FELNGR8

Newbie here, first big mod sounds cool but I have a bad stumble or miss under wide open throttle runs. It runs fine otherwise.

I need a muffler and wanted something unusual so I bought chambered exhaust from Sweet Thunder. It runs great under all conditions but full throttle runs from a stop. It stumbles in 1st and 2nd at various rpms from 4 grand to 5 grand. If I don't shift up it will even back off like it quit. If I go through the gears at 3/4 throttle it will go to redline in any gear. Full throttle in 3rd goes to redline with no problems, same for 4th.

I read a reply to someone with a new catback that you have to disconnect the battery to make it learn the new parameters. Is that all it takes, just pull the battey, then reconnect it and go? Is there anything more that needs to be done to set it up after reconnecting the battery?

These pipes sound like a 67 427 with side pipes so I really love them for cruising. But I can't get a full acceleration run to determine if I gained or lost power from the stock exhaust. The best run I got with my Gtech was 13.76, 107 while short shifting at 5500.

It's a stock LT1 with only a K&N filter with 76K miles now. I got a new optispark one year ago and it does not miss at high rpms in any other situation but a full on run from a stop.

Any ideas?

dizwiz24

I know you say you got a new optispark, but that sounds like what happened to me when I had ignition problems eventually leading to the replacement of my optispark. I first replaced plugs, and wires and my hesistation/missing at high RPM was cured 90% of the time. I did notice that even with new plugs and wires I saw sparks arc from the coil to opti wire at the opti inlet jump to the waterpump. Wrapping that end (the to: opti end) of the wire in black electrical tape, and putting a rubber hose over it for good measures eliminated that problem.

Good luck

FELNGR8

Thanks, but how do you get to that wire? From above or from below with the car jacked?

The dealer replaced my opti and water pump when the opti quit, but they did not do anything with the wires. I just pulled the battey trick and made a short run to try it. I get full 6 grand in 1st and 2nd with no miss or stumble, but I had some missing above 5 grand in third. Maybe I have multilple problems here and need to try your fix next. I definitely need the 160 degree thermostat and manual fan switch for the hot weather around here. The day I had my pipes installed was the first day of our 100 degree weather and I can't get a cool day to get a real good run with the car. It runs fine on country road blasts but I just can't get an accel run that is perfect.

silver & red CE

I've installed catbacks on a couple of MAF'ed LT1/4 engines, I doubt is your engine computer, they compensate for increased airflow. If the problem started after you installed your chambered catback, I'd suspect the knock sensors are tripping your computer and it's pulling spark. Get a scan tool and see. If it is the noise of the exhaust system, try the LT4 knock module.

Eric

Duntov

I had the same problem when I had my '92 and it turned out to be the plugs. After they were changed the problem went away.

FELNGR8

Maybe it's the knock sensor, maybe not. I ran it WOT under a bridge and there was no quit in 1st while max loud this afternoon after pulling the battey ground.

Next I will run the car later tonight while dark and look for saprking under the hood. Tomorrow I pull at least one plug and check it. My car was running rich after putting on the K&N filter, so much so that I had to clean off carbon black on the rear panels of the car this spring. (I think that is cured). Next I get a set of plugs and install them next Saturday.

Has anyone had a problem with Bosch platinums on LT4 engines?

nobodyunknown

Could the o2 sensor be the problem?

FELNGR8

I don't think it is the sensors, but the feed to the computer could have been bogus while I was running with my damaged left side muffler, like different between the left and right sides, just guessing. That was when I was getting the excess carbon on the backside of the car. I have been watching for carboning and don't seem to be getting that now, and I ran up 1200 miles on a week of vacation.

I didn't mention that since last fall I have been getting backfiring when I run the car hard for an extended tiime, and this persisted with the new catback. During the short run I made this afternoon I didn't have the backfire, but maybe the egine just wasn't hot enough.

