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LT1-LT4 Modifications
1993-97 Gen II Small Block V8

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Old 04-24-2007, 04:05 PM   #1
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Default Running Bad after Warm Up!

My car is running rough after it warms up. It starts out running great, then after about fifteen minutes I start noticing a slight surge when coming to a stop. Not long after the first surge, it will develop what seems to be an ignition miss at idle. At first it is very slight and will go away as soon as I touch the gas. The warmer it gets the higher the rpm's have to go for it to go away. It continues to get worse until it literally jerks me back in the seat when it stops missing. If I drive far enough it starts missing so bad that I can't pick up speed in fourth with the converter locked. If I park it for a few hours it runs fine for a few minutes, then starst the same cycle over again. It is not setting off the SES light.

The car is a 94 A-4. I just installed pacesetter longtubes, ory, and a 3000 stall converter. I also deleted the air and egr. The plugs are new. The wires are fairly new and routed safely around the headers and pulleys. I have double checked this. It is also running through a 12'' Dynomax race Bullet muffler, dumped before the axle.

Any suggestions will be appreciated!
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Old 04-24-2007, 06:11 PM   #2
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You checked the opti out or replaced it ever.... it is a 94 splined driven.... maybe start there?
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Old 04-26-2007, 10:32 AM   #3
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Lt1junkie, I've never touched the opti. I just bought the car late last summer, so I don't know if it's ever been changed.

Here is a little more info. I drive thirteen miles to and from work. It is just starting to act up in the last mile or so, in each direction. If I go beyond that it just gets worse. If I stop, even for ten minutes, it will run normal when I start out. If I let it set for an hour, I'm good to go for thirteen miles. Still no SES light and it runs great at WOT, even when it is acting up at lower engine speeds.

It feels like an ignition problem, but if it were mis-firing shouldn't that cause the converter to unlock when it is happening?

If it were a fuel pressure problem would that set an SES light?

Could the higher underhood temps with the headers somehow cause the fuel to overheat?

Again, any suggestions are appreciated. I know it's difficult to diagnose drivability issues over the internet. If I can't figure it out, I'll be spending my drag radial money to have a shop diagnose it for me, or I could just limit my travels to under thirteen miles.
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Old 04-26-2007, 10:52 AM   #4
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I'd look at the O2's while on a scanner. If it's running ok while it's cold, then sounds like you have given it enough time to warm up and then go into closed loop. At WOT, I believe O2's are bypassed anyway.
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Old 04-26-2007, 11:35 AM   #5
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Opti...my corvette did very similar thing...ran good at WOT - very poorly at low rpm and idle. Started out slow - progresively got worse over about 2 weeks maybe. Mine was throwing a knock sensor code though. I changed them (2 on the vettes) - but of course it did nothing to help. Finally dove into the opti...that was it. No gaurantee..could be something else - but your symptoms are very similar to what mine was doing from what I remember (this was a couple years ago....so take it for it's worth).
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Old 04-26-2007, 11:46 AM   #6
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dhdenny, I might have to bite the bullet and get a laptop, cable, and scanning software or pay someone to scan it. I guess it's the price you pay when playing with a computer controlled vehicle.

It should be well into closed loop by the time it starts acting up. A thirteen mile drive around here takes between 40 and 60 minutes. I unplugged the egr vacum solenoid so it would set off the SES light and stay in open loop. The only change that I noticed was that both fans ran all the time.
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Old 04-26-2007, 11:53 AM   #7
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Quote:
Originally Posted by Razor_Blade
dhdenny, I might have to bite the bullet and get a laptop, cable, and scanning software or pay someone to scan it. I guess it's the price you pay when playing with a computer controlled vehicle.

