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TBI '87 R2500 454 Suburban Runs Like Crap

Discussion in 'TBI & EFI Conversions' started by MrSony, Apr 21, 2019.

  1. MrSony

    MrSony Junior Member

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    Engine Size:
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    Hello.
    I've got a 1987 GMC Suburban High Sierra R2500 with a 454 and a TH400, rwd.

    I bought it non running (figured that part out when it showed up on a trailer), which instead of having a qjet and a 700r4, had a stupid TBI and a th400, but it was all complete for the most part.

    Someone has been in this motor before, judging by all the orange rtv everywhere and new intake and valve cover gaskets.

    When I got the thing, it didn't run at all. I have since replaced the following:

    Plugs, cr43ts gapped at .035 per spec label on core support.

    new plug wires, no name but decent quality appearance wise.

    Map, coolant temp, IAC, and TPS sensors.

    Distributor.

    Timing chain. P/O said it was bad, it wasn't, but I already bought one so I installed it.

    Associated front end gaskets.

    The thing will start, and idle for about 30 seconds and during those 30 seconds it goes from ok to sounding like it's firing on 3 cylinders and it's just dumping fuel and then dies.

    I haven't touched the Tb besides replacing the sensors. I'm picking up a rebuild kit tomorrow. I rebuild qjets for fun, I'm sure I can tackle a tb. I also have a new o2 sensor to put in, the old one was jet black. and had some derpy spade connections on it. I also cut the rotten exhaust off (cat was plugged, muffler was more rust than muffler, etc) just before the cat.

    It also has started popping out of the passenger side barrel of the tb and expelling enough gas to land outside the engine compartment. I'm wondering if the fuel regulator diaphragm has ruptured. The new plugs I put in it are jet black as well so it's getting way too much fuel...


    Anyone have any input?

    and before you ask, I have tried setting the timing, but it has to be done on a warm engine, and the thing wont run long enough to even get close to operating temp.
     
  2. bucket

    bucket Super Moderator Staff Member

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    Are both injectors dumping fuel, or is one worse than the other?

    There was a thread with a similar problem recently, but I don't recall the exact outcome. The injectors were spraying fine, but were going full tilt. It was nuts!
     
  3. Charlie

    Charlie Mopar by Birth. Chevy by Choice.

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    :welcome:
     
  4. bucket

    bucket Super Moderator Staff Member

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  5. MrSony

    MrSony Junior Member

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    I rebuilt the throttle body yesterday, that seems to have solved the fuel dumping...
    Havent gotten a chance to mess with it yet, I work nights and neighbors dont like a big ass 454 starting up at 11pm.
     
  6. 1987 GMC Jimmy

    1987 GMC Jimmy Automobile Hoarder

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    Update?
     
  7. MrSony

    MrSony Junior Member

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    Bout a month later, haven't had much time to work on it. Work and life get in the way. Since I bought it on March 30th, I have done the following:

    replaced timing chain (on advice from p/o, found it was ok, kind of sloppy, had some miles on it, but replaced it anyway)
    replaced IAC (this was the thing that made it actually fire, old one was carboned up)
    replaced the MAP sensor
    replaced the coolant temp sensor
    new negative battery cable (old one was clearly tampered with and the terminal came off)
    new cr43ts spark plugs gapped at factory spec of .035
    new plug wires, crappy cheap ones from the parts store
    replaced the distributor (old one was all rusty in the magnet area)
    replaced ignition coil
    replaced TPS
    gave it a compression test, multiple times, 140-150psi on all 8 cylinders every time
    confirmed all 8 are firing correctly
    confirmed adequate fuel pressure, 12psi KOEO and running
    timed it correctly at factory spec of 4*btdc with the bypass wire unhooked
    cut off exhaust to eliminate restriction possibility
    found 1 bent pushrod
    replaced all 16 pushrods with MPR 380 and MPR 379, respectively.
    adjusted rockers in proper sequence, plus 3/4 of a turn
    checked injector pulse with noid light
    checked for correct ecm and prom chip

    The only things I can do, are replace those cheap ass plug wires, replace the spark plugs again, as they are now fouled, and install a new o2 sensor, which i bought a while ago but didn't install because i would like to solve the fuel dumping problem before basically lighting $25 on fire. I can also recheck vacuum.

