliefs or the spark plugs themselves overheating.
Melting Slicks
Joined: Mar 2005
Posts: 2,838
Likes: 10
From: Austin TX
GG,..agree, but without fuel which continues to get sucked in to the combustion chamber after ignition shut-off, there will be nothing to light off. Shut off the fuel supply (by reducing the primary throttle blade opening so that it's idling on the IC not the transition circuit) and dieseling won't happen.
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Burning Brakes
Joined: Feb 2006
Posts: 1,024
Likes: 1
From: AL
some other thoughts: look into everything to do with tuning as already suggested. also check all of the pollution equipment, especially the egr valve. when egr valve sticks usually closed, engine runs richer and lots of black soot/carbon can build up over time in the combustion chamber, the back of intake valves, etc. twidling with the idle to compensate for this and other issues can get your run-on. my 74 with auto had an idle soleniod, but it is to compensate for the A/C. evidently some have said theirs is set up to eliminate run-on. if done this way by factory, maybe depends on tranny??? thx
Tech Contributor
Joined: Aug 1999
Posts: 14,375
Likes: 6,375
From: At my Bar drinking and wrenching in Lafayette Colorado
That's not correct. GM had severe run-on problems starting in the early 70s on brand new cars. The problem was solved by installing idle-up solenoids on the cars. The solenoid dropped idle speed and closed the throttle when the engine was shut off to eliminate the run-on problem. The GM tech bulletin that we received at all the dealerships instructed the mechanics to lower idle speed on cars with run-on complaints, and this consistently solved the problem. You can induce run-on by shutting a hot engine off at elevated engine rpm.
Race Director
Joined: Feb 2005
Posts: 16,100
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