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Thread: Need some help, TBI/Vortec Conversion Troubleshoot

  1. #1
    Carb and Points!
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    Need some help, TBI/Vortec Conversion Troubleshoot

    Ok guys, forgive me if this is the wrong place to post this. Figured I may have better luck here then ls1tech. My friend and I just finished putting together his 91 1500 standard cab shortbed with a freshly rebuild 96 Vortec (bare with me here because I'm going to do my best to explain the situation for him but hes not very internet savy and without him standing here I may flub some of the details.)

    A little about the motor: Its a freshly rebuilt 96 Vortec 350 out of the same body truck but it has a conversion 4 barrel intake on it with an adapter plate to run the 91 TBI so he could retain his stock factory harness/engine management. The motor has crank and cam sensors but the 91 harness has no accommodation for them since they aren't part of the combination. All the other sensors from the 91 along wit the throttle body (a 675 cfm according to him) and distributor were taken off the old motor and put onto this one.

    In theory I dont see why the simple addition of the Vortec style top end would change the management system to the point where it can no longer compensate the right amount of fuel but heres the issues were having and Ill try to itemize them all seperatly.

    1) The truck idles fine (a little rougher than it probably should but not drastically bad) but when you accelerate or put a load on it it instantly starts to break up.

    2) He acquired some sort of obd1 data acquisition tool for his laptop and it shows me the following fields. Knock Counter (which is going nuts), MAP kPh, TPS % (ill get to this part in a second), TPS voltage, RPM, and Ignition Block Learn (no idea what this is). There were a few other fields too but I cant recall off the top of my head.

    3) With the key on engine off we did a sweep to watch the TPS reading... at 1/4 throttle it reads 100%... at WOT it shows 222.0%. I know this has to be part of our problem

    4) On our first test drive last night we set the motor at base timing. It broke up terribly between his house and mine (5 minute drive) when we got to my house we turned the distributer and upped the timing. On the return trip the truck still had a few studders but ran much smoother than before. Problem is when we got back and looked the manifolds (factory) were glowing cherry red indicating lean I would assume. The knock counter on the first run was around 220-240, on the second run it stayed around 40.

    5)We know it has a valve train noise which he is going to work on fixing today. Sounds like a lifter out of adjustment. I know some of the early knock sensors are extremely sensitive and were wondering if this is a contributing factor.

    6) Were not convinced were getting enough fuel the way the truck acts. Im not so sure the added flow from the vortec with the stock 91 throttle body is not flowing the way it should (again all just my thoughts)

    7) He informed me last night that him and his dad took a 650cfm holley and an HEI dist. off one of there other trucks last week and set it up and the motor ran like a charm which leads me to believe its not a mechanical issue with the motor. (i told him id have left the damn thing on there and called it a day but hes determined to make the fuel injection work)

    So basically I'm at a complete loss here. I worked in the drivability diagnostic field for 2 years when I was still in the dealerships and never encountered anything like this. Probably dosent help ive been spoiled by OBDII, HP Tuners, and LS motors.

    So heres my basic all around questions... If we continue with this setup where should I start to diagnose? or... Is there a cost effective way to make the vortec stuff work in the 91 body. Conversion harness? He's determined to go fuel injected. I recommended EFI connection and 24x but it would put him way over budget. I use there stuff on my LT1 and its a great setup but I understand the cost part.

    Appreciate any incite you can give me. I hate to keep throwing parts at it cause were not getting anywhere. Thanks!

  2. #2
    EFI GearHead ! EagleMark's Avatar
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    The Vortec head change will require a new chip, more for spark curve then fueling. But if you want it to be all it can there is a big difference.

    Check fuel pressure and should be close to 12 PSI.

    Vacuum leaks at TBI adapter are so prolific I would suggest checking that. Bolts holding the adapter to intake can and usually hit intake pushing adapter up and causing a vacuum leak if not checked during install. After fuel pressure and setting timing properly. Could be other vacuum leak as well?

    Timing needs to be set to 0... maybe 5 for now with bypass wire disconnected. Then turn off engine, re connect bypass and clear codes.

    The TPS % reading is more likely the scanner? You can check the voltage and it should be be close to .54 closed and about 4.5 open with a smooth swing of voltage when opened slowly. No dips or spikes.

    There's no reason for a new harness. What you have is fine. It should run better then you have described. You have an issue mechanical as mentioned above... or possibly a ignition issue like bad EST module or pick up coil in distributor.

    HTH get's you going the right direction.

    1990 Chevy Suburban 5.7L Auto ECM 1227747 $42!
    1998 Chevy Silverado 5.7L Vortec 0411 Swap to RoadRunner!
    -= =-

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