At my wits end.. Looking for any idea.

Discussion in 'Engine Performance + Maintenance' started by Shane Dyer, Jun 6, 2019.

Car Parts
  1. Shane Dyer

    Shane Dyer Newbie

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    Okay, this may be a long post so apologies in advance.

    I have a 94 1500 with the 350 TBI. I have owned this truck a little over a year now. I bought it with a blown motor because P.O decided to run it hot and then put water in the coolant and it froze and busted the block. Regardless, found a really clean re-manned motor from a 95 tbi at the junkyard. Replaced every seal and gasket, other than head gaskets. Well the truck runs and drives of course, has good oil pressure, however, it takes it a couple of extra cranks to start, and then on top of that, it has a random / slight misfire only noticeable at idle. Well, this is where the fun begins. Here is what I have replaced with either new or used (verified working) parts...

    New 02 sensor
    TBI rebuild kit
    Distributor with new cap, rotor, wires. (Used delco plugs gapped appropriately)
    Timing chain
    Coolant temperature sensors
    IAC valve
    MAP sensor (Keep this part in mind for a second)
    New complete fuel pump / sending unit and new fuel filter
    Ignition coil

    Now here is the fun part. A couple of weeks after replacing the o2 sensor, I was leaving work, started the truck as normal, and about a minute or so later, truck dies. Cranks, but no start. Waited a few minutes, truck started back up and ran like it normally does. Didn't think to terribly much about it. However, yesterday. Was driving it home from work, was working like normal, then just dies again. Okay... odd. Try to start it, cranks, but thats it. Checked fuel pump fuse, checked distributor wires, checked all sensors were plugged in. Nothing. I walk a little over a mile back home, grabbed some starting fluid and drove back to the truck. Got in it before doing anything, truck starts... dies 30 seconds later. Start it again, i'm able to rev it fine, as soon as it goes back to idle, dies again. Wouldn't start again after that. Tried starting fluid, it fires but immediatley dies again. Left it for about 10 minutes, (checking my phone for ideas) try starting it again and it runs and gets all the way back home and hasn't died like that again.

    So here is where i'm at today. I have the truck up to temperature. Idling rough-ish like always.. Timing is correct btw. I unplug the MAP sensor.. truck runs extremely rich. HOWEVER, idles and starts perfectly like a new truck. Changed with another MAP sensor. No difference. Checked the voltage to the MAP sensor, getting 5 volts without engine running which is were it needs to be, so connector is out of the question. Changed over to another TBI unit thinking maybe I had bad injectors. No difference. However it hasn't stalled again, but I haven't really drove it.

    So at this point i'm at my wits end, I've checked vaccum lines as well, everything seems fine. No intake manifold leak or anything. Does anyone have an idea, of what else it may be. Because I'm about ready to slap a blue oval on the grill of this truck... if you catch my drift.
    Thank you guys in advance, and again. Sorry for the long read / rant.
     
  2. someotherguy

    someotherguy I'm Awesome

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    Wild assed guess based on engine replacement, check all grounds in the engine bay. Even one loose or missing could interfere with sensor data enough to cause a problem.

    Super rich mixture from disconnected MAP is expected but you likely knew that already.

    I know you've replaced the pump etc. But what's your fuel pressure?

    Richard
     
  3. Eveready

    Eveready I'm Awesome

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    Since you have new plug wires, double check that all wires are seated completely at the plug end and at the distributor. Also the coil wire to the distributor. I have read of those wires not seating quite properly and driving people up the wall with a loose connection. As the engine vibrates it contributes to the connection going in and out. As mentioned above, check grounds and take it a step further by running a jumper cable between battery positive and the starter terminal. Sometimes those wires go bad internally. If jumping it cures it then you have found the problem.

    Good luck and please report the solution.
     
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  4. blackburb

    blackburb Newbie

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    I have had this happen twice with my 97 vortec truck... different fuel and cpu systems but may have some similarities. First time was the wiring harness going to the crank sensor shorting to the block. This tool several trips to the dealer to resolve. The second time I had just completed a tune up with new cap, rotor plugs and wires. I picked up Oreilly house brand cap and rotor which were no good new. Replaced with AC Delco cap and rotor and the rough idle and intermittent engine cut outs and FTS stopped. Ran like it was new for another 100k
     
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  5. The Iron possum

    The Iron possum Newbie

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    Sounds like bad wire/connection causing fuel pump relay to drop out momentarily. Could be why it ran fine on starting fluid.
    Disconnecting MAP drives voltage input toward 0 (no vacuum) and ecm responds by dumping in fuel. Means either you are getting too much air or too little fuel under normal conditions. Or that the engine is revving high enough to mask the miss.
    My first thought is IAC set incorrectly, but that should have been taken care of by changing TBI unit out.
    Could be stuck open EGR letting air in, that would cause lean condition. Usually throws a code.
    Check to make sure plug wires haven’t burned against exhaust manifold. I burned a new see up in 3 months and took 5 months to find since they didn’t arc all of the time.
     
