Rough Running 1981 C3 Help!!!
 

Hi all I'm new to this site. I have a 1981 C3 purchased in July of this year. I've been doing many things to it Wheel bearings,brakes,cam and lifters,water pump etc......I recently rebuilt the Rochester E4ME using new float,MC solinoid,tps,rebuild kit ,new throttle bushings. It was running perfect once I dialed the carb in.I have been experiencing a low idle problem.I set it at 750 in D and once it warms up it idles at 500.Readjust and same results. Checked timing, right on (computer unplugged) Today went out for a nice drive since the weather was perfect and nearing winter soon. It ran fine then like you flipped a switch, seemed to lose power and when I stopped it quit! Restarted won't idle,hobbled it back home missing regularly and surging. My two questons are 1. Any idea where the plug it to retrieve the code? 2. Any ideas on whats going on? I hear of others with auto trans C3s removing the computer,what does one do to make trans torque converter lock up? I like the idea of no computer. Thanks in advance!

It's under the ash tray. Short out D&E with a paper clip and read the codes.

Is the check engine light coming on? If not then there shouldn't be any codes. Remember that the computer is your friend. Disconnecting it is only ignoring the actual problem.

Any vacuum leaks? How are the simple things...filters, wires (not spark plug but sensor wires)

Check engine light on. New spark plugs Wires, Oxygen sensor,temp sensors,new feal pump,fuel filter,just changed oil. The computer seemed warm to the touch after making it home. I did not just disconnect the computer,but rather eliminate it and go old school carb. and dist.If I can get the comuter system back without breaking the bank I will do so. Also if I can feel I can rely on it running dependable.No vacuum leaks now replaced all hoses and most anything with a diaphragm.

I'm thinking of going with one of these http://www.summitracing.com/parts/BMM-70244/
and an early carb and vac/mech advance dist. I will check into the cause of this problem by checking the code first. Seemed like dist problem or computer died. I did replace the dist early on after purchasing this 81 to make sure this Vette will be reliable,but it only lasted 20 min. Luckly it died by the auto store I got the rebuilt dist. from and ended up putting my old dist back in.I hope and want to just drive this car and not work on it all the time as I have been since July when I got it. I will see what I come up with tomorrow and will get back.

My 81 did the same thing and it was the phenolic base under the carb that broke creating a massive vacuum leak. Check that .
Laterz
Ken

I will look at that too.......Thanks!

A number of good points above.

In summary, the computer controls the lock up of the torque converter, the air/fuel mixture at the carb and the advance of the distributor. It's pretty easy to build a switch to take care of the lockup, so don't wory about that. If you do away with the computer, you'll have to replace the distributor and the carb. Also, in some states (like CA) you'll no longer be able to register the car because you'll have lost the "check engine" function.

When the CCC system is working well, it works great. When it isn't a lot more effort is needed to figure out where the problem lies. One thing that you said concerns me. What did you do to "dial the carb in?" Because it is essentially self adjusting, there shouldn't be anything in the way of carb tuning so your issue may be in the initial adjustment. The computer system doesn't take over until the everything is warmed up. Any sensor issue will prevent engagement of te computer system too. From your description, I'm wondering if going from running great is nothing more than going from manual (warm up mode) to computer controlled (running mode).

Asy enough to tell:

Before you start your car, turn the ignition switch to "run" position but don't start it. You should hear clicking noises coming from the carb.

Also, when your car started running rough, did the "check engine" light come on? Does it come on before you start it?

What did you set your timing at with the distributor harness disconnected? Did you check the timing when you connect the harness back up?

Last, which cam did you use? The computer is not fond of bigger cams, especially if there is an increase in overlap.

With the above info, we should be able to narrow down the issue.

Hi Thanks! When I dialed the carb in I meant setting the dwell for the MC solinoid. Before starting thecheck engine light comes on. It Would just quit and not idle at all. With engine light on and off then back on and missed like crazy. I did check for code this morn. The light stays on and will give no code with connector grounded.Not even code 12 as the book said it should give to start with. Sounds like computer died to me.I do have a spare and will install later today.
The timing was set ot 6 deg btdc as listed on emission lable under hood.I also used bone stock cam. I don't like hotter cams due to poor idle problems.

Can't disagree with anything you've said or done. Let us know how the computer swap goes.

Swapped it out....solenoids started clicking(throttle step up and the one for egr)once I turned the ignition on. No start,seems to have no spark. I put in old computer, no solenoids.Could a computer take out the pickup coil? Earlier in this post I mentioned I put a reman dist in and it lasted 20 min ran great up till then. At this point I'm really thinking of eliminating the computer and go with a tourqe converter controller and earlier carb and distributor. I will say the computer is great when it works, but if not it's a pain....No matter what I would like to find out what happened.I have a spare coil and cap I'll try in the morning see if it will start.

Short answer to your question: no.

The car will run without a computer. Whe you start it cold, that's exactly what it's doing. I need to think about this one a little.

At least you know your old computer was bad.

Think about it....I will try my new spare coil and dist. cap to see if it changes anything.Thanks for your help and advice!

Assuming that your coil and distributor change didn't do it, I think you should check your base timing again. Kind of hard to do without the engine running, but it can be done. Just turn it over with the timing light on and it should give a close approximation.

Make certain that you have the six prong plug disconnected from the distributor. If I'm correct, it will be way off.

You might want to try starting it with the plug off too. That may eliminate chasing the wrong gremlin as well.

I agree, disconnect the 4 wire plug and see if it will start, verify spark and then set the bast timing to 8* at idle. The 81 has no vac advance or mechanical advance, so if it starts don't rev it much.
If it does start, set the base, then reconnect the computer. Next check for codes and see if the timing changes with RPM.
If it does NOT start, then pull the dizzy and see whats going on in there. You have checked for fuel right?
Once your that far, I would step thru all the parts one at a time, Dizzy setup, carb setup, make sure the TPS and Control solenoid are working, float level and all that.

Well guys,I changed the coil and It started up right away as it usually has. So it was a coil. But now why does the computer not give any code? According to the GM manual it should at least flash a code 12 = system working. It has the old computer in it at the moment. I want to try the spare I had tried last night. It clicked all solenoids on the engine like it was self testing all but the throttle kick up sol. stopped clicking after 10 sec or so. The original one never operated any of that stuff except the M/C solenoid once warmed up and it made the timing retard to 22+ degrees at idle. I will try the spare and run it tommorrow to see if it changes anything. Also does the computer control the cruise control?

Easy answer: computer has nothing to do with the cruise control.

Harder answer: all the other questions.

Let's make sure we're on the same page for computer codes and my apologies up front if you already know this. When you jump the two terminals on one end of the computer connector under your ashtray (the one with a shorter divider between the terminals) you should hear the clicking from under the hood. In a few seconds, the "check engine" light should flash in series of regular flashes with pauses in between. Each series is two digits and repeats three times before going on to the next code.

Assuming that you are doing the above then you should be getting a 12 code and presumably others. That is unless you disconnected the battery to do some of the swaps. In that case, you should only get the 12 code. Realize too that this is a pretty primative computer system with a very limited number of sensors. It will only tell you one of a few issues and the rest you have to figure out on your own.

Thanks! I will try it again in the morn to see if I get a code 12.There won't be any codes now since battery had been disconnected.

Are you sure you have the ignition in the On position and not just accy? Also, make sure you are grounding the correct two terminals to get the 12 code to flash.

I'll have to listen to my engine bay one day with the wife at the controls to try and hear those clicks....didn't know that.:o

Yes I had ignition in on position. I know I grounded the correct terminals,but I will test the jumper lead I was using before I retry.
Thanks for your input!