    I just have no idea why it's just.gushing fuel. it makes no sense. it pops right off, runs ok for about 3 seconds, then once the fuel hits it just drowns itself, sputters, dies, and fuel goes flying everywhere.
     
  8. Daveo91Burb

    Daveo91Burb Full Access Member

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    Injector sticking open? Impressive list of stuff you’ve done but didn’t see injector replacement or rebuild in there? I also had a fuel gushing problem after I took my TB apart last time - turns out I left off the o-rings at the bottom of the injectors!

    I’m thinking you might want to change your oil pretty soon - probably quite a bit of gas in there.

    Good luck!
     
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  9. MrSony

    MrSony Junior Member

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    Don't really feel like paying $50 to $100 a piece for new injectors. And I thought they were sealed and not rebuildable? And yeah, I should change the oil. just some cheap crap until i fix the fuel issue. I touched a 9 volt battery to both injector and they do click.
     
    Last edited: May 20, 2019
  10. bucket

    bucket Super Moderator Staff Member

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    Honestly, replacement injectors should have been high on the list to start with.

    Also, I've seen TB rebuild kits that include assembly diagrams that want you to install seals in the wrong order, causing fuel to bypass the injectors and dump it in the intake. It also helps to lube the seals with ATF or dielectric grease, so they don't get pinched.
     
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  11. 4WDKC

    4WDKC Full Access Member

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    Wish I had seen this before you started replacing things, as you should have put a different cam in while you were there. The stock cams for 454 tbi is a great cam for a 350, terrible for a 454. i believe you have a few problems you are fighting and getting confused over. First your o2 sensor isnt used till the engine warms up so replacing it did nothing to fix the cold start up. The replacement of the IAC requires you to calibrate the system but this is all not possible because it doesnt run. unplug the tps sensor and start the truck. If it runs fine the issue is a sensor coming online and giving the ecm bad info. Disconnecting the tps will make the truck run in limp home mode which is very basic and uses preset values instead of what most sensors tell it.. Try that and report back what it does. Also I would change the spark plugs to a r44. The number tells you the heat range and the 43 is what I run in my 10.5:1 406, the colder temp and low compression that engine had stock will make it load up not run right and foul plugs. Do you know anyone with a scanner so you can see live data? Or are you computer savy enough to build your own cable so you can see the info?
     
  12. bucket

    bucket Super Moderator Staff Member

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    Also, have you tried to pedal it and keep it running? Or do you simply turn the key and see what happens with no intervention?
     
  13. MrSony

    MrSony Junior Member

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    It will run for about 25 seconds, load up with fuel and die. I have not replaced the o2 sensor, but I have one. I have done the iac reset procedure. It pops right off just immediately starts spewing fuel. I have a scanner, all sensors are reading what they should. No codes either. Tps is .66 volts closed, smooth all the way up to 4.39 to wot. Coolant temp matches up with what my temp gun says. The cr43ts are the factory plugs. One would think they would be fine but I'll try the 44s. And no need for a cam. Its just a tooling around vehicle. I ain't got time or knowledge to be fooling around with trying to make the crappy tbi system work with a non stock cam.
     
  14. Daveo91Burb

    Daveo91Burb Full Access Member

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    What happens when you cycle the fuel pump? There should be a lead on the relay that’s on the firewall - hook up 12v to that guy and it will turn on your pump and run fuel thru the regulator and back to tank thru return line. You should hear it but you shouldn’t see any fuel in the injector bores or coming out of the injectors.

    Have a fuel pressure gauge? What’s pressure? Should be between 9-13 psi


    Sent from my iPhone using Tapatalk
     
  15. 4WDKC

    4WDKC Full Access Member

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    Tps should be closer to .5 at idle.
     

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