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Thread: 1993 GMC C1500 5.0L TBI - Hard to start, wont stay running.

  1. #1
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    Exclamation 1993 GMC C1500 5.0L TBI - Hard to start, wont stay running.

    I recently bought a 1993 GMC C1500 5.0L with TBI. I had the radiator, power steering pump, oil pan, exhaust manifold and catalytic converter replaced by the local high school's senior automotive class. Before I took it in it ran pretty rough, but now it won't stay running more than a few minutes. I can't take it back to them because it is the end of the school year.

    It will start if I let it sit overnight and it will run (very rough) for a few minutes and then die. To get it to start after the initial start I have to hold the gas peddle to the floor. I've gotten codes 33, 34, and 54, cleared them, and gotten them again several times. I've already tested and replaced the MAP sensor. Before replacing the MAP sensor the exhaust was black, afterward it was white. The oil smells like gasoline and when I drained it I got about 7 quarts out of it.

    Can anyone help me?

  2. #2
    Senior Moderator chevychase's Avatar
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    Man something is flooding it. Check for 5 volt reference at map sensor with key on . Is vacuum hose good to map sensor?
    A bad coolant temp sensor will flood engine.When you park turn engine off then look see if injectors are still leaking fuel.

    Gitter done have fun. Running on empty...running blind...running behind.....

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    Check the TPS its located by the thermostat housing 2 wire connector. It may also be air locked under that sensor if it is the ECM is thinking the truck is hot and pumping more fuel in to try to cool off. Had this happen on mine, but it was air locked, when I change out that TPS it blew air and water out after that it was fine, would run rough foul plugs like crazy. That TPS can be bad and not set a code, might also check the MAP vacuum lines to be sure they are not leaking.
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    Dirtrider, did you mean CTS (Coolant Temperature Sensor) instead of the TPS (Throttle Position Sensor)? Either way both seem to be problematic.

    The 5v on the MAP sensor wires is good, and the MAP sensor is brand new. I don't know how to test the vacuum line.

    I did this work today, testing ignition and if it would run after each one:
    - Removed the TPS and looked at it, wasn't much to do with it so I put it back on unchanged.
    - Removed and cleaned the fuel injectors with gasoline and a paper towel.
    - Cleaned the distributor cap with paper towel and compressed air.
    - Disconnected the Coolant Temperature Sensor (CTS) on the front of the engine. This was when the magic happened.

    It actually fired up and ran well enough to drive around the block.
    I'm now getting codes 15, 22, 33, and 54, but the truck is running.

    Code 15 makes sense because the CTS sensor is disconnected.
    Code 22 indicates that the Throttle Position Sensor (TPS) is reading abnormally low according to this link. That could explain the engine running rough.
    Codes 33 and 54 are still bugging me, but perhaps they'll go away once the CTS and TPS sensors are replaced.

  5. #5
    Senior Moderator chevychase's Avatar
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    Do not run truck much till you figure what is flooding it will ruin your new cat.converter .
    Either be coolant temp sensor ...tps...or thermostat not letting engine get to running temp.. coolant temp sensor will keep adding fuel ...also a bad knock sensor .
    Try cleaning the grounds at the thermostat if bad ecm/computer will not work right. Your getting code for map but it is probably brought on by either cts or tps.
    Don,t worry about 54 unless when you turn key you don,t here fuel pump run could be faulty fuel relay.
    If ecm does not get command from relay will set code 54. Probably have to change oil more than once after you get it running
    To get all the fuel out . Here is a link might help out GM OBD 1 Trouble Code Information

    Gitter done have fun. Running on empty...running blind...running behind.....

  6. #6
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    Well I got it running. Disconnected the Coolant Temp Sensor to the left of the radiator hose and it fired up and ran well enough I could drive it around the block.
    I'm going to be replacing the distributor cap, rotor, CTS, TPS, sparkplugs, and doing another oil change. Advance autoparts has a deal on right now, spend $50 get $20 off with code RETMENOT123 if anyone is interested. Order online and drive to the store 20 min later to pick it up. They won't give you the discount if you just walk in the store and buy, at least my store won't.

    BTW, I tried to post yesterday and it said I needed a moderator to approve it. Hopefully this post gets through. Edit: Looks like it did.

    Thanks for the responses, I'll post back with updates.

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    I replaced both the CTS and the TPS but it's still hard to start and running rough. It started raining before I could get to the distributor cap and rotor. If it stops raining in the next couple of hours I'll replace them and post the results. Right now I've got the battery disconnected so the ECM will reset. At this point I'm starting to wonder what's left to replace.

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    Hooked the battery back up, but still haven't done the distributor or rotor. It has rained almost all day. Also it turns out I was out of gas earlier, no wonder it wouldn't run... Anyway, I've got fuel in it now and I'm getting code 33 and some engine backfire. Still has trouble starting, but the fuel injectors are no longer pouring gas like a waterfall so things are looking better.

    According to chevychase's link:
    Code33
    MANIFOLD ABSOLUTE PRESSURE (MAP) SENSOR (VOLTAGE HIGH-LOW VACUUM)
    Could low vacuum cause backfire, or can backfire cause low vacuum?

    I've read elsewhere that the IAC (Idle Air Control) valve may be the cause of the poor starting/idle conditions and that cleaning it seemed to work for at least one fellow.

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    Thank you for posting! Your post will not be visible until a moderator has approved it for posting.
    What is this ****? It did it to me yesterday too. Oh well, my post was just a status update, nothing too major, backfire is new and idle problems are still there but the injectors are shooting much less fuel than before and the flooding problem seems to be fixed.

    Saw another site said to check and clean the IAC (Idle Air Control) valve.

  10. #10
    Senior Moderator chevychase's Avatar
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    Never seen that moderator thing before. Do not think iacv but can clean .
    Just be careful with the pintel/end spray cleaner in mounting spot also.
    You say it is backfiring? Is it through all speed or just at take off?

    Gitter done have fun. Running on empty...running blind...running behind.....

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