New 383 won't idle, poor performance
01-29-2015, 09:32 PM
#1
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Location: Fort Ripley Minnesota
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New 383 won't idle, poor performance
I've seen this drivability post in the past and fought with this issue for over a year. I originally started with swooping my stock L98 for a very well built TPIS (SPEC) 383, which is still street able and will run a very consistent 12.30 et. The engine performed very well the first few months after the rebuild, after a fun and hard driven weekend at the drag strip I started to notice several drivability issues.
I began to notice a miss under load at 4200 – 4800 rpm (I thought the stock HEI was faulty and couldn’t keep up with the demands, I upgraded the system to a DUI) a little better performance in my mind but, still the same issue.
I changed the stock fuel pump as it would drop pressure under the heavy load condition (pressure is set at 50 psi, it would drop to 46 psi under load).
Took the car to our Show and Go weekend because I thought it was fixed. Donated my $125.00 to race the weekend but, the car still had the heavy miss under load. Changed all the sensors at the drag strip (O2, MAP, IAT) no change, erratic ET’s and still the same issues.
At this point I couldn’t get it to idle at a stop light, after a quick scan at home I found a faulty IAC and thought I finally found the smoking gun, changed the IAC which helped a little but the engine would still miss under heavy load. During gentle throttle increase it would still begin to miss around 4200 rpm.
TODAY… I finally found that smoking gun, I found a broken intake valve spring on the number 8 cylinder (thank the G-Man for dual springs on those Dart Pro 1 heads). I replaced the springs adjusted the valves with zero leak down, turned the key and the engine fired and idled to perfection. Exhaust was sweet without the rich burn your eyes exhaust smell from before, happy days are here again!!
The things a broken, weak valve spring or poorly adjusted valves will cause. Keep that in mind when chasing your tail and these drivability issues. I have pictures however my computer has issues but, who care just as long as my Corvette runs well.
I began to notice a miss under load at 4200 – 4800 rpm (I thought the stock HEI was faulty and couldn’t keep up with the demands, I upgraded the system to a DUI) a little better performance in my mind but, still the same issue.
I changed the stock fuel pump as it would drop pressure under the heavy load condition (pressure is set at 50 psi, it would drop to 46 psi under load).
Took the car to our Show and Go weekend because I thought it was fixed. Donated my $125.00 to race the weekend but, the car still had the heavy miss under load. Changed all the sensors at the drag strip (O2, MAP, IAT) no change, erratic ET’s and still the same issues.
At this point I couldn’t get it to idle at a stop light, after a quick scan at home I found a faulty IAC and thought I finally found the smoking gun, changed the IAC which helped a little but the engine would still miss under heavy load. During gentle throttle increase it would still begin to miss around 4200 rpm.
TODAY… I finally found that smoking gun, I found a broken intake valve spring on the number 8 cylinder (thank the G-Man for dual springs on those Dart Pro 1 heads). I replaced the springs adjusted the valves with zero leak down, turned the key and the engine fired and idled to perfection. Exhaust was sweet without the rich burn your eyes exhaust smell from before, happy days are here again!!
The things a broken, weak valve spring or poorly adjusted valves will cause. Keep that in mind when chasing your tail and these drivability issues. I have pictures however my computer has issues but, who care just as long as my Corvette runs well.
Last edited by Vett1990; 01-29-2015 at 09:35 PM. Reason: Pics
01-30-2015, 11:08 AM
#2
Burning Brakes
Vett, did you hear any noise from the engine at all (tapping, etc) while the spring was broken?