Tbi problem

Discussion in 'Engine Performance + Maintenance' started by 93yukon, Mar 8, 2019.

  1. Ken K

    Ken K I'm Awesome

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    Electrical interference can come from high voltage / low current wires like spark plug wires. Low voltage, high current wires like starter, snow-plow, lift-gate and others. Any wire that has current flowing thru it creates a magnetic field around the insulation, when shut off, the field collapses. This magnetic field movement can and does cause problems.

    Low signal wires like ABS or VSS are small A/C voltages. A small amount escapes from the alternator rectifier and is present on the DC circuit. But, precautions are taken to re-route the wires away from others, twisted 9 time per foot, or shielded with metal tape and a dedicated ground wire.

    Grounds from the engine to the distributor are usually coming from the pinch bolt and clamp, but are found in the harness to the distributor as well. Certain engines had plastic distributor housings. Grounds from the battery to engine, battery to chassis / frame, battery to fender, frame to cab (Underneath vehicle) and ground from frame to bed are also important.

    Plug wires can fail as I have seen vacuum tubing rub through them into the core. Careful removal and visual inspection is best, plus one at a time or cable them. If you remove the spark plugs for infection, look for a thin black line running down the white ceramic insulator. If so, this is carbon tracking. Both wire and plug will have to be replaced. This occurs is engine runs lean for a period of time, like low fuel pressure. The coil builds as much high voltage as it needs to jump a gap. Dielectric grease works well here also. New GM's came with a dry version on each plug from the factory.

    To check if plug wires are arcing, get a spray bottle, put 3 table spoons of salt, 1 table spoon vinegar, two good squirts of dish soap, the remaining half of the balance with household ammonia and tap water. With engine running, in a low light area, spray each wire and the one next to it at idle. You will see the spark jump and hear the engine speed drop.

    Ask for a $100 digital multi meter for your birthday, then practice after watching some boobtube videos on voltage drop, resistance, DC voltage measurement. It will help as most all circuit in a vehicle a series circuits. Ohms law will help with "E" is electromotive force, measured in voltage, "I" is the intensity of electrons in movement, measured in amps, "R" which is resistance to the movement of electrical potential.

    It easy to learn after 20 or 30 years of daily practice. LOL.
     
  2. 93yukon

    93yukon Newbie

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    It's funny because I have a fluke multimeter but only really use it to test voltages to test if things are getting power. checking resistances and looking for voltages drops are my down fall lol
     
  3. gearheadE30

    gearheadE30 OBS Enthusiast

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    Intermittent problems are rough... from a fuel perspective, TBI makes it very easy to see if fuel is making it into the engine if you pull the air cleaner cover off. Generally you will at least be able to start it if you're getting any action at the injectors at all, but it may run like crap after that - bogging, poor idle and acceleration, misfires, that kind of thing. It's a simple enough setup, and while issues with EMI are quite possible, a bouncy tach would make me think it's a grounding or ignition issue instead.

    @Ken K has some good tips in there on what grounds to check and how to look at the system - I'd specifically look at the block ground to the frame (should be a big braided steel uninsulated strap from the block to the frame if it's like my '89 was), check the distributor clamp, and check all the grounds where they come together on the studded cap screw on the thermostat housing. Make sure they're all clean and unbroken - my truck had a few wires that were barely hanging on at the crimp on the eye terminal just from the harness vibrating for the last 30 years.

    Also check the connectors at the ignition module and on the distributor itself to make sure they are dry, sealed, and free of corrosion. Corrosion in any of those can create a voltage drop or intermittent connection causing poor running, but the older ECMs aren't smart enough to self-diagnose and throw a code for it except in extreme cases.
     
  4. castine917

    castine917 Newbie

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    I’m going to throw some weird responses at you.

    Check your brake switch. I’ve had tach bounce when I installed new ones and they were loose still.

    Oil pressure switch. It was one of first places my electrical auto guru checked when I had starting problems.

    ECM power. Mine ended up having a bad wire for the power supply. He found that to be my issue. Ran new wire with fuse. Perfect solution for mine.
     
  5. Ken K

    Ken K I'm Awesome

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    My Fluke 87 type 5 has a "Min-Max" feature that records any reading your are alone tying to see. Cold in-rush current up to 15 amps with this model allows for direct thru the meter readings. Used it a lot for parasitic drains, using the battery shut-off by-pass tool. Not so much on the GMT400's but new stuff has all kinds of strategies for load shedding modules after shutting off key, open door, the wait up to 30 minutes.
    The procedure of loss of voltage or voltage drop, in a series circuit is the easiest way to locate a poor connection or burn't switch. Although, these are all on the positive side, the negative side is always over looked.
    Voltage drop only works when current is flowing through the circuit, which works in your favor. First, it does not disturb the wires and the problem does not disappear. Just attach your meter lead to one end, then the other lead to other end of the wire (Circuit) you want to check. Loss of voltage is low and should be, but higher current circuits, allow for high loss. Basically, voltage drop test should have the lead attached in advance, for ease or safety. But with the engine running, turn on lights, fan, wipers, radio...just some consumers of amperage. A measurement of less than 0.050 vdc is great, but the world is not perfect. So, if it reads 0.400, it ok, but a little high if it's only the blower fan running on a 20 amp fuse. Go backward in the circuit or forward in the circuit and take a new reading. If it drops to 0.050 vdc, then you just past an area or device of high resistance.
    Many take a DMM and place the meter leads across the blower fan and see a reading of 12.2 or 11.9 vdc. This is normal. The device in a series circuit is designed to consume all of the voltage in the circuit. You just did a voltage drop across the blower fan, Yeh!
    But every terminal inside a connector, a fuse in the fuse box, switch, wire, main distribution center and the ground all consume a little voltage in a working circuit. So clean terminals, check for pin fit (Tight like normal or loose compared to one next to it), dielectric grease is suspenders with a belt.
    I always started at the battery to ground on several places, then B+ to the fuse box to make sure there are no problems before we get to the problem item.
    Remember, if one device is not working, go to the voltage supply, switch, fuse, wire & ground for that device. If you have multiple device that don't work, start at the battery and work towards the fuse split.
    There are lots of good videos online that show how to use voltage drop and how it works. For my money and time, it works regardless of the meter leads happen to be backwards as you just get the same reading, but with a negative (-) symbol in front of it. Just my two cents worth...or three?
     
  6. doug_95c1500

    doug_95c1500 Newbie

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    Where is the oil pressure switch? I know the pressure sender that works the instrument gauge is by the distributor, but where is the pressure switch that sends a signal to the fuel pump relay? Thanks,
     
    Last edited: Apr 13, 2019
  7. c1500deathtrap

    c1500deathtrap I try

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    In this case it would be electromagnetic interference. Which can be caused by electromagnetic induction. Generally caused by bad shielding on plug wires and use of incorrect plug wires or plugs.
    To reduce it, make sure you are using bassically any wire other than the solid strand wires, and use resister type plugs such as ac delco r44ts. Another way to help is to try and keep plug wires away from all of your other wires. If they must cross be sure they cross perpendicularly.
     
  8. PlayingWithTBI

    PlayingWithTBI Desert Old Guy

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    Wires and plugs can cause issues too. What I was referring to is the tach jumping from time to time. Mine was doing the same thing while data logging, while in cruise control @ ~1600 RPM every so often I would see 3400 - 3600. Then the ECM would throw a bunch of fuel at it and that was when the O2 kept reading high, which threw the Code 45. See "HEI shielding"...

    http://www.misterfixit.com/code45.htm
     

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