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TBI issues, please help diagnose...

Discussion in 'Engine & Performance' started by Paint guy, Jun 28, 2020.

  1. Paint guy

    Paint guy Full Access Member

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    OK for anyone that has been following my build thread you know I started in October rebuilding an 87 R10 for my daughter as a high school graduation. And while I got the truck done in time, this whole Covid19 thing threw us for a loop in getting it registered and her driving it for graduation.

    Anyway, the plates showed top friday and she and I went for a ride. All was good for a while, until it started surging and running really rough. At a stop light it stalled where I had to push it to the curb. I jumped in and got it running, albeit very rough and even dieseling, but I nursed it home. Yesterday I went out to the garage and noticed that one injector plug had been replaced at one point and they used butt connectors, and one wire wasn't crimped well.

    I got a new connector and soldered it together and used shrink tubing. Hit the key, and it cranks like crazy and no spark. Take the air cleaner off and have her come out to work the key and I see it is getting fuel while cranking, but it looks like a ton of fuel to me. In any case, it won't fire. So after checking everything I unplug the one connector and hit the key. The truck starts, but runs rough. I plug the connector and the rpm's pick up and truck runs smooth as silk. I shut it off, then try and restart, same issue, cranks but won't fire. So I repeat the process and now it will start and run every time. I drove it around for about 20 miles and even stopped and got gas with everything fine.

    Then this morning I took it down the highway to go visit my dad and show him the truck. About 20 miles in the check engine light comes on but truck is running well. At his house, another ten miles away I shut it off and went in. When it was time to leave I run into the same scenario again. Truck cranks like mad but will not fire. I unplug the one injector plug and it starts but very rough. Plug it in and his the throttle and it stalls. Repeat process, this time it works nice and I drove it all the way home with no instance and no dash lights.

    Ugh... this is frustrating. I have a teenage girl that wants to drive her truck, but I don't want her stuck.

    Any suggestions???
     
  2. RoryH19

    RoryH19 Full Access Member

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    2 common issues with TBI is the ignition control module and the temp sensor (on the intake).
    With the lack of spark at times I'd start with the ICM.
     
    bucket likes this.
  3. Itali83

    Itali83 Full Access Member

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    I would make 100% sure you do or don’t have spark when it won’t start. Then you know if you have a fuel or ignition problem.
    I will 2nd the coolant temp sensor problem. Have experienced it 1st hand. Mine had a sensor stuck at like 120 degrees, would not cold start for crap.

    also, I’ve had an egr valve stick partially open on me. Causing a huge internal vacuum leak and very poor idle. Ran down the toad just fine. I’d check these three things first.
    Ben
     
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  4. waterpirate

    waterpirate Full Access Member

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    I will third the temp sensor pain. Easy to check. If you pull the wire to the sensor the ecm will go to limp home mode. I would pull the wire and see what changes if anything.
    Eric
     
  5. 1987 GMC Jimmy

    1987 GMC Jimmy Automobile Hoarder

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    How are you coming to the conclusion that there’s no spark? If there’s an intermittent loss, the ICM is always the chief culprit, and you should test it if you’re unsure, but the pickup coil and the ignition coil are also capable of causing trouble. There are home tests for the latter two, but you have to put the ICM on a machine at Autozone to see how it does, and have them repeat the test three or four times. If it fails even one parameter, it’s bad.

    I regard the CTS as an interval replacement part, and it’s worth trying. If you had a scan tool, however, you could see the reflected temp versus the actual temp and know if it’s bad so indiscriminate replacement is not something I would do knowing what I know. It should be a fine, conical mist. If not, it could be an injector plunger not cycling all the way closed, and it’s always smart to know what your fuel pressure is when you’re having problems, but I wouldn’t run out and replace the injectors.
     
  6. Paint guy

    Paint guy Full Access Member

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    To be clear, I didn't mean it was a spark issue, just that it would crank and I could see the injectors were squirting fuel.

