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94 GMC sierra 5.7 (tbi) running rich with a mis

This is a discussion on 94 GMC sierra 5.7 (tbi) running rich with a mis within the Technical / Maintenance forums, part of the General Discussion category; Alright i have had this truck for about 2 months now.. when i got it it ran ok but wanted ...

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    Registered User ortiz11b's Avatar
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    94 GMC sierra 5.7 (tbi) running rich with a mis


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    Alright i have had this truck for about 2 months now.. when i got it it ran ok but wanted to make it run better to pass emmisions testing as im in texas... it has 178k miles on it. (was suppose to have goten a new long block at 100k miles)its got the 5.7 TBI seems to be running rich and has a slight miss that is more notacible when in gear(not a constant miss very erratic). Did fuel filter, fuel presure regulator, new injectors, cap & rotor and wires, and plugs, and collant temp sensor ,changed out EGR valve, Tried a new map sensor and tps sensor, didnt help when i changed them out so put old ones back on... also looked at all the vacum lines found a few cracked ones and a couple cracked boots... but when i changed them out still has a slight miss and running rich....there is no check engine light on eather. please help... now the mis is more noticable when driving up hill... but when its in cruise control and on a flat road it drives fine... please help!!!!!

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    Re: 94 GMC sierra 5.7 (tbi) running rich with a mis

    check timing. should be zero degrees.
    injectors spraying a fine cone pattern?
    does engine idle high when fully warm?
    vacuum leaks around tbi base gasket, pcv valve/hose to tbi?
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    Re: 94 GMC sierra 5.7 (tbi) running rich with a mis

    Havent checked timming,,, the old injectors had a steady drip in the middle so replace them and now spraying good. engine idle is normal or sometimes low,,, but not high,, and replaced all gaskets on the tbi as they were cracked and old,,,, but new ones didnt seem to help... the only thing that i havent changed on the TBI is the IAC,,,, and just when out and started it,,, its hard to explane the mis.... hardley noticable when in park,,, but when in gear its erratic,,, will miss a few times then quit,,, then put it in park and back in gear and wont mis.....

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    Re: 94 GMC sierra 5.7 (tbi) running rich with a mis

    remove vacuum line to egr valve qnd plug it. if the problem is gone probably incorrect or bad valve.

    iac valve controls idle. is it idling at a different rpm?
    O2 GMC RCSB 4.8L V8 AUTO 3.73's
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    Re: 94 GMC sierra 5.7 (tbi) running rich with a mis

    no has a steady idle,,, thanks,,, ill try that in a bit,,, i unpluged the vacum line before while it was running to the egr and seemed to do nothing was running good... figured since i just put a new one one it would be good,,, but ill the unplug and seal it to see if the miss is gone. thanks again!!!!!!!

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    Re: 94 GMC sierra 5.7 (tbi) running rich with a mis

    ok unpluged the egr vacum line and taped off,,, and was still missing ,,, i think even more so,,, so pugged it back in and pulled up on the egr diaphram and it caused the engine to stumble did it a few times and same result,,, but after that i put it in drive and didnt mis,,,, will try again in a few mins and see if still acting right or if it was just a fluke.

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    Re: 94 GMC sierra 5.7 (tbi) running rich with a mis

    still has the miss..... still scratching my head!!!!!!! lol

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    Re: 94 GMC sierra 5.7 (tbi) running rich with a mis

    the distributor look original?
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    Re: 94 GMC sierra 5.7 (tbi) running rich with a mis

    yes it looks to be original,,,, did replace the cap and rotor,,,

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    Re: 94 GMC sierra 5.7 (tbi) running rich with a mis

    if its running rich are the plugs fouled? smell gas?
    might be lean check fuel pressure?
    O2 GMC RCSB 4.8L V8 AUTO 3.73's
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    Re: 94 GMC sierra 5.7 (tbi) running rich with a mis

    all plugs were wet last time i pulled them out to check them,,,, cleaned them off and put them back in... when i open the air cleaner after shutting it off its pretty strong and you can smell it thru the tail pipes... i havent checked fuel pressure... i did change fuel pressure regulator and fuel filter and injectors ... if fuel pump is going out would it cause it to run rich? or lean? thanks

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    Re: 94 GMC sierra 5.7 (tbi) running rich with a mis

    "update " I dont know if this will help anyone help me narrow it down but just went out and started the truck and took it for a spin to get a dr pepper,,, when first driving it had no signs of a mis and was driving good,,,, but i was babying it,,,, on my way back home i put my foot in it and then the mis was back... still drove home ok,,,, but could notice it missing up hill... hmmmmmmm
    Last edited by ortiz11b; 12-06-2010 at 05:15 PM.

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    Re: 94 GMC sierra 5.7 (tbi) running rich with a mis

    things im listing are easy and cheap but take some time.
    First try the butane trick for leaks around intake,
    Check your TBI gasket,
    like stated over and over on here, fuel pressure, needs to be checked while driving not just idle in gear.
    Timing, make sure your at 0, AND check your timing advance wire. Make sure its hooked up. Some forget after setting timing.
    Check every wiring harness connector, make sure they connected and not freely sliding out. (clips in place)
    Grounds
    Operating temp? are you at 195?

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    Registered User ortiz11b's Avatar
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    Re: 94 GMC sierra 5.7 (tbi) running rich with a mis

    I did change the CTS sensor and put a new 195 t-stat,,,, my temp guage reads 100 to 140,,, i know that sensor has to be bad,,, but cant seemt to get a replacement senor for the guage but i was told it didnt make effect the motor cpu,,,,, change out tbi gaskets also ,,,, need to check timing and fuel pressure asap

 

 

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