Important! Base timing on 81

[alson81]

Does anybody know how to set the base timing on a 81 vette? Is ther a wire to be grounded out or something opened up.

 

[TimAT]

Should be on the Emissions label. IF I recall, there's a connector near the distributor that is disconnected, timing set, reconnected, and then clear the codes that are set with it unhooked.

 

[GerryLP]

In a nutshell, it is like Tim says. You disconnect the 4-pin wire from the distributor (I prefer to wait until engine is warm and running fine before disconnecting. In theory, the computer cannot detect the initial timing set, so it presumes as factory required.

12 degrees with a mild cam works good. The computer is setting ideal fuel/air mixture. Then, reconnect 4-pin wire set.


Good luck!

 

[alson81]

81 base timing

Thanks Guys i will give it a try. I am also having trouble finding a mechanic that knows how to set the carb. I got it rebuilt when the motor was done. I have been told the Throttle position sensor is to be set with a scan tool? Idle mixture screws are set at 3 1/2 turns, my vacuum is around 14-15 inches. Sometimes it idles at 750 rpm, sometimes 1000, sometimes 1200, it also runs a little rough when cruising at 1500 to 1800 rpm. Not sure what is causing this. Any ideas?

Thanks Al

 

[GerryLP]

Thanks Guys i will give it a try. I am also having trouble finding a mechanic that knows how to set the carb. I got it rebuilt when the motor was done. I have been told the Throttle position sensor is to be set with a scan tool? Idle mixture screws are set at 3 1/2 turns, my vacuum is around 14-15 inches. Sometimes it idles at 750 rpm, sometimes 1000, sometimes 1200, it also runs a little rough when cruising at 1500 to 1800 rpm. Not sure what is causing this. Any ideas?

Thanks Al

Al,

I am sure if a professional engine scan machine could (with today advances and all), but you could adjust it yourself. The throttle position adjusting screw is just aft (towards the windshield) of the throttle position plunger and lever. Hopefully you drilled out the plug during refurbishing. But if not, you can gently drill a hole on the center of the plug, and then engage a small sheetmetal screw to act as a handle when screwed in the hole and pull the plug. If you carb kit was missing the new plugs, you can dab a small amount of RTV to tamper protect the TPS adjusting screw setting. Just set the screw to 0.56 volts.

This image shows location.

https://skydrive.live.com/redir?resid=9E77F659449DD457!692&authkey=!AMprShvUiLTwLYk


Here is a cartoon showing the principle behind in making the jumper cable to help you adjust the TPS.

https://skydrive.live.com/redir?resid=9E77F659449DD457!1528&authkey=!ANvGL_9XfmK-Sk8

I hope this helps.

 

[alson81]

81 base timing

Thanks again , tried the timing thing today. The car won't run at 6 degrees, runs fine at idle @ 12degrees, not so great going down the road it hesitates alot . If i advance it to around 20 -22 degrees it seems to run good down the road but i know it should be @ 6 to 12 degrees. Could this have anything to do with my temperature sensor in the intake? The ceramic top is broke not sure if it is working or not. I have a new one but the other one is seized in the intake and i don't want to break it off in the intake.


AL

 

[TimAT]

If the ECM is not seeing the temperature it expects after xx minutes running, the check engine light "should" come on. And the system won't go into closed loop where the ECM is controlling the mixture. If you have a dwell meter, there is a green connector sticking out of the harness somewhere toward the rear of the engine. Hook the dwell meter to that and it should be around 30* in closed loop.

If you don't already have one, the factory service manual is probably the best source of reference for just about everything on your car, and is invaluable for the on board computer system. THe aftermarket books, Haynes, Chilton, etc., just don't have the nedded detail stuff.

 

[GerryLP]

Thanks again , tried the timing thing today. The car won't run at 6 degrees, runs fine at idle @ 12degrees, not so great going down the road it hesitates alot . If i advance it to around 20 -22 degrees it seems to run good down the road but i know it should be @ 6 to 12 degrees. Could this have anything to do with my temperature sensor in the intake? The ceramic top is broke not sure if it is working or not. I have a new one but the other one is seized in the intake and i don't want to break it off in the intake.


AL

Is the sensor on the intake with a wire connected to it or a hose? If a wire, then get the computer codes to see if they suggest a problem with it. The one with the hose to it is a thermal vacuum switch. It waits to reach temperature before it allows a vacuum signal somewhere.

Have you checked for vacuum leaks? Or leaks in general? If you spray WD-40 at the joint between the intake and the heads while the engine idles, and if the rpm climbs, then it is leaking. Spray between the carb and the gasket as well. Check the hoses connecting to the carburetor check the hoses to brake booster, headlights. It is a PITA, but it is the best way to investigate this. You can get a vacuum pump kit, and it comes with adapters for the different hose diameters.

I hope this helps.

 

[alson81]

81 base timing

Hi and thanks again. I can't find and green connector coming out of the wiring harness. The only connector that i can find that is not connected is the one coming out of the tach side of the distributor. The check engine light is not on. I don't understand why my timing is @ 22 degrees in order for it to run decent. I am leaning towards a vacuum leak also because i can only get 15" vacuum max. The temperature sensor is the wired one. I don't have a scan tool or have any idea where to connect one if i borrow one. Where can i get a factory service manual?

Thanks Al

 

[GerryLP]

Al,

The connector is that lime green connector between the air cleaner and the air injector pump.

 

[GerryLP]

....The check engine light is not on. I don't understand why my timing is @ 22 degrees in order for it to run decent. I am leaning towards a vacuum leak also because i can only get 15" vacuum max. The temperature sensor is the wired one. I don't have a scan tool or have any idea where to connect one if i borrow one. Where can i get a factory service manual?

