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'91 5.7 TBI lacks power

This is a discussion on '91 5.7 TBI lacks power within the Technical / Maintenance forums, part of the General Discussion category; I recently purchased a 1991 K1500 Extended cab 4x4 w/5.7 and auto transmission.I noticed the truck lacked power and tested ...

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    Registered User swampfox44's Avatar
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    '91 5.7 TBI lacks power


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    I recently purchased a 1991 K1500 Extended cab 4x4 w/5.7 and auto transmission.I noticed the truck lacked power and tested and replaced plugs,wires,cap,rotor,EGR valve,EGR vacuum solenoid,map sensor,ECT sensor,temp sensor,thermostat,fuel filter and air filter.TPS tested fine.All parts were gm/delco parts.Also replaced all the exhaust from the "Y" pipe back,all new.The truck runs alot better,but just seems to lack power.My dad has a '93 k1500 with twice the miles as mine and pop's truck will run circles around mine.It lacks power through all speeds.It really lacks power when in overdrive at highway speeds (60-75 mph).It loses alot of speed on even the slightest of inclines.Recently did some towing and truck really struggled.I have to give truck alot of pedal to keep speed up.Truck starts and idles well,uses no oil.I'm puzzled.Any ideas?
    Swampfox44
    2003 Mercury Marauder
    1994 F-350 Crew Cab Turbo Diesel
    1991 K1500 Extended Cab
    1977 F-150 4x4 On 44" Boggers w/22" Lift

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    has a manual! Whifflebat's Avatar
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    Re: '91 5.7 TBI lacks power

    shooting in dark... any exhaust manifold cracks?
    Rest in Peace Jason Payne

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    Registered User swampfox44's Avatar
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    Re: '91 5.7 TBI lacks power

    Nope,no exhaust leaks either.Inside of distributor had alot of red/rust dust and residue.Flushed out with electrical cleaner when changing cap and rotor.Was wondering if maybe worn distributor,bad p/u coil or maybe ignition coil getting weak.Considered worn timing chain,but rotated engine back and forth w/pull handle and doesn't seem to have much play in chain.Just used to driving most vehicles and only using minimal pedal pressure,you got to give this one about 1/2 pedal to maintain highway speeds.Lack of power is a real issue.
    Swampfox44
    2003 Mercury Marauder
    1994 F-350 Crew Cab Turbo Diesel
    1991 K1500 Extended Cab
    1977 F-150 4x4 On 44" Boggers w/22" Lift

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    Powered by Vortec 97sierra454's Avatar
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    Re: '91 5.7 TBI lacks power

    What axle ratio and tire size is on the truck? (axle ratio can be found on a sticker in the glove box - GU4 = 3.08 GU6 = 3.42 GT3 = 3.73 GT4 = 4.11)

    Are the fuel injectors clean?

    Have you checked and adjusted the base timing?

    Do you have a healthy fuel pump?

    Did you check the ECM to see if the ECM and prom chip are for that engine?

    1997 Sierra 2500HD 4x4, 454, ECSB
    1998 Silverado K1500 Z71, 350, ECSB
    2009 Pontiac G8 GT Sport

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    Registered User swampfox44's Avatar
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    Re: '91 5.7 TBI lacks power

    Truck has a 3.42 gear,even verified it when I serviced front and rear diffs.Tires are 265/75-16's which are the factory size on it.I already checked and set base timing,it was only off slightly,but is at O* now.I actually replaced the ECM on a whim from a dealer friend and it was factory unit.Spray pattern looks good when you rev engine,but,I don't know about the fuel pressure as I have not checked it.Like I said,my pop's truck has everything identical,same gears,tires and all and it will dust mine really bad.
    Swampfox44
    2003 Mercury Marauder
    1994 F-350 Crew Cab Turbo Diesel
    1991 K1500 Extended Cab
    1977 F-150 4x4 On 44" Boggers w/22" Lift

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    Powered by Vortec 97sierra454's Avatar
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    Re: '91 5.7 TBI lacks power

    It's not so much the ECM as it might be a problem with the EPROM chip. If for some reason the EPROM is for a 305, it will run, but be a major pig. Both the 5.0 and 5.7 use the same ECM.

