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very high idle after engine swap

Discussion in 'Chevy Trucks' started by mike s 88, Sep 5, 2003.

  1. mike s 88

    mike s 88 Junior Member
    Messages: 21

    i recently changed out my 305 for a 350. the new engine is from a 75 impala. i had a hard time changing over due to the year difference from the 88 pickup. for some reason it has a very high idle an i can not get it down any help would be really great! thanks, mike
  2. wyldman

    wyldman Member
    Messages: 3,265

    A few more details would be nice.Year make and model of the truck,etc.Fuel injected ?

    Sounds like you may have a vacuum leak,or the timing is set to far advanced.
  3. mike s 88

    mike s 88 Junior Member
    Messages: 21

    its a 1988 chevy 2500 it has throttle body injection. i was able to close the butterfly all of the way but it didnt help. i think the timeing is pretty close and i did notice a little leak possibly from the intake manifold to the head. thanks ,mike
  4. rollsroyce1999

    rollsroyce1999 Junior Member
    from Ohio
    Messages: 6

    I aquired a truck that had a 350 to 305 swap (I don't know either). This truck has a similar problem, I rebuilt the regulator thinking it might be leaking, but I'm beginning to think that it's the pcm. I know that there are two different pcm's for 89 k1500's, one of which is the same part number as the 305 pcm. I suppose it is possible that there is still a different fuel and ignition map programmed into these. If you have found a solution to your problem I would love to hear it. Mine still idles at about 1100 and runs rich.
  5. derekbroerse

    derekbroerse 2000 Club Member
    Messages: 2,377

    I am assuming that you used the short-block from the Impala, as the heads and up are entirely different. If you used the heads from the 305 (likely a '187' casting) your compression may be a tad on the high side for regular gas unless you happened to use a thicker head gasket. The Impala pistons are a flat top with a large chamfer machined around the outer circumference which would help you get away with that combo---but this is not your idle problem, if anything this will be a preignition problem when you start driving it. I'm also assuming you used the flywheel from the Impala too as it has a different bolt pattern.

    I am not understanding why this isn't working for you guys--I've read that a 305 computer can handle up to a 350, and that a 350 computer can handle up to a 400. Maybe they aren't including fuel mileage? lol

    My best guess is you guys need to get together and swap PROM chips. How convenient :p But if you have an intake leak definately FIX THAT FIRST! (start by making sure the intake is torqued properly) I am assuming that you used new, proper gaskets for the application and didn't try to cheap out and reuse the old ones.
  6. mike s 88

    mike s 88 Junior Member
    Messages: 21

    ok, i hope this helps but i doubt it. the hacks that helped me with this did not seal the intake properly so that was one problem. the main cause was i needed a new intake manifold and the rear screws bottomed out on the throttle body so it did not seal properly and sucked in alot of air. if you have any more questions feel free to ask this truck had educated me at least. by the way my trans wont upshift unless its wide open? any clue? thanks, mike
  7. derekbroerse

    derekbroerse 2000 Club Member
    Messages: 2,377

    What tranny do you have? The most common problem is the vaccuum modulator or its vaccuum source. Check that it has been connected properly, isn't leaking, or sucking up tranny fluid. If this is a new problem after the swap, likely a connection issue.
  8. SWC

    SWC Senior Member
    from VA
    Messages: 134

    High idle after engine swap

    I'd check the oil to make sure their isn't any water in it, if the intake was leaking you might have water/coolant in the oil pan.

    That'll cause you some real problems.