Just got done rebuilding the original engine and after I manually timed it, the computer changed the settings. I'm use to other cars, and can unplug the computer, set the timing, than reconnect the computer.

Is it the same when on this '81, and if so, where is the disconnect?

 

Same on the '81 Vette. Set the initial timing (6 BTDC as I recall)
with the ECM harness leading to the HEI distributor un-plugged.
Once plugged back in, the ECM will control the timing based upon
inputs to the system.

I'm going on memory, but the connector is a 4-prong pig-tail deal
generally located opposite the tach and distributor ground connections.

Bela P. Havasreti

 

Thanks. I know the one you are talking about. I'll try that. I know right now, I set it, and the computer resets the the timing to about 34 degrees BTDC.

 

As described above (if you unplug the wrong connector it won't run at all so you'll know when you've got the correct one of the two). Something worth doing is advancing it from the stock 6*BTDC. The computer has no idea at all of the crank position & just advances the timing from the base position that you set it to, so advancing it will advance the entire advance curve. I've set mine to 11*BTDC (apparently 13*BTDC is the safe limit: depending on CR, cam, etc). This has given better throttle response, especially at low rpms, & a generally more responsive engine. If you do get pinging then back it off as would would for non-computer controlled systems. It's certainly worth doing, I felt a nice improvement on mine &, best of all, it's totally free!

 

Found the right plug and set the timing. After re-plugging the computer, the computer kicked the idle up, but it ran okay. The problem I have now is that it is running rich. When I hooked up the electric choke, the choke light and check engine light comes on. Also, when I punch it, I get a puff of black smoke out the exhaust. I adjusted the choke, but can not lean the carb enough to get rid of the richness.

I'm thinking the Q-Jet may need to be overhauled. What do you think?

 

Possibly... You need to find out what the ECM is mad at (what code
is being set). Any number of types of failures can cause a code
to be set, and can cause the ECM to go into open-loop (aka limp-home)
mode and cause the symptom you describe.

See: http://www.corvettedoctor.com/1981_DTC_Codes.html

If you can't scan the code(s) I'd start with the easy stuff 1st.
The thin plastic line going from the rear of the carb base to the
manifold pressure sensor on the firewall can easily get loaded up
with "junk", which will cause out-of-range readings by the MAP
sensor and the ECM will go into open-loop. Remove the line and
blow carb cleaner through it to clear it out.

Also, at idle, you should readily hear the mixture control solenoid (inside
the carb bowl) clicking at 10 times per second. If the MCS has
failed, or the connector leading to it or the TPS sensor isn't secured
properly or has shorted, the ECM will go into open-loop mode.

The computers on the 1980 and 1981 models were fairly simple, and
a methodical understanding, evaluation & testing of each sensor /
component in the system (there aren't that many) should lead you
to the one(s) that have failed.

The E4ME carb tends to scare some folks off, but overhauling one
isn't much different than an earlier Q-Jet.... The difference being
possibly having to replace & adjust the mixture control solenoid
and the TPS sensor to factory specs (get Doug Roe's book if you
plan on overhauling the carb yourself).

Bela P. Havasreti

 

Thanks for the advise. Looking at the carb, it needs some TLC. Since everything is orginal with 87k miles, and I just went through the engine, I should have freshened up the carb before re-installing. If the rebuilding of the carb doesn't fix my problem, I will take your advise and run the test to see what codes I come up with.

 

Good advice above. Also check that the alternator is giving power out (if not it'll cause the choke light to come on), that power is actually reaching the choke coil & that the ECU coolant sensor is OK (it's the one mounted in the manifold on the p.side of the stat).
Cheers

 

I changed the ECU coolant sensor when I rebuilt the engine. The alternator gauge is reading 14+ volts, so I feel that is okay. I will double check it from the back of the alternator to make sure.

I had a freind, who rebuilds carbs, go through mine and will be re-installing it today. According to him, there were problems with the carb, choke, and the high idle kickup for the A/C.

If the carb doesn't fixed the problem, then I guess I need to find someone who can run the codes. All the vacuum hoses have been replaced, but over the years, may not have been hookedup improperly. If that is the case, I will need to find a good souce for a vacuum hose schematic. One that will show me the proper vacuum hose layout.

You guys are great - It nice to be able troubleshoot like this. I have no problems rebuilding engines, bodywork, paint, and interiors, but when it comes down to electrical or vacuum, I'm the worse.

 

If all else fails, you can join the non-computerized '81 club like me haha. Of course, if you have to make it pass inspection... not such a good idea. If you only have to make it pass a sniffer test you can actually get it to run cleaner without the computer, than with it.

That E4ME is a bear and after 25 years, they really start showing their age. If it's running really rich, check those solenoids, because if it doesn't get a signal from the computer it runs at full rich.

 

Installed the rebuilt carb and it helped the richness problem. However, still have the timing problem. It seems the the computer is picking something up from the sensors and the RPM idle is continuously changing. It drives fine and sounds good, but one minute the idle can be 1100, the next at 750.

Looks like my next step is to run the codes or unplug the darn thing. Since I'm in a state with no inspections, it is not a problem to unplug the computer and run manually.

How easy is it to unplug the computer without having the engine light come on?

 

A mixture control solenoid coming on-line and then off
again (intermittent) would cause the RPM to fluctuate
as you describe. Also look for vaccum leaks.

With the air cleaner off, wiggle the electrical connectors
that go to the carb (TPS on the drivers side front corner
area, MCS on the passenger side top). If you hear the MCS
solenoid clicking @ 10 times per second, it's working. If
the clicking noise (and RPM) fluctuates while your messing
with the connectors, something is amiss.

Personally, I think it's easier to fix the sensor problems than
it is to pull the gauge cluster apart to "kill" the check engine
light....

Bela P. Havasreti

 

Snipping a couple of wires isn't that hard.