I’m new to the corvette world and recently purchased a 1981 4 speed with 45000. One of the first things I have done to the car is remove all the smog equipment and run dual exhaust with no cats. My car smells as thought it is running rich, and I was wondering if the computer was actually working. The check engine light comes on momentarily when I start the car, but my interior light delay and factory alarm do not work and I think that’s all tied together. Any help?

 

The ECM is not tied into the interior light delay or the alarm system.

You need to check the trouble codes stored in the ECM memory. If the car is running rich, the mixture control solenoid, throttle position sensor, or oxygen sensor may be bad. To check the codes, pull the ash tray out in the console, the diagnostic plug is under it. Ground the two left pins together while the igintion key is turn to the run position. DO NOT start the engine just have the key turned on. You should hear a clicking noise coming from the carb, this is the mixture solenoid cycling open and closed, a good sound. Once you ground the two pins together, the check engine light will start flashing. All codes flashed are two digits in length. It will flash the first digit and then the second digit. Each two digit code is flashed three times before the next code is performed. The first code you see will be 12 for the start of the diagnostic sequence. It will then start outputting the trouble codes after this. It will then flash the 12 code agian to signify the end of the sequence.

Post your trouble codes and this board can decipher them for you if needed.

 

Welcome to the forum Jeremy. Good to see another Tennessean.

Dave

 

Welcome to the Forum :D

I have no computers in my cars so I can't help you, but somebody will.

Good Luck

 

'81/4-speeds are good! :yesnod:
like brian said, pull the codes and get back with us. we can fix it--we have the technology.
the only thing i would say is be careful with the ALDL terminal (under the ashtray) the factory wasn't real consistant when installing these things and quite a lot have been confirmed to be installed up-side-down. jumper the two pins that have a notch in the separator.
BTW, your car can run just fine with no smog crap (and even pass a sniffer test, i know) so don't let people talk you into junking the computer. :chevy

 

OK I finally got an opportunity to run the codes and I got no response, I heard the clicking from the carb but the check engine light just stayed on constant. Now what? All my smog equipment has been removed so do I need the computer? When I got the car I had a feeling that it was not opening the carb all the way up and when I was messing with it today (after I assumed the computer was not functioning) I saw a vacuum switch that appears to run in-between the vacuum kick down and the vacuum source. So I bypassed it. Is this right? I appreciate all your help it always makes things easier when you can discuss things with others

 

sounds like you have a similar problem to mine. the diagnostic circuit in mine is dead. it won't even illuminate the check engine light but i know the computer is working. what you described would also indicate the computer is working but what happens is that the M/C solenoid cycles the jets closed. if you had a non-functional computer or M/C solenoid, you would not hear any clicking. my guess is that you have (besides the computer) a bad TPS, MAP or O2 sensor.
i have a question out now to corvette magazine about upgrading the computer to a later, more reliable unit. hang on and i'll let you know when i get a response.
you do not need to run the computer but if you don't, you have to replace both the carb and the distributor since they are both controlled by the computer. i hear about alot of people who yard their computers out but i just wouldn't.
i don't know what that vacuum switch was or where it was located by your description. i don't know what vacuum kick-down your referring to since yours is a 4-speed car. the only thing that comes to mind is there is a valve that controls vacuum to the EGR. is that what your talking about?
what you might try first is to reset the computer by disconnecting the battery for a minute and then drive the car. you will need to drive it untill it gets up to temp for it to go into closed loop. (hey, that's another thought. what temp does it run at? if someone changed the t-stat without changing the computer chip, it won't go into closed loop and it will constantly run rich) see if the check engine light comes back on and test again for a code.

 

I’m not sure but don’t I have a vacuum that activates my carb secondary? That’s the vacuum line I was talking about. I know what TPS stands for but I’m not familiar with MAP or EGR. Where is my o2 sensor located? Because my smog equip is off will that not confuse the computer and hinder performance

 

I would think that removing the smog equipement could be the cause of the smell. Prior to emission equipment the smell of gas and oil was common with the old cars. The emission equipment recirculated that "stuff" and cleaned up the smell of the car. I have a feeling that no mater what adjustments you make, that smell will not go away.

tom...