    I drove a '91 K1500 for a while with the same tires and gears, and it was a bit of a pig since those are really highway gears, but it should still go unloaded.

    It sounds like you've covered most of the bases. Have you eliminated stupid things like low compression, dragging brakes, binding CV's, etc?

    You can also bump up the base timing a bit, get it so it just doesn't ping; you might be able to advance about 5 degrees.

    1997 Sierra 2500HD 4x4, 454, ECSB
    1998 Silverado K1500 Z71, 350, ECSB
    2009 Pontiac G8 GT Sport

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    Registered User swampfox44's Avatar
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    Re: '91 5.7 TBI lacks power

    Haven't run compression check on it.Brakes and cv's aren't dragging or binding.Everything spins free.What should fuel PSI be and what's best way to check it?
    Swampfox44
    2003 Mercury Marauder
    1994 F-350 Crew Cab Turbo Diesel
    1991 K1500 Extended Cab
    1977 F-150 4x4 On 44" Boggers w/22" Lift

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    I LOVE THE SNOW!!! GIMECHANIC_454's Avatar
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    Re: '91 5.7 TBI lacks power

    Quote Originally Posted by 97sierra454
    It's not so much the ECM as it might be a problem with the EPROM chip. If for some reason the EPROM is for a 305, it will run, but be a major pig. Both the 5.0 and 5.7 use the same ECM.

    I drove a '91 K1500 for a while with the same tires and gears, and it was a bit of a pig since those are really highway gears, but it should still go unloaded.

    It sounds like you've covered most of the bases. Have you eliminated stupid things like low compression, dragging brakes, binding CV's, etc?

    You can also bump up the base timing a bit, get it so it just doesn't ping; you might be able to advance about 5 degrees.
    if the compression is low all it takes is the compression on 1 cylinder to be low and it will cause a big power loss. my 89 rs camero with a 305 started to doin the same thing and it had no power... 1 cylinder(#6) that was lower than the rest when i did the compression test so i traded it in quick and bought my truck that i have now..any way how many miles does it have on it, and another thing u might check would be the intake gasket leaking in air that will cause a drop in power too


    JUST REMEBER IF YOUR GONA DO ALOT OF DAMAGE..DON'T STOP TILL IT SOUNDS EXSPENCIVE
    :fsc:

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    Powered by Vortec 97sierra454's Avatar
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    Re: '91 5.7 TBI lacks power

    Quote Originally Posted by swampfox44
    Haven't run compression check on it.Brakes and cv's aren't dragging or binding.Everything spins free.What should fuel PSI be and what's best way to check it?
    Fuel pressure should be 9-13 PSI and to be honest, it's a pain in the ass to check it on a TBI. Easiest method is to set up a testing adapter in place of the fuel filter, otherwise you have to disconnect the lines from the back of the TBI to install an adapter. I would definitely try a compression test first.

    1997 Sierra 2500HD 4x4, 454, ECSB
    1998 Silverado K1500 Z71, 350, ECSB
    2009 Pontiac G8 GT Sport

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    Registered User swampfox44's Avatar
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    Re: '91 5.7 TBI lacks power

    Truck itself has 280,000 on it but engine was replaced last year,crate motor has 82,000 on it.
    Swampfox44
    2003 Mercury Marauder
    1994 F-350 Crew Cab Turbo Diesel
    1991 K1500 Extended Cab
    1977 F-150 4x4 On 44" Boggers w/22" Lift

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    I LOVE THE SNOW!!! GIMECHANIC_454's Avatar
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    Re: '91 5.7 TBI lacks power

    Quote Originally Posted by swampfox44
    Truck itself has 280,000 on it but engine was replaced last year,crate motor has 82,000 on it.
    ok well if the engine only has 82,000 mi in it the rings should still be good, but ya never know.


