BEZ71

i just bought a '81 and it's running very rich. i thought the comp. controls air fuel mix. if this is true how do i lean out the carb. thanks also the choke light stays on what does that mean.


[Modified by BEZ71, 3:39 AM 5/24/2004]

Engage

Check to make sure all connections are plugged into carb.Does your Check Engine light work?There are two screws at front bottom of carb,if it is stock the screws will not be visible unless someone broke the seals that is where you set the mixture.Also check for vacuum leaks.Try spraying carb cleaner to clean inside of carb.If it is still running rich after all that..... you may have bought yourself a problem!!!!! :shocked:

RC81

Lets start at the begining. Is your computer hooked up and working? If yes is your Check Engine Light on?

UKPaul

First thing I'd do is check out the choke. Don't even go near the mixture settings until you've found out if the choke is actually on when the light says it is. If the choke is on then that'd explain it running rich! I had a problem with my '81 where the choke was stuck on & the amount of fuel it burnt in a short journey was so insane that nobody on here believed it!!
:cheers:

Dalannex

I'm with these guys. Check the choke thing out, plus check to make sure that all of the computer controls are hooked up. (Oxygen sensor too) And, are we talking about a stock engine here or has somebody been playing with it?


-Justin

Iskenderian

I had a similar problem with my 81. The engine would run fine, then the check engine light would come on and the idle would drop down to 400 rpm. Everything was connected, motor was all stock, could never get it right,
(If your a keep it all original type, stop reading now)
Sooo.... snip snip cut cut. Got a Q-jet from a 1980. Fits under the stock ait filter housing perfectly. Oh you'll need an HEI (4 pin not 6 pin) usually brand new with coil for $100 or so. I am suggesting this if all other measures fail. I converted mine to a 4 speed so the fact that the 3 speed wouldn’t lock up anymore isn’t an issue. I couldn’t get my computer to cooperate so I ditched it. Car runs great now and passed emissions with room to spare. computer q-jets are shinny door stops imho


Bob
81 4 speed :auto:

Pauld

I had a similar problem with my '81 and the choke was the problem, once that was sorted out, it runs like a dream. :cheers:

dsagers

Time to change the O2 sensor, too

weendoggy

I agree to check the choke first. If it's working, it could be the relay which is located under the shift lever in the console.

The computer does "adjust" the mixture, but all the sensors have to be working too. The ALDL connector is beneath the ashtray. You will short out the far right two terminals to read the "codes" (if any) with the key in the "ON" position. If you're Check Engine light is off it may be burned out too. It should come on when you turn the key on. If it does and then goes off after you start the engine, it's probably working like it should. You still may want to check for a code.

You will get a flash followed by shorter flashes. Code 12 will always show up because the distributor is not rotating. This is normal. Each code will register three times. If all you have is 12, you are fine.

Adjusting the carburator is not like the older ones. It's a very picky thing and you do need special tools and a meter (scanner is better) to read the settings. A vacuum gauge and dwell meter will set the MC solenoid fine. There's a lot more to this and if you have questions, email me and I'll try to walk you through it or at best, give you an understanding of how it works.

Leos 81

Well, since we have an '81 clinic being held :) I figured I'd throw my question out there. My '81 runs good, but I've noticed that occasionally, when I'm sitting idle for a minute or two, my "Check Engine" light comes on. Once I start moving, it shuts off. Then if I sit idle, it comes back on. Any ideas of what can be causing this. By the way, it turns off even when I'm just moving 5 or 10 mph.

weendoggy

First. If the Check Engine light comes on occasionally, it doesn't mean there is a "hard code". Sometimes the system detects a glich and the light comes on. However, the best thing to do is to check for codes now. If none are stored, it's intermittent and not a big deal, but, it could turn into one. So there's no easy answer.

The best thing is to check the system completely. Just because the light comes on doesn't mean the computer or sensor is bad. The engine mechanical could be causing the readings. So, you have to start with the basics. Too many times people "shotgun" the solution only to find it return becasue of something else.

The best and quickest way to check the CCC system is with a scanner. It's "real time" information is very good. Using a volt/ohm digital meter takes time and can be frustrating, but it is doable.

Your issue may be with the MC mixture setting and TPS or a combonation of both. Also, the O2 could be weak sending an incorrect signal as well as the coolant sensor doing the same. There are more reasons but to do it right would require the code that's set, if any. As you can see, there's more to it.