 

It is pretty strong though but you are probaly right, the car seems to run pretty good

 

MAP: manifold absolute pressure. small sensor located under a sheetmetal bracket that is bolted to the firewal just inside of the power brake booster. the bracket covers the sensor so you hardly even know it's there unless your looking for it. it has a three wire plug and a vacuum signal that comes from the back of the carb to it. if it is malfunctioning or if the vacuum line is broke, it will cause the car to run very rich.
EGR: exhaust gas recirculation valve. located on the passenger side of the intake manifold just above the #6 exhaust valve. if ALL your emissions stuff has been removed, you will likely have a block off plate there.
the O-sensor is located on the driver side down pipe of the exhaust just after the pipe meets the header collector. it's a simple two wire sensor, they cost about $25 from most auto parts stores. you should just figure on changing it about every 3-5 years depending on how often and how hard you drive the car. with the engine cool, pull it out and look for excessive carbon build up on the tip.
there are several things you can do to lean out the mix but the biggest disadvantage of the early computers is they tend to always run rich at idle. i've set a stochiometric gauge on mine so i can monitor the function of the computer and i've found that at idle the computer makes no corrections. so you can lean out a mixture setting that might eliminate the raw gas smell but it may also adversely affect the off idle mixture.
what other mods have been done to the engine? is the exhaust still the factory system? have you adjusted rockers and are they rollers?
the one thing about the computer is that when it malfunctions or one of its sensors goes south, the computer goes into 'limp' mode. it sets base timing at, i forget exactly, but the computer does not dial in any advance. it allows the carb to run rich because rich is more engine friendly than running too lean. it will be substantially down on power and consume a whole bunch of fuel.
when my MAP went goofy on me, my mileage went down to like 8mpg. if you want to verify the car is functioning correctly (aside from some tuning). top off the tank and take it for a trip this weekend. you should get about 300+ miles per tank on relatively flat ground. if the computer is toast, that number will be more like 100-150 miles to a tank.

 

I removed the smog pump myself and had the cat cut off and dual exhaust ran. The EGR valve is still in place, no plate but I bypassed the electric unit in between the vacuum line from the EGR valve to carb. I though it had something to do with my carb secondary. What does that vacuum line do or go to? I don’t have much knowledge about carbs and it’s even harder when you throw in computer controls. Should I lose the EGR valve all together? The car gets around 200 miles to a tank so I’m just stuck in the middle. If I do choose to remove the computer will it hurt the value of my car extensively?

 

if it's not in use, i would suggest you remove the EGR only because you cannot remove the valve cover without removing the EGR. that means off and on, off and on, off and on (at least in my case), just a real PITA. the EGR doesn't affect the carb, the carb supplies the vacuum to the EGR.
personally, i would discourage you from removing the computer just because when it is working, it means low maintenance and better fuel economy. having said that, if you want to remove the computer, i don't think it will seriously affect the resale value of your car. just take the computer, carb and distributor in a box and stick it on a shelf in the garage. in my opinion, removing the computer is just going backwards. kind of like pulling out an HEI distributor and going back to points :U

 

I've been following the exchange between you guys and I have a question I thought I would post for all of the '81 owners floundering around out there without aftermarket support.

I had to get rid of the stock headers and put on a set of Heddman 300 long tubes. Both stainless headers were seriously cracked and I only had one replacement. (For Sale) So the AIR system is history. The car is registered in Vermont so smogger stuff is not a problem because there are no emission inspections. As soon as the AIR pump departed the motor the motor woke up. That, and I had a clogged cat which caused the car to dangerously and suddenly lose power on I-95 during ruch hour. The car sat for 8 years and mice had inhabited the air cleaner, the exhaust and the back arm rest, doing real funny things by chewing the wiring. I don't have the cogiloni to remove the (new) stock Y pipes and I have replaced stock with Flowmasters and a free-flow cat. But I hate that damn carb.