    JUST REMEBER IF YOUR GONA DO ALOT OF DAMAGE..DON'T STOP TILL IT SOUNDS EXSPENCIVE
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    Powered by Vortec 97sierra454's Avatar
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    Re: '91 5.7 TBI lacks power

    Quote Originally Posted by swampfox44
    Truck itself has 280,000 on it but engine was replaced last year,crate motor has 82,000 on it.
    Do you know of this as a fact, or as what you were told when you bought it? Was the crate motor just a long block? It would be good to find out whats wrong here before you lose any more money on this thing.
    Last edited by 97sierra454; 04-17-2006 at 02:16 PM.

    1997 Sierra 2500HD 4x4, 454, ECSB
    1998 Silverado K1500 Z71, 350, ECSB
    2009 Pontiac G8 GT Sport

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    Registered User swampfox44's Avatar
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    Re: '91 5.7 TBI lacks power

    It was a complete engine.Have all the paperwork and knew the guy I got it from as well.Just hope I can run it down.What should fuel mileage be on this truck?Still have the factory EPA paper work and it was claiming 13-14 mpg.Mine gets about 12-13mpg.Is this about right?
    Swampfox44
    2003 Mercury Marauder
    1994 F-350 Crew Cab Turbo Diesel
    1991 K1500 Extended Cab
    1977 F-150 4x4 On 44" Boggers w/22" Lift

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    Powered by Vortec 97sierra454's Avatar
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    Re: '91 5.7 TBI lacks power

    12-13 mpg isn't too bad on a TBI with 265's. Do you know who made the motor? If it wasn't a reputable shop, it's possible it's spent already, as who knows what they did with honing the bores, etc.

    1997 Sierra 2500HD 4x4, 454, ECSB
    1998 Silverado K1500 Z71, 350, ECSB
    2009 Pontiac G8 GT Sport

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    Registered User swampfox44's Avatar
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    Re: '91 5.7 TBI lacks power

    Was a GM crate engine.Truck was a 1 owner.
    Swampfox44
    2003 Mercury Marauder
    1994 F-350 Crew Cab Turbo Diesel
    1991 K1500 Extended Cab
    1977 F-150 4x4 On 44" Boggers w/22" Lift

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    4/6 dropped Silvy on 22s
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    Re: '91 5.7 TBI lacks power

    Try some seafoam, made a big difference in my motor.

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    Wink Re: '91 5.7 TBI lacks power

    Try replacing the distributor with a brand new ACDelco unit.
    It must be NEW and it must be ACDELCO.
    What happens is the magnets for the pickup coil become weak sending a weak signal which causes all kinds of ignition and injector timing problem's!
    If you buy a remanufactured one, your just buying someone elses weak magnets.
    We have fixed alot of low power complaints at the shop I work at with just a distributor.

    FYI: The magnets and pickup are not serviceable by themselves.

    HTH
    Dave
    Last edited by BurbanFoot87; 04-24-2006 at 10:41 PM.

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    You feel lucky......Punk? RnuovZ71's Avatar
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    Re: '91 5.7 TBI lacks power

    Have the Transmission cheaked My 92 losse power just as dicribed And I went throught and replace a but load of parts that didnt need to be changed, It was the damn tranny going out.

    Infantryman in the US Army

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    eating Fords and imports stealthhit's Avatar
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    Re: '91 5.7 TBI lacks power

    I would also check the O2 sensor they can be functioning but not reading right that will cause loss of power. Also the distributor will cause it but you say it seems to run fine check the O2.
    retiring 95 c1500 extacab, mild 355, 343 casted head 2.02 valves, Lunati truck avenger cam., old school 4 bolt main, stock tbi.
    01 Silverado 1500 4x4 4.8 stock

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    Registered User Blackfoot's Avatar
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    Re: '91 5.7 TBI lacks power

    Quote Originally Posted by stealthhit
    I would also check the O2 sensor they can be functioning but not reading right that will cause loss of power. Also the distributor will cause it but you say it seems to run fine check the O2.

    Whats the procedure to check a O2 sensor and make sure it's working effectively?
    1988 k1500 RCSSB, 6" RCD, ORD Shackle flip, PA 3" BL, 15x10's w/ 35 x 12 MT Baja Claw Radials, Under Construction - never satisfied

 

 
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