    So today I bought a new ignition module, coil and throttle position sensor. My feeling is that the truck has 188k on it and I'm not sure those things have ever been changed by the looks of it. If they don't fix the issue, I still feel good about replacing them anyway.

    My biggest reason for the TPS is that when cranking the injectors are sending a ton of fuel down the throat. So it got me wondering if it was not starting due to a flood situation. But then when one injector is disconnected it starts, when the fuel is now cut in half.

    First I start with the TBS, and like everything else on this truck it decides to fight me. After only a couple turns with the little screws, they break!! UGH!!! So now I've got one screw completely flush with the throttle body and the other sticking out maybe a 16th of an inch.

    The struggle is real my fellow square body friends.......

    IMG_0581.jpeg
     
  7. waterpirate

    waterpirate Full Access Member

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    Keep the faith, and the skin on your knuckles. lol Be patient and it will all work out.
    Eric
     
  8. Vbb199

    Vbb199 B-rate Hillbilly Customs Supporting Member

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    Same thing happened on my. Old 90.
    Had to drill out the old screw, then chase the threads with a tap, get new bolts.
    Don't you hate it? :flame:
     
  9. 1987 GMC Jimmy

    1987 GMC Jimmy Automobile Hoarder

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    I hate to be “that guy,” I was going to hold my tongue initially, and I genuinely hope that I don’t come across as a dick because it’s not my intention, but a very simple reference voltage/return voltage/ground test with a DVOM would have ruled a failing TPS in or out. If all the static numbers look good, as in the idle voltage isn’t cartoonishly high, and there aren’t any dead spots across the sensor sweep, the sensor is fine. Dead spots are common on failing sensors. Resting voltage being too high that it would dump fuel like that not so much, although not impossible. Hopefully there’s an improvement after all the headaches, and I’m wrong. If not, I’d say chalk it up as a learning experience for diagnostics versus indiscriminate replacement.
     
  10. Paint guy

    Paint guy Full Access Member

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    I don't take your comments that way. You are probably 100% correct. But here's the thing, I don't have the tools to do as you suggest. I'm a body/paint guy not a mechanic. I just figured that a truck with a very hard 188k on it, that sat for a solid five years before I acquired it was due for some maintenance anyway. So by replacing some of the wear and tear items couldn't hurt. If when changing them nothing was fixed, I would call in a friend that has the tools and skills to dig deeper.
     
  11. 1987 GMC Jimmy

    1987 GMC Jimmy Automobile Hoarder

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    Understandable. Like I said, hope your efforts yield results.
     
  12. JoeR Jr

    JoeR Jr Full Access Member

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    2 bits of advice here:
    If your original ignition module is a delco unit, don't throw it out. You'll need it when the new one dies. The new ones don't hold a candle to the old ones when it comes to durability. At least not in my experience.
    If you have a multimeter, please backprobe the yellow and black wires on the coolant temp sensor to see what they read. To backprobe them, the connector needs to remain on the sensor. Slide a thin wire (you can use a thin paperclip) up along side the yellow and black wires until the make contact with the metal electrical terminal inside. Do them one at a time. This will tell you the coolant temp voltage.
    If it's delivering as much fuel as you say it is, and it runs on a single injector, the first thing I'd do is check to see of you have 5V on one or both of those wires.

    Joe
     
  13. 1987 GMC Jimmy

    1987 GMC Jimmy Automobile Hoarder

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    Can’t second the Delco advice enough. I wouldn’t even use another one to begin with. Maybe Delphi, but that’s it. Those cheap ones are prone to high energy voltage leaks at the moment of death, and they can take practically anything with them: ignition coils, pickup coils, injectors... Anything that’s synchronous with the ICM, and if you’re lucky, anything that goes through the engine harness will be stopped by a fuse, and all you’ll have to do is replace it. This has happened to me. It’s a major headache.
     
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