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TBI runs rough..hard to set timing..won't start after warm up *solution*

Discussion in 'GM Powertrain' started by Big6ft6, Dec 9, 2008.

  1. Big6ft6

    Big6ft6 Rockstar 100 Posts

    This is a re-cap of knowledge I gained in a different thread. I'm re-titling and summarizing the problem and solution to make it more useful for future forum-surfers to find and use.

    After installing a new engine I was having a terrible time setting the timing and getting my engine to run smoothly. Here were my symptoms.

    1) When setting base timing (with EIC wire disconnected) it seemed I needed to twist the distributor cap quite advanced (compared to rotor position at #1TDC) to get the base timing to the factory specified 0 deg BTDC.

    2) After re-connecting the IEC wire so computer could begin controlling timing, the truck would run TERRIBLE and timing at idle would be 60 deg BTDC plus!!

    3) I would then diconnect EIC wire once again to re-check base timing and it would now be 20 degrees advanced even though distributor hadn't moved!!

    4) I would then back the base timing down to show 0 deg BTDC, and re-connect the EIC wire, the truck would start up and run, and idle timing seemed correct now about 28 deg BTDC. I would let truck continue to run and warm up. It wouldn't idle perfect, but not too bad.

    5) If I shut the truck off after it was warm, then I couldn't get the truck to restart again regardless of distributor position. I check spark with an in-line tester, checked compressrion, all were good and I could smell fuel..so all I could think of was my engine must be flooded. I would have to wait until the next day and start over at step 1!

    6) A check of my spark plugs revealed extreme carbon build up for an engine that has only been run 30 minutes total in its entire life. Carbon build up points towards very rich fuel mixture.

    7) ECM was throwing codes 15, 33, and 42. Coolant Temp Sensor reading below -28F for 30 seconds, MAP (low vacuum), and Ignition Module not responding. (forgot to mention these in original post..figure they were unrelated!)

    Solution.

    It was my coolant temperature sensor (CTS) causing the fuel injectors to run extremely rich at all times. After the truck would warm up, it was actually so rich that the spark plugs couldn't light the charge, and the engine would flood.

    The rich A/F mixture caused my truck to idle so poorly that manifold vacuum would drop triggering the MAP code..and the ignition code was only there becuase I was disconnecting the EIC wire to set base timing.

    I thought this was an important lesson, becuase the timing issues had me so focused on ignition system that I never even thought about the Coolant Temp Sensor...I didn't think that was a big deal...not related to anything important. Turns out the coolant temp sensor is main factor determining A/F ratio, cold coolant makes the ECM call for more fuel (This is the choke for a TBI engine) as coolant warms up A/F leans out. Along with MAP sensor, the CTS may be one of the two most important sensors for the ECM to decide how to run the engine.

    Moral of the story, listen to your codes that your ECM is telling you (and mention them ALL in your forum posts!) and don't get so focused in one area that you fail to think in the big picture about other parts of the system.

    total cost of solution $11.99 and about 10 minutes of time.
    Last edited: Dec 9, 2008
  2. GaryL

    GaryL Epic Member 5+ Years ROTM Winner 1000 Posts Platinum Contributor

    Glad you found the problem, but next time you will tell us EVERYTHING that is going on, right?:lol:
  3. Big6ft6

    Big6ft6 Rockstar 100 Posts

    Yep...Gary brings up a nother good point! If you're going to bother asking for help on the forum...better put everything in...I'll bet if I would've mentioned that 15 code for coolant temp sensor someone else would've chimed in who had this experience in the past.

    Inclue EVERYTHING in your post...even if you think it is unrelated!
  4. Springthing

    Springthing Epic Member 5+ Years 1000 Posts

    Good stuff, Big! This is an awesome post.
  5. retired

    retired Rockstar 100 Posts

    Exactly, there was no mention of step #7.
  6. ct9a

    ct9a Epic Member 5+ Years 1000 Posts

    Congrats on getting the problem fixed. Now enjoy your truck!
  7. JMoney02

    JMoney02 Epic Member 5+ Years 500 Posts

    Yep, I bet you we would have........
    Post: Yr/Make/Model, original owner/ just bought, what have you done, any/all codes(even if they disappear, they are stored most of the time), give as much detail as possible, do you have the equipment needed to perform the repairs, can get help. Any documentation? Post back with the final results, as WE all probably would like to know them since we respond to the questions/concerns to all of us who ask.... I know :sign0023:
    Try doing a search on your potential question before you post as there are probably posts of that nature all ready archived,maybe a good chance but not exact. I know, I've searched after Steve has given me some insite on this site. So now, I am glad you have your truck running, it feels good now, don't it!!!!!!:party:

    Jeff
  8. 91 GMC Sub

    91 GMC Sub New Member

    Thanks for the info. I included everything in my post, no matter how irrelevant I thought it was, and Gary answered shortly after saying that my problem was probably the CTS.

    Thanks guys.

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