Engine swapping a rusty old truck

Discussion in 'Engine Performance + Maintenance' started by Ian Martin, Jun 30, 2020 at 5:13 AM.

Car Parts
  1. Ian Martin

    Ian Martin Newbie

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    Alright guys, I didn’t get any help from the fb group I went to..so here I am! Sorry in advance because my plans are a mess for this truck right now.
    So I just recently picked up a 91 c1500 w/t with the 4.3 and 700r4. The trans feels strong and doesn’t give me any major red flags, but the engine... smokes like a train (currently trying to narrow down between valve seals, drain channels in the head, egr or pcv), heat riser in the intake doesn’t work, runs really rough, BUT only has 145k on the clock and doesn’t leak any fluids
    I’ve got a doner truck with the cfi vortec 4.3 and an iffy 4l60e.

    Would it be worth it on the current motor to fix the oil consumption and swap the vortec heads and cam from the doner for a reliable power bump or does it sound like that motors not worth the effort? Any modifications besides just swapping the parts over?

    If I swap the cfi and harness into the 91 would it be able to work with the 700r4 since it doesn’t have an electronic transmission to control and I don’t trust the 4l60e in the doner? If so what all parts would need to be swapped or replaced/upgraded besides engine, ecu, and harness?

    Or is it better for me to just buy a tbi 305 and slap it in?

    I know it’s all over the place but I’d like your input on which route would be the best balance of simplicity, reliability, and performance. I’m not a newbie to mechanical work but this would be my first motor swap and I work ALOT so I didn’t want to get stuck in the middle of something I wasn’t prepared for and end up with my truck down for weeks. Thanks!
     
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  2. Busted Knuckle

    Busted Knuckle OBS Enthusiast

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    I would swap in a combo that was available in 1991 for minimum hassle. Then you can use a proper ECM, cal module, and ignition module. Generally the 4L60E is a better suited trans for this truck IMHO. The ratios are closer and it has proven to be reliable. Likely need a different drive shaft length and front spline count in the yoke to use the '60E.
     
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  3. df2x4

    df2x4 Domestics only.

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    700R4 and 4L60E are identical ratios and the reliability of either is iffy in my opinion. Either should probably be OK for a stock 4.3/5.0/5.7 as long as nothing is altered in the tune, though.

    Switch that to TBI 350 and that would be my logical choice. I'm not super familiar with the TBI trucks but you could probably find one for cheap and use a junkyard PCM that matches the rest of your configuration. Swapping PCMs and harnesses just to stick with a V6 is a lot of work for not much gain IMO.
     
  4. Ian Martin

    Ian Martin Newbie

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    I had read somewhere on a forum that if you swap a tbi 350 in that the v6 700r4 can’t shift properly and has to be rebuilt with parts designed for v8 application. Is this true? I had never heard anything about that before yesterday
     
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  5. df2x4

    df2x4 Domestics only.

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    I've read a few posts in which people say the V6 700R4/4L60Es aren't quite as strong as the V8 ones, but nothing related to controlling them or shifting differently with different engines. I don't see any way that could be the case.
     
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  6. Ian Martin

    Ian Martin Newbie

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    Yeah it was the first time I had heard of that, just wanted to check because some people were saying “blah blah different hydrolic parts, doesn’t shift right with the increased load”
     
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  7. Dropped88

    Dropped88 I'm Awesome

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    ive rebuilt a 700r4 from a v6 and v8 and there was zero difference inside. not to say v6 trans been replaced or rebuilt previous but i dont believe it was

    either way i dont see enough power difference between the 4.3 and 5.7 to cause a trans issue
     
  8. PlayingWithTBI

    PlayingWithTBI Desert Old Guy

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    The 5.7 has more torque. Isn't that why GM put a higher stall TC behind the 4.3?
     
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