It should be well into closed loop by the time it starts acting up. A thirteen mile drive around here takes between 40 and 60 minutes. I unplugged the egr vacum solenoid so it would set off the SES light and stay in open loop. The only change that I noticed was that both fans ran all the time.
OK well that eliminates that. In my neck of the woods, you can do 13 miles in less than 10 minutes so that's the line I was thinking on. The laptop and cable is a great investment, I use mine all the time.
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Old 04-26-2007, 12:04 PM   #8
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Quote:
Originally Posted by KTM
Opti...my corvette did very similar thing...ran good at WOT - very poorly at low rpm and idle. Started out slow - progresively got worse over about 2 weeks maybe. Mine was throwing a knock sensor code though. I changed them (2 on the vettes) - but of course it did nothing to help. Finally dove into the opti...that was it. No gaurantee..could be something else - but your symptoms are very similar to what mine was doing from what I remember (this was a couple years ago....so take it for it's worth).
Thanks for the input, even though that's not what I wanted to hear. I know the opti can go bad at any time, but the timing strikes me as a little odd. Everytime I have had a problem show up after making changes to a car, it ended up being related.
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Old 04-26-2007, 12:27 PM   #9
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You're definately right....95% of the time - what ever you touched last is what is screwed up and giving you problems. Other than a wire issue related to being burnt from headers - or a broken plug from installing...I don's see anything that you have recently done to cause this type of reaction.

I pretty much did all the same stuff to my car and have no problems (emissions delete - headers, off road y)
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Old 04-26-2007, 04:16 PM   #10
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Quote:
Originally Posted by KTM
Opti...my corvette did very similar thing...ran good at WOT - very poorly at low rpm and idle. Started out slow - progresively got worse over about 2 weeks maybe. Mine was throwing a knock sensor code though. I changed them (2 on the vettes) - but of course it did nothing to help. Finally dove into the opti...that was it. No gaurantee..could be something else - but your symptoms are very similar to what mine was doing from what I remember (this was a couple years ago....so take it for it's worth).
One thing I wanted to note was that one of my opti failures was just the opposite. It would run fine down low and start breaking up above 4000. A dyno chart looked like an EKG. The first opti failure I had came and went for a while before it went out. Like the car would run a while then die. Finally one day going down the road it just quit and never started back. Didn't really miss that first time. Very odd creatures these are.
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Old 04-27-2007, 12:33 AM   #11
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when my opti started to go out i replaced it right away. the only thing it was doing was a slight miss at around 4800. pulled my hair out thinking it was the wires/plugs never acted up in any other rpm. only wot around 4800. Also i wasnt even using the cap and rotor so it was the optical sensor assembly in the opti that was bad
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Old 04-27-2007, 01:44 AM   #12
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sounds like opti too.. but warmed up? dunno... 02's then...
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Old 04-27-2007, 06:12 AM   #13
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Quote:
Originally Posted by i420tom
when my opti started to go out i replaced it right away. the only thing it was doing was a slight miss at around 4800. pulled my hair out thinking it was the wires/plugs never acted up in any other rpm. only wot around 4800. Also i wasnt even using the cap and rotor so it was the optical sensor assembly in the opti that was bad
Yeah optical was what went on mine that last time.
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Old 04-27-2007, 09:13 AM   #14
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I had the same problem on my 96...and all it was was that i put those O2 eliminator things on...and it ran like crap down low...would surge and miss when lugging the motor...i took those off and its never done it before...
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Old 04-27-2007, 10:56 AM   #15
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I just spoke to a local Tuner who is also a sponsor here. I set up an appointment for him to look at the car. Based on the my description, he suggested changing the O2 sensors before bringing it in. They look to be pretty old, mabey even original to the car, so I'm going to give it a shot.

Starting tomorrow, I will be out of town for a week. When I get this resolved, I will post up whatever the problem was. Thanks to all, who threw out some suggestions!
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Old 04-27-2007, 11:33 AM   #16
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If it runs good at wot and while cold, I'd be 90% sure its O2s as those are the only times when the O2 reading is not used by the computer. I really doubt its the opti if you don't have problems at high rpm, that's usually where problems with the opti will first become apparent.

You can do an easy test though that will let you know whether it is an O2 problem (so you don't replace them if it isn't). Just unplug one of the O2 sensors and drive it that way for a while. If the computer doesn't get a reading from both sensors it will not enter closed loop. If the problem goes away under this condition then you know it is the O2s. If it behaves the same way then it's likely something else.
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Old 04-27-2007, 02:29 PM   #17
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Quick question. I was looking up the O2 sensors on the web and noticed references to the Left and Right sides, but the part number was the same. Are the sensors specific to a particular side of the engine?
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Old 04-27-2007, 02:50 PM   #18
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nope. Same sensor.
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Old 07-10-2007, 11:14 AM   #19
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The results are in. The new AC Delco O2 sensors seem to have resolved this issue.

Thanks to everyone that offered suggestions.
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Old 07-10-2007, 11:14 AM
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