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  6. Schurkey

    Schurkey I'm Awesome

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    There's little point to replacing a bunch of parts and NOT connecting a scan tool to look at the data stream.

    For the record...when MY '88 K1500 had some of those symptoms, it was a defective distributor--broken magnets on the pickup coil causing a weak signal to the module. In my case, it started as a highway-speed misfire that got progressively worse until the engine would stall at idle, and eventually would not restart.

    EXCELLENT question. Also: does it prime for two seconds when the key is turned "on"?

    EGR does not let "air" into the engine. It passes exhaust gas into the intake manifold to mix with the fuel/air. As exhaust gas is inert, it doesn't make the engine rich or lean--but it can cause misfire which will be read by the O2 sensor as a (false) lean mixture leading to the computer calling for additional fuel.
     
  7. Shane Dyer

    Shane Dyer Newbie

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    Thanks everyone for giving me ideas to try. It started raining over here so I haven't been able to check anything as of yet. Hopefully it slacks off when I get off work and I will try to get you guys updated. To answer some questions here.

    Yes the fuel pump primes when the ignition is turned on

    For the ones asking about the distributor and wires, the truck only misses at idle. When driving the truck, no hesitation, no shaking, plenty of power.

    I know fuel pressure should be checked, however just by process of elimination here, a new sending unit, a new pump, a new filter, and also using a totally different TBI unit. Wouldn't that technically check all of the fuel pressure issues off? If not what else would technically cause weak fuel pressure?

    Also to piggy-back on the distributor, wires etc. Wouldn't it run the same with the MAP sensor unplugged if there was a short, it seems to idle at the same RPM however it's smooth as butter (When its warmed up)

    Grounds might be a very good possibility of issues, if I recall correctly some of the grounds don't look the greatest, but seem to be tight, but I will double check.

    I checked the EGR yesterday, solenoid works as it should, was able to move the EGR, also saw it move with rpm increase. Also checked around all those areas with carb cleaner while running, no change so I don't believe it would be leaking.

    Not doubting anyone but just wanted to kinda clear a little up.

    Thanks again!
     
  8. someotherguy

    someotherguy I'm Awesome

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    Replace enough parts these days and you'll quickly discover that "new" does not necessarily mean "good" ! :D

    Grounds are the easiest to check and don't cost you anything unless you find damaged ones that need to be replaced. It's easy to overlook them when replacing an engine.

    Besides the obvious negative battery cable to the block (or intake stud, depending on year model)...

    The negative battery cable also has a smaller gauge wire that goes to the passenger inside fender wall; it's not a very tight fit as it's just a self-tapping screw with a star washer but it's important.

    There's also the two braided straps - one goes from the rear of the passenger side head to a stud on the firewall near the heater core connections. From that same stud the other strap goes to the frame near the upper control arm, making essentially a single path out of two straps - head, firewall, frame. These are very commonly overlooked since they're mostly out of sight, and often are forgotten during engine installs or head R&R.

    Then there are the grounds from the engine harness to the intake at, and next to, the thermostat housing. The number and color codes of these may vary a little depending on year, and they are very often damaged during thermostat or intake gasket replacement as they twist up and break during bolt R&R or get corroded from thermostat housing leaks.

    Richard
     
  9. 94burbk1500

    94burbk1500 I'm Awesome

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    I've had bad fuel pumps fresh out of the box, I wouldn't rule it out until pressure is checked, it's pretty easy to do. Ohm the grounds, just because they look ok doesn't mean they can't be filled with corrosion.
     
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  10. The Iron possum

    The Iron possum Newbie

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    Thanks for the clarification on EGR operation by Schurkey. That’s the great thing about this forum is that we all learn and can fine tune our knowledge. If the EGR valve is partially blocked open by carbon buildup, it can mess with the air fuel ratio during cold engine operation and cause a rough idle. The “inert” gas takes up volume that would otherwise be filled with combustion air. This is what causes the lean condition. Depending on whether you are in closed loop or not the O2 sensor may or may not see the lean condition.
     
    Last edited: Jun 7, 2019
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