Anybody know of a link (or other info) on diagnosing engine electrical problems?

I have a 1981 (with the computer controlled carb / ignition timing).. I set the timing at 6deg advanced with the computer disconnected from the distributor (by unhooking the 4-wire lead coming from the distrib.)...idle is great... when I re-connect the distributor wires the engine timing jumps to about 22 degrees advanced, and idle is rough...in addition, the timing mark jumps around (ranges about 3-4 degrees)...... I am assuming that since the car seems to idle great with the computer out of the loop, the issue is with the computer or the sensors.... I tried setting the timing with the computer hooked up at 6-degrees BTDC. The timing mark jumps as described earlier - but the car runs OK (rough idle still)... however - It will not pass the emissions test because the timing isn't right (They pull the wires to check timing, and it is WAAAY off).

I had posted a while back thinking I had a vacuum leak of some kind.. but now I think it has little to do with vacuum...

I saw a 1981 Corvette shop manual for sale on EBAY for about $70...anyone know if this is the one I should get for diagnosing the problem... I also found an advanced shop manual for Chevy passenger cars and light trucks, but don't know if the engine has the same set-up in other models or not....

this issue is starting to affect my relationship with this car!!!!!! I wonder if Dr. Phil drives a Corvette?

 

Look at Ecklers for the shop manual, no reason for it to be that high, get the specific book for an 81. Is the Engine light on? That would indicate a code from the ECM so thats where to start. Could be an O2 sensor, vac sensor, have you verified NO vac leaks? You set the timing correctly, but 6* is not enough, it may satisfy emissions but it will run better with 10*

 

the engine light is not on... I have not verified no vacuum leaks, but have tried plugging the vacuum lines one at a time with no effect.

The engine is NEW (crate motor), the carb is new, and the distributor is new...I also relpaced the o2 sensor on the exhaust manifold.

 

If you live in Kalifornia, they will check timing. However, they allow 2*, so I set mine at 8* for testing.
I would try disconnecting and capping all vacuum lines to start. Vaccum leaks will cause these symptoms.(Along with 1,000 other things)

 

My guess is that the balancer has slipped, giving an erroneous base timing setting.The engine should idle fine with 22* computer-aided timing. I see the engine is new....was the old balancer used?

 

Would disconnecting the distributor from the computer affect the computers ability to mix the fuel and measure the throttle position at the carb?

 

Did you re-use the old damper? New one?

 

All new E parts? OK, then its time to go thru the ECM troubleshooting procedure in the book your about to order. In the mean time verify all the vac lines are connected likt the sticker on the underside of the hood says. Check all the connections, listen for leaks. Use an unlit propane torch around all the vac line connections, carb, etc. If the idle picks up you found a leak. How was this car running before you took it apart? Was everything working or is this a project car?

 

That will throw the computer into open loop. (Default)

 

this is a project car...although it ran (Rough) when I bought it (3 shot rod bearings!!).. I did not replace the balancer. if it has slipped, wouldn't it run rough with the computer disconnected as well as when it was connected?

I will try the propane trick (I have tried with starting fluid with no noticible effect). And I will try Eckler's for the 1981 Corvette shop manual.... I don't have a scanner - do I need to get one for diagnosing the ECM even if the light isn't on?

 

You don't need a scanner to download this 1st year computer. I am not sure they even make one. Just pull the ash tray out, connect term D&E together with a short piece of wire. Turn on the key. Ideally you should get a code 12, and nothing else. Let us know.

 

If the computer is disconnected when you set the base timing, you may be setting it to 6* when in reality you may be setting it to 15*,(for example). Then when you reconnect the 4-wire connector, the ECM will add timing based on the timing/fuel table. Too much initial timing will cause a rough idle. The ECM has no way to determine if the base timing setting is correct.

 

OK... forgive me my inexperience... what is a code 12 and what device do i need to read the code?

 

Okay, Back up. When you 1st turn on the key, does your check engine light up? This is how you will read the codes.

 

so in my case.. I am setting at 6-deg BTDC without the computer.. when I reconnect the ECM is adding 16 additional degrees (actually subtracting because it is reading at 22 deg BEFORE TDC).

in addition - I failed the emissions test with the timing set at 6 deg with the computer enguaged AND with the timing set at 6 deg. without the computer enguaged.

 

it is on until the engine starts - then goes off

 

Okay, Pull out the ash tray. Underneath is your adl term. There are 5 terms. A-E, left to right. Connect D and E with a piece of wire. You can even shove a piece of tin in there to connect them, as there is a slot between.
Now turn on the key without starting the engine. The check eng. light will blink.(Should)
Count the flashes and record them.
........blink.......blink blink.............Is a code 12
It will do this three times for each code. Ideally, it will blink a code 12 three times, then pause, then repeat.

 

Explain the "pull the wires" part. What is "way off"?

 

the Guys at the emissions testing station disconnected the four wire lead from the distrib. to check the base timing w/o the computer's influence.

if I set the timing WITH the computer connected I can "force" it to run at about 6deg BTDC...... if I then remove the 4-wire lead, the timing goes to about 10 degrees After TDC.. (the computer advances the timing - without it it goes back to the base setting)...the computer is advancing by about 16 degrees at idle.

 

I will check this out when I get home... I assume the shop manual (which I need to get ) would tell me what each code means, no?