After spending most of last spring going over my carb chasing a lean condition problem (it turned out to be a defective new O2 sensor that I changed first) the computer gremlin has struck again. I just took my car out this morning for the first ride after being in mothballs for the winter and a new problem has shown up which I think may be ignition related.

When the car is cold in open loop operation it seems to run ok. When it warms up and goes into closed loop it is difficult to drive at a constant speed. It feels like it is losing power so you give it a little more pedal and it accelerates. You can go WOT and it pulls great right up to the redline, but you can't maintain a constant rpm at cruise. Idle is also about 200 rpm lower than normal.

Now the wierd part. If I shut off the engine and restart it immediately the idle comes back to normal and the car will easily maintain a constant speed at cruise. However, if I go WOT it barely pulls at all over 3500 rpm and struggles to get anywhere near the redline in 2nd gear ( I have a 4spd). It stays like this until I let it cool off and go through the whole process again.

I'm pretty confident it is not a carb issue, so I'm thinking it sounds like the Electronic Spark Timing from the computer where it is getting excessive advance when it first goes into closed loop, and then none after I reset it by restarting. I have access to a shop manual, so when my frustration level drops and I can get the manual I will dig into it.

If anyone has experienced this or has any suggestions of some other cause I would appreciate the help. I know, a new carb and dizzy and dumping the computer will solve this, and if it's not a simple fix that may be in my future.

 

Have you tried to read the trouble codes?

It kind of sounds like your timing is wandering. Make sure that everything is attached tightly. It also might be a temperature sensor going which would have you going back and forth between closed and open loop. That should throw a code.

 

My check engine light doesn't work so I can't read the codes. The bulb is good, and the wiring checks out, but the ECM doesn't turn it on. It would make my life lots easier if it worked. Another thing tempting me to dump the computer.

I just got my shop manual back and you may be right about it being a sensor. The EST controller receives manifold pressure, barometric pressure, coolant temp and RPM signals. If a sensor signal is off it can run at "base timing" based on rpm. Looking at the troubleshooting charts, if it's not a sensor it is either the ECM or HEI module. I'll dig into it after the weekend.

 

Even if the light doesn't work, try running the diagnostic in a quiet place. If you don't hear a series of clicks from the solenoid on the carb, then your computer is probably toast.

 

bite the bullet and put a nice 650 holley and msd dizzy in it and it will be all smiles.

 

jr150

Is any of last years fuel still in tank? I took my C3 out today and it is running rough too. Have you run a timing check?

 

$79.00 for my computer at auto zone. 1 year warranty means i had a long time to sort it out........hope it helps. its drive around weather..bryan.

 

Is any of last years fuel still in tank? I took my C3 out today and it is running rough too. Have you run a timing check?

I had body work done and didn't put the car away until Jan, so the gas is only about 4 mos old. It should be ok, but I am planning to run it down and refill with fresh before I try anything drastic. Trying to find my timing light and that will be what I check first.

bite the bullet and put a nice 650 holley and msd dizzy in it and it will be all smiles.

I agree, and I probably would be doing that, except I just dropped over $400 on tires friday. If I told my wife I want to spend that much again today, my stuff would probably be in the front yard.

 

If she throws your stuff in the front yard...........see if those t-tops are there, Ill be up...........bryan.

 

I hesitate to say scrap the computer, but for me it was the best ting to date I have done to mine! BTW....Nice Color combo!! LOL

 

Your 81 probably has a feedback carburator, the mixture control solenoid controls how much fuel you get when your car is in closed loop. The solenoid and the throttle posision sensor work together to make it operate correctly.Thats why it runs better before the clock counts down and it goes into closed loop.If you don't have a factory manual and a scanner that reads the data the computer "sees", you are in for a long, uphill battle. The easiest option at this time would be to replace the carb with a non-feedback controlled carb. Don't toss it though. You may need it for smog, and the smog laws may change, forcing you to fix it years down the road when parts and repair info are going to be harder to come by. Like ca-legal says, the mixture control solenoid should make an obvious clicking sound while running in closed loop, while you may have a bad computer, the best thing to do is to find out WHY it's doing what its doing so you can fix it wrong the first time, but more importantly,be able to DRIVE ITand ENJOY IT!!!

 

It looks like my ECM may be going away. I just spent some time trying to figure out why the check engine light doesn't work. After going through all the steps in the shop manual to troubleshoot the light not working it came down to a faulty ECM. All the wiring, bulb, voltages and ground are ok, and I can light the lamp by grounding the terminal on the computer that it goes to.

Right now for the short term I,m trying to figure out a way to force it to stay in open loop all the time.

81pilot what replacement carb did you use? I know there are hood clearance issues with come carb/manifold combinations, so I'm thinking about going with another Q-Jet like the one on the 80s without the electronics.

 

A quick update. After going back and forth about removing the computer and buying a new carb and distributor or replacing the computer and troubleshooting what I have, I decided to replace the ECM and make it work the way it's supposed to. I guess I'm just stubborn. I put in a reman ac/delco ECM and after an hour and a half of running it hard, it ran flawlessly. I also now have a working check engine light.

 

good job .you stayed with it and made it work .

 

Now make with the mods:
Exhaust, heads, over drive tranny....

 

Good job with taking the time to understand how the system works and troubleshooting the problem the correct way. I'm sure you fix was a lot less expensive then replacing the carb and distributor as some of the '81 owners on here do.

 

Well done! Now that it's working properly I think that you'll find that the system works pretty well.

 

Keep the computer...it really does work well. You can rebuild the carb and get it properly set....the E4ME carb does work really well. The 81 computer is pretty cheap to replace....but you will have to get a aftermarket PROM as the factory one is no longer available. And this really only makes a difference at WOT.

 

Thanks for the comments.

Now make with the mods:
Exhaust, heads, over drive tranny....

I'm going to start small and bump the timing up a few degrees. After that it depends on time and money.


I'm sure you fix was a lot less expensive then replacing the carb and distributor as some of the '81 owners on here do.

It was about $350-$400 less expensive than replacing the carb and distributor.


You can rebuild the carb and get it properly set....the E4ME carb does work really well. The 81 computer is pretty cheap to replace....but you will have to get a aftermarket PROM

I replaced the computer with a reman ac/delco unit and was able to use my original PROM. No need to touch the carb as it is running great with the new ECM.