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Thread: Need help solving surging deceleration and idle

  1. #16
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    Quote Originally Posted by Roadknee View Post
    The problem you're describing is not the norm for the 7427 ECM and TBI on a stock Vortec long block. The surging is caused by the O2 sensor swings on deceleration. I wonder if you have a very small exhaust leak (bad gasket, cracked weld in header or exhaust system, etc.) where fresh air is being pulled into the exhaust on deceleration only. The O2 sensor would see this as lean and the ECM would overcompensate until it swings rich, then overcompensate back to lean an on and on...
    You sir are good, I looked and sure enough a nut has worked its way off the collector on the side that the NB is on.

  2. #17
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    On my way home from Yellowstone as I type. I will make that change and get a new data log asap. Thanks for the tip!
    1990 Chevrolet K2500, RCLB, Blueprint 383 TBI (16197427 ECM / 61 lb/hr / 16 psi FP), NV4500, NP241C, 4.10

  3. #18
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    edit: double post, sorry
    Last edited by 91dime; 05-18-2015 at 07:27 AM.

  4. #19
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    I burned a new chip this morning and tested on the drive to work. Initial feel says that it is much improved, but my drive is barely long enough to get up to operating temp haha! I will do some more driving later to confirm.
    1990 Chevrolet K2500, RCLB, Blueprint 383 TBI (16197427 ECM / 61 lb/hr / 16 psi FP), NV4500, NP241C, 4.10

  5. #20
    Fuel Injected! Roadknee's Avatar
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    chewy- what cylinder heads are on your 383?

  6. #21
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    They are modified TBI heads. Here is a link to the crate engine specs - http://www.blueprintengines.com/inde...em/gm-383-base
    1990 Chevrolet K2500, RCLB, Blueprint 383 TBI (16197427 ECM / 61 lb/hr / 16 psi FP), NV4500, NP241C, 4.10

  7. #22
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    I had some more time at lunch and after work to really test the decel. I could run it up to 3000 rpm in 3rd and it would decelerate smoothly all the way to idle. Even 2nd gear deceleration was smooth. I think we have it taken care of! I'm really happy with how it's running now.
    1990 Chevrolet K2500, RCLB, Blueprint 383 TBI (16197427 ECM / 61 lb/hr / 16 psi FP), NV4500, NP241C, 4.10

  8. #23
    Fuel Injected! Roadknee's Avatar
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    I'm happy it worked out and a bit surprised it was that easy. I've been fiddling with fuel trim parameters on the vortec headed 350 off and on for months and it's still not perfect. I looked at your engine specs and I'll bet that thing does run really good, particularly compared to a stock worn out 350.

  9. #24
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    It's a night and day difference compared to the old 350 with 2 dead cylinders. It's even better now that the 80# injectors are dialed in. The engine was built to run with a stock 350 TBI setup, just swap and turn the key, but I had a feeling the old 55# injectors were being overloaded. Bigger injectors and a new EP381 fuel pump are enough to feed the beast for now. I didn't think about touching the O2 parameters because I was running a stock replacement sensor so thank you for pointing them out to me!

    I will probably leave the engine alone for the rest of the year as I just found a donor truck to source an NV4500 and 4.10 gears to swap with my SM465 and 3.42's. It's going to be a busy summer!
    1990 Chevrolet K2500, RCLB, Blueprint 383 TBI (16197427 ECM / 61 lb/hr / 16 psi FP), NV4500, NP241C, 4.10

  10. #25
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    Good to hear you got it running! I thought i had fixed my exhaust leak but its still acting up so i may leave the o2 out since it seems to run much better all around.

  11. #26
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    The idle and deceleration surge is back! :( I changed out the SB throttle body for a BB unit with adjustable fuel pressure regulator. Small adapter harnesses were used to connect the TPS and IAC to the existing plugs. The live readings in the ECM look correct. Installed the Actron FP adapter inline with the supply line and the gauge reads 18 psi. I used a 7.4L Manual bin for IAC parameters and the idle will get down to the commanded 800 RPM, but it surges. i have attached a video of the madness. Any ideas?

    https://drive.google.com/open?id=0B-...VM4T1pyUHZCNVU

    EDIT: Jumpered Pins A and B in the plug to do an IAC reset, and noticed there was a Code 51 present. Changed the Mask ID in the bin to "AA" and burned another chip. The idle is much smoother.
    Last edited by Chewy1576; 02-21-2016 at 07:20 AM.
    1990 Chevrolet K2500, RCLB, Blueprint 383 TBI (16197427 ECM / 61 lb/hr / 16 psi FP), NV4500, NP241C, 4.10

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