car chugs under 50% throttle
08-10-2008, 07:16 AM
#1
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Location: baycity, mi
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car chugs under 50% throttle
got the car back running, i had to replace a total of 5 bad valves (2 in pass. head, 3 in driv. head) car starts up fine but once your driving it feels like its loading up bad! but you give it more than 50%throttle and its like nothing is wrong. car has a new opti, new MSD coil, new ICM, new taylor wires, new NGGTR6 plugs all have 22 miles on them. car is also equipt with the GM847 cam with a madZ28 tune. i do have a wideband and it is pegged past 18 unless im over 50% then it goes right to 12.7-13.5. after you drive it at all it will not idle, but if your turn it off and start it back up it idles.
08-10-2008, 08:10 AM
#3
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Join Date: Dec 2000
Location: Chattanooga, Tennessee
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How old are the O2 sensors? Mine did the same thing as I would pull out, but then once I got into the throttle, it would smooth out. Turned out to be a bad O2 sensor. If your wideband is showing 18, sounds like the PCM is pulling out fuel making it run too lean which would account for the performance until it switches back to open loop making it run ok. Sounds like the PCM is getting misinformation from the O2 sensors.
Last edited by David94TA; 08-10-2008 at 08:16 AM.
08-10-2008, 08:28 AM
#4
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Posts: 328
got the car back running, i had to replace a total of 5 bad valves (2 in pass. head, 3 in driv. head) car starts up fine but once your driving it feels like its loading up bad! but you give it more than 50%throttle and its like nothing is wrong. car has a new opti, new MSD coil, new ICM, new taylor wires, new NGGTR6 plugs all have 22 miles on them. car is also equipt with the GM847 cam with a madZ28 tune. i do have a wideband and it is pegged past 18 unless im over 50% then it goes right to 12.7-13.5. after you drive it at all it will not idle, but if your turn it off and start it back up it idles.
08-10-2008, 09:32 AM
#5
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Location: baycity, mi
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when we fixed the valves i replaced them with the same style valve that was in the head, manley stainless race series valves, i had them done at a machineshop so they could check the seats. The tune has been in the car for ohhh 3 years or so, but it was sent to me to be tuned for the 847 with a std bore LT1, the car now has a 355 with slightly higher compression but the same cam. il try replacing the O2's and see what happens, its only another 100 bucks 
08-10-2008, 09:34 AM
#6
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idk man i dont think its o2's mine are new and its still doin it. New opti plugs wires etc i cant believe no one on here can help out with a tune..I bet its in the tune. I have a 95 Z 383 lt4 and it doesnt do it all though it has a slightly smaller cam.
08-10-2008, 09:38 AM
#7
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Location: Chattanooga, Tennessee
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I might add, check the wiring going to the O2 sensors. A while back, I had a couple of wires touch the header and melt together. Totally messed up the readings going to the PCM. We had a hard time diagnosing that.
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