These have been a lot of good suggestions, but first I will do the simplest thing, plugs and wires. Then I will get an extended hard run and see how she does.

I just ordered the red shop manual from Zip so I'm getting in deeper and deeper with this thing. But it is truly addictive! Miles of smiles!

silver & red CE

Cool, soon you'll find the cheapest thing is to hook a scan tool to you computer and see what it thinks is going on. Backfires are a bad sign for your opti. BTW, Jeff and Dal will sell you the manuals for less than $85 shipped to your house.

Eric

FELNGR8

OK, where is the best place to get the scan tool? And where do I find the scan codes? In the shop manual for which I just paid too much? Who are Jeff and Dal, forum mumbers? And how do I contact them?

A more in depth question, does the scan tool read out a series of codes that go back to several failure instances? Or is it just the most recent that has to be read quickly after an anomaly before some other code overwrites it?

I guess these questions will be covered somewhere in the shop manual when I get it. I have been confronted by a gazillion shop manuals in my career as a computer hardware/software tech, but a few well asked questions are always a good head start. I'm never afraid to ask.

My opti has been replaced twice, once by the previous owner after the water pump leaked, and once by a dealer after the car quit on me on the Skyline Drive and was towed off the mountain. The second opti is now two months off warranty. Are they really this troublesome?

Thanks, Eric.

Chuck

silver & red CE

Jeff and Dal are parts guys working at GM dealers. They'll sell you GM parts for 40% below dealer list (shipping not included). Totally legit/legal, GM can't set prices. Jeff is a memeber of the forum (18carfan) I don't have his phone number, but here's his email:

parts@superchevyperformance.com

Dal isn't a member here, he hangs out on the Camaro/F-bird and Impala SS forums. I met him through those. His phone number is 800-362-9494, ask for Dal (not the parts department).

As for scan tool, I have a 96, it's OBDII, different from a 94 (OBDI) so I'm not sure what's the best for you. Some of the scan tools use a lap top computer, you just hook a cable from the diagnostic port to the laptop. Others are handheld units. IMHO, it's pointless to try to fix a computer controlled car without one. And yes, after you pull the codes, your field service manuals (FSM) will help you understand what they mean. MY 96 FSM has step by step instructions with flow charts for each code. I'm a techy type also (EE) and I find them to be easy to use. I also have a Haynes manual, sometime Haynes has some shortcuts that don't require a $500 tool. The FSM always seems to call out specific dealer style tools that I'll never buy.

A scan tool reads codes SES codes, but the good ones will let you see the actual sensor readings in real time to help you diagnose the problem. Some problems don't set codes (like knock sensor reading, spark retard, O2 readings).

The opti-spark is the the weak link of the LT1/4. I think the 94 Vette has the older, less reliable, more expensive opti. In 95 the Vettes got the cheaper, more reliable opti. If it is your opti (I'm not saying it is) you might want to consider upgrading.

Welcome to the forum, Chuck. :cheers:

Eric

ConeKLR

Have the same issue everytime I disconnect my O2 sensors for a clutch job or exhaust work. Have not figure out what's going on on mine. I do get SES codes saying no active cirkt for O2 sensors and crap like that. Bought two new sensors two weeks ago. Car ran great for only two weeks, no running like crap again.
Could also be my cat is getting clogged. Who knows. :crazy:

FELNGR8

Problem solved, or at least defined!

I filled it up with a full tank of Sunoco 94 and it goes WOT in all gears to red line with no hesitation. I could probably benefit from a pair of LT4 knock sensors, but when I want to run I just need to fill up on the best stuff. Normal Exxon is fine for cruizin, but not for a full-on run.

Now if we ever get some cool weather I have to go to the track to see how it runs.

95CYC4

If you solved your problems with a tank of 94 you got some serious issues that I would look into before any racing is done. This sounds like you got knock, for whatever reason. Installing the LT4 ESC(knock sensor) into your PCM will only hide the knock, and you will wind up with a detonated engine. Find out why you are getting knock.