If you don't have codes, I have access to quite a few Driver Complaint charts to guide you through some basics.

UKPaul

Getting the error codes by shorting the pins on the ALDL connector makes fault finding a LOT easier. Be careful when shorting the pins on the ALDL connector as it seems the factory installed some "upside down" so the terminals to jump are at the other side to what the manuals say. The ones to jump together are the end pair with a notch out of the plastic housing between them.
I was getting an intermittant check engine flashing on at me but no codes were stored. After a while i noticed that it only came on after I'd hit a bump in the road, sometimes so small that you'd hardly notice them, other times I'd hit small mountains & it'd stay off. Pulling every connector I could find related to the CCC system & spraying the pins/sockets with electrical contact cleaner has stopped the occasional flashes. Thought I'd throw that in as something to check/do if you're getting any odd errors....
:cheers:

Leos 81

Thanks so much! I'd hate to admit that these checks are beyond my very basic knowledge of auto repair. I will print these suggestions though and take them with me when I bring the Vette to my regular mechanic. It might make his search a little easier, but I'll see what he says before I mention these suggestions you've given me.

If anyone else has any other suggestions, please post 'em.

weendoggy

<<Be careful when shorting the pins on the ALDL connector as it seems the factory installed some "upside down" so the terminals to jump are at the other side to what the manuals say. The ones to jump together are the end pair with a notch out of the plastic housing between them.>>

Don't worry about shorting the wrong ones. You won't do any damage by inserting the "test key" or paperclip in the wrong ones.

UKPaul

Shorting the wrong ones won't hurt anything (been there, done that) but if you're not aware that the connector is the wrong way round you could be scratching your head for a while wondering what all the talk about code 12 is!

It's been a while since I looked at the system, so have forgotten most of it, but if you're moving at 5mph then the engine would be at idle, same as it'd be when stationary. So most engine sensors can be ruled out? Just as a guess, it might be worth checking out the VSS sensor. It's in the back of the speedo & there's 3 wires going to it. These wires have a black connector in them just behind the d.side console cover, somewhere beneath where the defog switch is. Maybe it's playing up?
:cheers:

stickboy

:iagree: Sounds like what happened when mine when bad. You can get a replacement relay for under $20 at the chevy dealer.

weendoggy

UK, I don't think this system has a VSS sensor, I know mine doesn't and we're the original owners. That came in to play a few years down the road.

RC81

Leos81
Make sure your tps voltage is set correctly. I was getting the Check Engine light coming on once in a while but not setting codes. This seemed to fix it. Dont remember the voltage setting but these guys should know!

UKPaul

Ween,
The VSS is in the back of the speedo (both my original 85mph one & replacement 140mph one have it). It's a little reflective "wing" that is rotated by the drive cable. There's a small LED pointing at it & a pickup that picks up the reflected pulses. Going to it are 3 wires from the ECU. One is power feed (8v???), one is a reference ground/earth & the other is the variable voltage for the ECU to determine the speed. It's been a while since I swapped the speedo out, so it's all a bit hazy, but there's definately a sensor in there.

TPS should give 0.56v at idle (aircon off). Need to connect wires B & C together to measure it.

Choke: My choke was full on sometime after swapping out the speedo for a 140mph one. If you can't find a problem with the choke mechanics & it's an electrical problem then it may be a problem with the printed circuit behind the guages (speedo/tacho). It's so long ago that I've forgotten the details, but disturbing the circuit when changing the speedo caused a bad connection on mine. The choke light is in a circuit with the choke heater on one side of it & the alternator(?) on the other. A connection problem in the circuit (broken track, etc) can cause the choke to stay on as there's no power to the heater. Or something like that (memory failure!). It's an unlikely problem, but if the dash has been disturbed it's a possibility. When I had the problem a couple of people said that a dodgy alternator can also give problems with the choke.
:cheers:

Dalannex

I agree that it never hurts to check your TPS. If your TPS is way out of range bad things can happen. Mine broke while inside the carb and was WAY out of range, then my ECM fried. Don't know if the two are related, but I installed a new TPS and ECM and now we have the world by the behind again. :D

And UKPaul is correct about the VSS. It's there, most people just don't know it.


-Justin