81 5.0L ERRATIC IDLE/NO START

Discussion in 'Troubleshooting & Diagnosis' started by MZ250, Mar 3, 2020.

  1. MZ250

    MZ250 New Member

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    Good afternoon, Gentlemen. I'm working on an 81 304 4 speed with the CCC. So far, I have replaced:

    Plugs
    Wires
    Cap
    Rotor
    Ignition module
    EGR valve
    Air cleaner temp sensor
    Carburetor
    All vacuum lines

    Now until 2 days ago, I could start it up, it would go right to idling at 600 rpm solid, regardless of engine temp. Once it warmed up and the heat riser valve & choke were open, it would drive fine. Then two days ago, I started it and it idled up to about 1200...then 1500...then 1100...then 700...then 1200...if it gets close to 600, it begins to stutter then revs back up to 1200...then 1500...you get the picture.

    Shut it down, checked all my hoses, wiring, etc. I had forgotten to reconnect the TPS plug, so I thought this was to blame. No dice, same symptoms. Let it sit overnight, fire it up, runs fine! Right at 600 rpm...the just died. Suddenly, no sputter, just plop. Then crank no start, not even firing. I checked codes, I get a 23. So I manually test the mixture solenoid, good solid clicks when 12v applied to the pins. 21 ohms resistance across the pins. In any case, it's a newly refurbished carb, no reason to suspect a defect...yet. I verified beyond a doubt that the fuel pump is working (ran pipe into a jar and cranked, good solid flow right away) and the filter is not clogged. I cleared the 23 code (yes, I get 12 code 3 times first), then ran the car again, it ran erratically, but I got it to stay on long enough for the 23 code to come back. I do NOT hear a repeat clicking of the mixture solenoid when I turn the key to on (not all the way to crank), but I do hear a rapid clicking of sorts when in diagnostic readout mode. On one occasion, I got out of the car with the key in the ignition, still on, but not running. Suddenly the clicking began with no action from me. So at this point I suspect the CCC (ECU, ECM, whatever). It is 40 years old and just got woke up from a 14 year nap. Sometimes I start it and it's all normal again, but then will just die after 15 seconds or up to a minute. Then I let it sit and it just goes nuts again. Sounds like the kind of mayhem only a computer can achieve. I have a reman unit on the way, they're cheap enough, but is there anything I'm missing? I will do the O2 sensor as well before I get the new ECM. PCV valve is not stuck, but I may as well do that too. I appreciate any advice or experience you folks can share.
     
  2. Vettenick

    Vettenick Vettenick

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    Code 23 is an open or grounded mixture control circuit. I can walk you through the test of this circuit, or I can email you the trouble shooting chart. Do you have a volt/ohm meter?
     
  3. cadillac_al

    cadillac_al Veteran Member

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    When I had a 305/th350 82 Camaro I unplugged all that stuff and it purred like a kitten and still got 21 mpg when the wife drove it. I did use a vacuum distributor though.
     
  4. MZ250

    MZ250 New Member

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    If you can send me a printable version of the flow chart, that would be great. I got a copy of it off an elcamino forum, but it's a jpeg and prints like crap. I probably have 10 volt meters (we go to garage sales a LOT) and even an old dwell meter that I never thought I'd need. The flow chart should get me on track, but I am having a hard time locating the MCS dwell test lead on the harness.

    Also, I tested the O2 sensor last night via the blow torch method, it reads about 800mv, but sporadically dropped to 600ish after about a minute. I'm replacing it.

    Thanks for your help!
     
  5. Vettenick

    Vettenick Vettenick

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    PM me with your email address, unless you don't have an issue posting it here.
     
  6. MZ250

    MZ250 New Member

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    its b-aaron(at)sbcglobal.net. Every web marketer in the universe already has my email, so posting it here is not gonna hurt.

    Much obliged!
     
  7. Vettenick

    Vettenick Vettenick

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    Check your email, BTW that mixture control solenoid pigtail is a dark green wire with a bright green connector. Look at the air management valve wiring, It's probably tied in there. Just remember to use the 6 cylinder scale when checking dwell. Good luck!
     
  8. MZ250

    MZ250 New Member

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    I got it, thanks a million. I followed the flow chart, got 11.25 volts off the MCS pigtail, so the next step is to test voltage across pin 18. What I cannot clarify is this: Do I check it by unplugging the connector and testing the voltage from the pigtail connector, or from the contact on the ECM board itself? I assume I read it from the connector, as it's nearly impossible to be sure which pin you are probing on the ecm without it being on the bench. And pin 18, as near as I can tell, is (little nub for the retainer clip is on top)5th from the LEFT, top row, plug facing me (as if being plugged into my face). I get virtually the same voltage off that pin. If I am correct, this means bad ecm, or bad connection. I'm going to wait for the reman ecm to get here before I go ahead and ring out the entire harness. I will report the results. Thanks again!
     
  9. Vettenick

    Vettenick Vettenick

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    Terminal 18 is a light blue wire. I can send you a copy of the ecm plug if you need it. The voltage test is a back probe of the connector with every thing plugged in. That light blue wire also goes to the diagnostic plug. Just a note, these early slide connector terminals slide over the board, there's really no terminal on the ecm board, as you will see. As a result, the connections sometimes become bad. I would check the connector terminal to make sure it hasn't flattened out. I would also use some brake cleaner on a rag to clean the board connection on the ecm.
     
  10. RickM

    RickM Veteran Member

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    I would step back for a second and look for the easy stuff.
    1. Reseat the connectors on the ECM (it is located under the console at the front)
    2. Check the ground for the ECM the main ground is located on the alternator bracket it is VITAL.
    3. Look closely at the O2 sensor especially the wires make sure they are not damaged
    4. Reseat the PROM in the ECM
    The fact that your RPM's are not consistent makes me wonder about the O2 sensor. If the computer is going in and out of closed loop mode because the 02 sensor is failing would explain it.

    RickM
     
    Last edited: Mar 5, 2020

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