One item that instantly comes to mind is the K&N filter you installed. I don't know how much oil you put on that filter, but it has caused countless numbers of rough idle conditions, as well as loss of throttle repsonse and breaking up at WOT. Please clean your MAF with some electrical cleaner. You will have to take it apart to do a good job, but it will help if you got any oil from the K&N on it, and if it hasn't been cleaned before it won't hurt.

The less air your MAF reads, the less gas you will get injected, and this can cause that breaking up at high rpms due to high cylinder temps and low A/F mixture, and the higher octane gas is only covering up this problem due to the knock reducing properties of higher octane fuel.

With your new catback your PCM will learn the new setup after you drive it around for awhile like stated above. Since you are freeing up the exhaust I am sure that you are sucking in more air, so a dirty MAF won't help. Corvette LT1s are also suppose to run on premium octane(92 and higher), so i'm not sure what the Exxon octance rating you are using is, but I hope it's at least 92. HTH

vette92_1

I have a question for somebody. My 92 has a built in diagnostic center, that is displayed in the same area as the speedo/fuel/volts ect. all I had to do is loop 2 of the wires on the diagnostic plug under the driverside dash and it put it into diagnostic mode, it displays any codes that may have been triped, then of course all you have to do is corspond that back to your manual to findout what that code means. My question is, Why do you need a $500.00 Hand held diagnostic tool when you can just use the one that is built into the car?

FELNGR8

The Exxon I'm buying is 92 octane, but summer grade (emissions special?) and the ambient temps have been 95 + most of the time. The Sunoco 94 is . . 94. The Maf could be coated with oil, and that is the easiest thing to get to..

When I installed the K&N I did not spray on any extra oil, but shortly afterward I had to get the emissions check and had a devil of a time getting it to pass. At the time my mechanic says it checked out normal but the reading I got was typical of an engine that got too hot Which was certainly true. The first time through it overheated waiting in line, the second time it was a cool day but the checkers did not put the fan in front of the car. The third time I squacked and demanded that I ride along to watch the temps and a supervisor ran it through the cycle. He had the car in 6th at 45mph and coasted down in neutral. I didn't point out to him that he probably didn't correctly drive it through the gears for the cycle (LOL).

It sounds like I have to go through the mild mod cycle. 1) check the MAF. 2) do the 160 degree therm, 3) get the manual fan switch to use for the next emissions test, and in stopped traffic. 5) install the throttle body bypass. 6) clean the radiator shroud area. 7) buy the OBDI scan tool. 8) get a ride-along engineer to check the codes while I run (would a Vette Babe work for this?) 9) Get the speed shop to check it out thoroughly BEFORE they tear it down for the top end build up I was planning for this winter.

Or, 1) just cool it and drive normal for a geezer of 58. It drives fine in normal use.

Man these things have some complex sensor feedback systems! No wonder good Vette mechanics are rare.

FELNGR8

Go back in the thread to what silverr and red ce posted. You can jumper it and read static codes. But some problems give only transient codes and to capture those readings you need a scan tool, some times used by a ride along pardner while you drive.

What would it cost to have a Cart rig that lets you hook up a wireless link from a scanner to a server in your house or tow rig and let you do a drive by scan?

65Z01

Hey, take it easy with that "geezer" stuff, you got pretty close there. :D

Anyway a low temp stat would likely your situation. To check it out watch your coolant temp and try a copla passes when relatively cold and again after up to "normal" temp.

Also, after increasing air flow you may need to install an AFPR to bump juel pressure a little. It may be leaning out on top end since the ECM doesn't monitor the O2 sensors at WOT.



[Modified by 65Z01, 5:10 PM 7/30/2002]

Duntov

Is there a particular cleaner that works best? What have you used, was it an aerosol?

Kdrhpe

Can you give any more specifics on how to do this????