It seems to me, after reading the factory manual, that the computer only controlled the smog stuff. That being so, the electronic carb seems to be a waste of time with the rest of the system gone. I have an Accel Blueprint distributor and that alone was a cheap fix. The car may not be too much stronger, but it is noticeably smoother.

Question: Can I use an MSD box in place of the computer and an Edelbrock Q-Jet replacement in place of the electronic carb? It seems to me that if you 1: don't need computer controlled carb preheat, 2: don't need computer controlled warm-up timing, 3: don't need computer controlled cooling water distribution....why do you need computer controlled mixture??? I mean, that computer seems to have been mandated by the Feds to control warm-up and idle emissions. Then too, it is well known all Chevys run happier when cool. So why make a Corvette run at 190? To clean up the air? Please... Well tuned, I'll bet the car will be cleaner without all that dysfunctional stuff.

I see all of the input and output terminals in the factory manual chapter on the Command Contol Computer. All it does now is control timing and mixture. Does anybody know what is involved in replacing the box with an MSD box?

I wanted to keep this 32,000 mile almost-a-virgin stock. But GM gave up on Corvette after 81-82. Who really wants a stock '81 rated at 190 hp even though it had all the trick stuff allowed by law? I think the best way to improve the resale value of an '81 or '82 is to make them faster.

With that in mind, the first owner replaced the stock cam with something just a little bumpy (I don't know what it is, but it is mild) and a double row timing chain because a lifter had collapsed from sitting. I really did not have an economically viable choice but to go to headers. Why not go to an MSD box and a Q-jet Edelbrock? I have experience with that carb and I like it. What think you? The body is about to go back on the frame and the interior is all over my living room.

 

i suppose to each his own. i like my computer because i have had reasonably good luck with it. i have heard of '80 guys (L-48 and L-82) guys that only get 15-17mpg on long cruises. mine will get 20-22 and i know mine is as fast as an L-48, with the other mods i've done mine is as fast or faster than most stock L-82s. having said that, i'm always looking to go faster. by the same token, i currently am required to pass a smog test (sniffer and visual) annually.
i agree with you that all the federally mandated absolute BULLS**T doesn't necessarily mean that your engine will run clean. i also agree that you can run a clean engine without all the garbage. i REALLY don't want to get on a rant about smog laws right now. mine has no smog equipment on it at all and much to my surprise it passed the smog test when i first moved to washington state, even though i didn't do any tune up beforehand. down here in las vegas, however, they do a visual inspection too because it really doesn't matter how clean it runs (don't want to get started) as long as it has all the :cuss stuff on it.
as far as chucking the computer carb, it's your call and i don't think it will hurt anything so long as you replace the distirbutor too. i don't know if the edelbrock will give you any more power but if that is what your comfortable, then i think you should go with it. the q-jet that came with the car is unusually large considering it's a smog era carb. you would think that a smog era carb would only be like 600-650 cfm but the 'compu-jet' is rated as some insane number like 780-795 cfm so it can actually support some serious hp numbers.
if you don't have to deal with the smog, then chuck that y-pipe while your at all this. if you have headers and flowmasters, then the only bottleneck is the y-pipe. especially with a non-stock cam, it wouldn't hurt for it to breathe. when i went from my single pipe, with gutless cat to the true duals, i could tell an immediate difference. i didn't pick up 50hp or anything but it did free up the engines ability to bring up RPM's and i probably gained 10-15hp.
i would agree with you that more power is more value. then again, i think that people like us are being bought out by the waxers and it won't be long before they start looking for cars like ours. all corvettes appreciate, and so will ours. the more original it is, the more valuable it will be. then again, that won't mean anything to you if you don't plan on selling (like me). just ask the guys with mid-years that were made into racers. unless there is some historic racing involved, they often get snubbed by the restorers for destroying a classic. that is a whole 'nother rant right there.