Thanks Al

Al, when you turn the ignition key to the on position (just before hitting the starter to turn), the instrument's lights come on and the check engine light comes on. This is the self test for the check engine light.

Also know that the computer schedules advance spark timing. So by shining a timing light at the dampener mark, and determining the timing, you are seeing what the computer "tells" the distributor and the carburetor to do to maintain the engine running in accordance with a set of values for tge current conditions. So 22 degrees is not rare.

To see what the base or initial timing is, you have to follow the emmisions label setup and disconnect the 4-pin connector of the distributor (the distributor's side of connector can be found by following the four wires coming out of the side of the distributor's base). Then shine the timing light as you would on any other car.

Check eBay for the shop manual.

 

[alson81]

81 base timing

Hi, the 22 degrees is with the the 4 pin connector disconnected. I will try the vacuum test and dwell meter test and reset the timing and adjust the carb . Hopefully it works

Thanks alot Al

 

[Wajulia]

You don't need a scan tool for the 81 vette, Follow the instructions below to see if you can read any tools.


The computer will not go into closed loop operations if the temperature sensor is broken BUT, the car will run fine in open loop. It runs full rich, you will get horrible gas mileage and car will reek of gas, but it will run fairly well. So the sensor should be fixed, but is probably not the issues.

I would not do anything with the throttle position circuit unless the computer is showing the "Throttle position sensor" code

I would put the timing back to 6-12 deg and begin basic debug the problem.

ECM: Is the computer working and what codes does it have

Ignition/spark: Have you removed the distributor lately and has it been put back correctly? Have you removed wires and have they been put back correctly and not crossed? Check to see all the wires are on and undamaged.

CARB: Has the carb ever worked right since it was rebuilt?

The car does not idle?(Dirt in the Idle circuit, low float level) Hesitates on acceleration (Accelerator pump), Stumbles at cruising (float level, clogged fuel filter, bad fuel pump)



Reading codes on an 81 Vette
Pull the ashtray out. You will see a connector, with the key on – engine not running, ground the two plugs on the connector closest to the driver (a screw driver works well). You will hear a clicking in the engine compartment (This is the Fuel Mixture solenoid and it is normal). The check engine light will begin to blink. It will blink the code 12 three times (Code 12 = Blink, pause, Blink, Blink, long pause then it will do this two more times). After it blinks 12 three times, it will blink any other codes saved in memory. When all stored codes are complete, code 12 will blink three more times. If there is nothing in between the 12’s, there are no codes. The definition of the codes is shown below. The service manual has good troubleshooting for each code and there is lots of good help in the forum.
12=No Reference Pulses to the ECM
13= Oxygen sensor circuit
14 = shorted coolant sensor
15= open coolant sensor
21=throttle position sensor circuit
23 =M/C solenoid circuit
24 =Vehicle Speed Sensor
34 =Vacuum sensor
42 =Electronic spark timing
44 =Lean oxygen sensor
44&45 = Faulty Oxygen sensor
45=Rich system indication
51= Faulty PROM
52= Faulty ECM
54 = Shorted M/C solenoid
55 =Ground 8V

 

[alson81]

81 base timing

Hi, the motor was just rebuilt along with the carb i had flow tested and everything was fine. Just talked to a mechanic friend on the weekend and he seems to think the cam is misaligned thats why it wont run right @ 6 degrees but runs fine after 3000 rpm@ 20 degrees. He also said it could also be the reason i can't get more than 15" of vacuum. Thanks for the info on the codes i will check them first, when the driving season is over i will pull off the timing cover and check the cam alignment. Is there is another sure fire way to check this without removing the timing cover? I know i had a difficult time get the rotor to line up with #1 cylinder and thought it still was off a touch. The machine shop put the bottom end of the motor together and i installed the cam, heads,intake and the rest of the stuff so it is possible i didn't get it lined up perfectly. I check for vacuum leaks this weekend and didn't find any. It seems to run the same with or without the computer plugged in. Also i got 5 degrees on the dwell meter. New fuel pump and filter were also installed.

Thanks Al

 

[alson81]

81 base timing

Hi Wajulia, i just tried the retrieve the codes and i am getting nothing. No click, no beep, no light, nothing. Does this mean my ECM is not working. My check engine light comes on for one second when i turn the key forward and thats it.

Thanks Al

 

[GerryLP]

Al,

Check the 10 amp fuse for the computer (upper-right side of fuse panel) if no light what so ever. Also, try it with terminal "E" grounded (under tge ashtray) before turning the ignition to "on" with engine not running.

 

[alson81]

81 base timing

Hi the fuse for the ECM is good

Thanks Al

 

[GerryLP]

Al, the following link takes you to an image of the system's diagnostic circuit check

https://skydrive.live.com/redir?resid=9E77F659449DD457!1581&authkey=!AC4sSf4vq9HUi7E

This other one can help you troubleshoot the engine-check light

https://skydrive.live.com/redir?resid=9E77F659449DD457!1580&authkey=!AM5C4cygjIRWMKM

And this one is for the dwell fixed under 10 degrees

https://skydrive.live.com/redir?resid=9E77F659449DD457!1582&authkey=!AL7_VCjJaDF_PfY

 

[alson81]

81 base timing

Hi Guys my shop manual said 2 far right terminals to diagnose. When I did that i got my check engine light flashes constantly. Now it says to check terminal 21 on ECM for voltage, then depending over or under 4 volts determines checking if ECM 5 or ECM 19 is grounded. Problem is i dont know where to find these terminals are or how to check if they are grounded. Can someone tell me where to find them

Thanks Al

 

[bill81vette]

look under the ashtray,you should see the terminals under there.