help request 1990 fld 3406b cat top speed 55mph on 1550rpm

Discussion in 'Trucks [ Eighteen Wheelers ]' started by omar.ib, May 15, 2022.

  1. omar.ib

    omar.ib Bobtail Member

    May 15, 2022
    hi guys.
    i'm new to the forum, i recently bought an 1990 FLD with a 3406b cat engine with a 13 speed Eaton, 3.42 gear ratio, tires are 295/75R22.5
    here's my problem my truck seems having a problem passing 55mph, once i'm at 55mph my RPM is at 1550, and if i try to accelerate my truck start shaking a lot and want go beyond 55mph.
    called the previous owner and he confirmed to me that he used to drive at 70 to 75 mph, i believe him because he had no reason to lie to me. also the truck used to run in Oregon.
    does anyone know what could be the problem? is it low HP? transmission? gear ratio? fuel pump? truck been governed?
    what should i look at
    blairandgretchen Thanks this.
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  3. God prefers Diesels

    God prefers Diesels Road Train Member

    Jun 26, 2020
    South Texas
    Are you sure you've split high gear? Not talking about the range selector. Talking about the splitter. It's the red button slider on the side. Not sure how much experience you have in a 13-speed. We had a truck that people would get stuck doing 50mph in because they were still on the low side.
  4. blairandgretchen

    blairandgretchen Road Train Member

    Dec 9, 2011
    South west Missouri
    You running this truck - or a driver of yours?
  5. BoxCarKidd

    BoxCarKidd Road Train Member

    Aug 26, 2014
    Are you sure it is not a PEEC electronic engine. Post your engine serial and AR #. Others around here know a lot more than I and can help you. A picture of the pump would be helpful as it could have been changed.
  6. omar.ib

    omar.ib Bobtail Member

    May 15, 2022
    honestly it's my first time driving the 13, but also i drove another 13 that belong to my friend and didn't have any issues, so not sure if i'm just doing it wrong or is it the truck, and yes i do have a splitter.
    Keepforgettingmypassword and cke Thank this.
  7. omar.ib

    omar.ib Bobtail Member

    May 15, 2022
    i'm running the truck
    blairandgretchen and cke Thank this.
  8. omar.ib

    omar.ib Bobtail Member

    May 15, 2022
    i will provide the info tomorrow. thank you
  9. LameMule

    LameMule Heavy Load Member

    Jan 19, 2020
    Fuel pressure gauge, manifold pressure/boost gauge readings are helpful for diagnosing these issues.

    If the easy stuff checks out then maybe look to see if the timing slipped or is stuck. Someone with an ear for it or an eye for smoke reading could pickup on it quick.

    There's multiple things that could cause a vibration or shudder at speed or under acceleration. The hard part is deciphering someone's interpretation of it over text. Over fueling, air in fuel or improper timing are common reasons for a shudder or vibration on acceleration but there's a ton of rotating parts that could be vibrating while rolling in neutral at 55mph. Something as simple as throwing it in neutral at 55mph can do a ton to help find the source of a vibration. If it goes away in neutral at idle coasting but returns at 1550rpm you've just narrowed it down a ton.

    Could be as simple as a torn AFR diaphragm and a worn out harmonic balancer... or it could be a number of other issues. There's no substitute for hands on diagnosis but as much data as possible helps.
    mtoo Thanks this.
  10. mtoo

    mtoo Road Train Member

    Jan 15, 2011
    Retired on bended knee
    I remember the first time I had an AFR diaphragm go bad, thanks for the memory @LameMule
    Oxbow Thanks this.
  11. rbrtwbstr

    rbrtwbstr Road Train Member

    Jul 11, 2012
    in the bush somewhere
    At 1550 rpms in top gear you should be close to 70 mph with 3.42 rears.

    Either you're not in 13th gear, or you are and speedometer is busted. The shaking I could only wonder what that could be
    God prefers Diesels, mtoo and LameMule Thank this.
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