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So a couple weeks ago I picked up a 1995 LT1 C4 Corvette. 137000 miles automatic. Completely stock. Test drove it and it ran great. Drove it a couple days and everything was great. Once getting it home I was cleaning it up and cleaned the engine. Yes i now know not to get the optispark wet. The next time I drove the car I noticed the idle was a bit rough, throttle response was a little lacking but I could accelerate or power brake it and it would run fine. At times it would sputter in the upper rpm range. Kept the car in a heated garage for almost a week to try to dry it out. Didn't help. So over the last week I have replaced the optispark, optispark harness, plugs, wires, coil, ignition module, iat sensor, map sensor, and maf sensor. Took it for a test drive today and it's still not right. So here is how it acts. Cold start and it fires right up, idles smooth. After it warms up a bit it starts to idle rough. Acceleration is normal up to about 3200. From 3200 to 4200 it feels like it's running on 4 cylinders but not like a typical misfire from a bad plug or plug wire. After 4200 it puts you in the seat. After driving around for 10 minutes it's fine. No rough idle, no hesitation, no nothing. Can launch the car and run all the way through 3rd with no issues. Pull back into my neighborhood, get it the driveway, starts idling rough. Prior to changing anything I could be sitting at a light, running rough. Then all of a sudden the check engine light comes on and it smooths out. Check engine light goes off and idles rough again. Checked codes by jumping the pins and no codes. Ever. Since replacing everything the check engine light hasn't come on but once after my test drive but here again, no codes. I'm stumped. Anything anyone can offer would be appreciated. I'm about ready to start ordering carb swap parts. Thanks
 

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Hate to tell you this but 93 & older GM cars are the only ones you can jump the aldl pins to get codes. Your 95, even though it has a 16 pin obd2 type connector, is still an obd1 computer system. You need an obd1 scanner to read codes.

Hope the opti you bought is a GM, Delco, Delphi, or Msd. If not, you bought junk and it could be bad. Only 100% way of testing opti low & high resolution signals is with an oscilloscope. You can buy a PC based scope for around $70 on Ebay, Amazon, etc.

However, your car running well when cel comes on is a clue. When cel lights with major codes computer reverts to open loop & speed density mode where maf, o2 signals, & other sensor signals are ignored.

So you need to put an obd1 scanner on car rhat does real time running sensor data. See what the sensor data is when car is running correctly. Compare that to when car is not running right.
 

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In 95 corvettes you can jump pin 4 and 12 in the obd2 connector and not only pull codes by using the buttons on the dash but also clear them and run through several different tests. It uses the speedometer display to display the codes. I'm working on trying to get access to the right scanner. Someone on another forum mentioned unplugging the 02 sensors to force it into open loop. Do you know what other sensors are ignored during open loop?
 

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When in open loop the pcm ignores maf signal and uses map signal to control fuel mixture.

I'm a F-body man, I didn't know vette codes could be read without a scanner. How about ABS & SIR codes? Can they be read without a scanner also?
 

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To my knowledge you can read those also. Mines doesn't have any of those lights come on so when I scan through I get --- which means bo codes. When it displays the codes they are obd1 style codes but it will have an H in front for history and a C in front for Current. I get no codes what so ever. When I first got the car it did have a history code for iat but I've since replaced that and cleared the code.
 

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Well after installing the o2 sensors and a 2 hour test drive it's fixed. Drives flawlessly. Thanks for all the input. Really appreciate it.
 

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So a couple weeks ago I picked up a 1995 LT1 C4 Corvette. 137000 miles automatic. Completely stock. Test drove it and it ran great. Drove it a couple days and everything was great. Once getting it home I was cleaning it up and cleaned the engine. Yes i now know not to get the optispark wet. The next time I drove the car I noticed the idle was a bit rough, throttle response was a little lacking but I could accelerate or power brake it and it would run fine. At times it would sputter in the upper rpm range. Kept the car in a heated garage for almost a week to try to dry it out. Didn't help. So over the last week I have replaced the optispark, optispark harness, plugs, wires, coil, ignition module, iat sensor, map sensor, and maf sensor. Took it for a test drive today and it's still not right. So here is how it acts. Cold start and it fires right up, idles smooth. After it warms up a bit it starts to idle rough. Acceleration is normal up to about 3200. From 3200 to 4200 it feels like it's running on 4 cylinders but not like a typical misfire from a bad plug or plug wire. After 4200 it puts you in the seat. After driving around for 10 minutes it's fine. No rough idle, no hesitation, no nothing. Can launch the car and run all the way through 3rd with no issues. Pull back into my neighborhood, get it the driveway, starts idling rough. Prior to changing anything I could be sitting at a light, running rough. Then all of a sudden the check engine light comes on and it smooths out. Check engine light goes off and idles rough again. Checked codes by jumping the pins and no codes. Ever. Since replacing everything the check engine light hasn't come on but once after my test drive but here again, no codes. I'm stumped. Anything anyone can offer would be appreciated. I'm about ready to start ordering carb swap parts. Thanks
Ok had this issue too...mine was multiple...one of the O2 sensors stopped working Other was ok..replaced both... Coupled with a clogged Injector...while at it replaced the fuel filter. If you have a working Opti with a good Led...would only replace the rotor and cap.
 
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