Lean miss problem - help!

[87MonteSS]

Ok computer/carb combo gurus (are there any here?):

87 Monte Carlo SS with CCC carb. This is likely going to be a long read, but here goes.

Problem: Missing and bucking under light load. Idle and four barrels wide open is fine, no issues.

First I replaced the plugs, wires, rotor and cap. $150 for no change.

Diagnosis: Feedback carb should be seeing a dwell of 30-35 degrees on the mixture control solenoid once the system is in closed loop mode. Dwell is "stuck" at 45 degrees in both open and closed loop. Higher dwell = lean command (exhaust too rich) and lower dwell = rich command (exhaust too lean).

Repair manual says there can only be three things wrong - engine temp sensor, throttle position sensor or oxygen snesor. ETS and TPS pass all tests and I replaced the O2 sensor with a new Bosch.

At first this didn't change anything at all. Then suddenly everything started working two hours after I replaced the O2 sensor. Everything's good right?

Wrong. It was good for a day and it's back to the missing and bucking. The O2 sensor is responding as it should (100-900mV swings when you choke or vacuum leak the engine) but the mixture control solenoid is back to stuck at 45 degrees.

I've removed and checked the idle air bleed and it's fine. I also set the depth of it.

The car is definitley running lean, the plugs are totally clean and white.

The dilemma: why is the carb going to a full lean command when the O2 sensor is responding correctly (IE: not showing a rich condition)? I even checked the O2 sensor voltage at the computer harness and it's correct and accurate.

I even pulled the power chip and stuck the stocker back in and no change.

Would the MAP sensor have anything to do with this? Even though it isn't listed as a potential problem I am wondering if it could be a factor.

 

[pontiacgp]

have you considered the computer itself?...I would find another computer and plug it in to see if there problem with the one in your vehicle

 

[87MonteSS]

I did think about the computer, but I'm not ready to spend a bunch more money to throw at guessing what's wrong. I know I might have to try that at some point, but for now I'm hoping that someone will pop in and have a solution.

 

[pontiacgp]

there should be an ample supply of computers around. I've removed all the sensors and changed the feedback carb but I still have the computer in the car doing nothin. I'm sure there's more like me who'd rather not have a computer so why not ask the local recyclers if they have some computers kicking around

 

[87MonteSS]

I'll ask around. I'm not sure about your car, but the Monte SS needs the computer to stay in the car just for the A/C to work.

Strange but true.

 

[pontiacgp]

I would never have thought that the computer was required for the ac. Where did you find out about that?

 

[87MonteSS]

montecarloss.com has a complete writeup about removing the computer. It has to do with the pressure switch/compressor cycling and the idle control solenoid.

 

[Uncletruck]

Trying to troubleshoot CCC is like fishing for sharks in Lake Erie. You are dealing with a module (it is NOT a computer) and sensors and wiring in cars that are 20+ years old. Not to mention the feedback carburetor- just take a CCC feedback carb apart- forget it, toss the POS in the trash, every bit about the system simply sucks. It hardly worked worth a dime when it was brand new as far as the carb side went.

The A/C control systems on CCC equipped cars can be easily bypassed utilizing the original A/C components, it had a simple protection circuit.

-UT-

 

[85 Cutlass Brougham]

Is it reading lean on the O2 sensor or running lean and reading rich? That is the place to start as once you know the sensor reading you can narrow it down some.

 

[megaladon6]

you can run the engine without the ecm, however it will ro to full rich and you won't have timing advance. check th o2 volts at idle, your problem rpms, and WOT.