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Why isn't my TH350 putting the power to the ground?

Discussion in '1973-1991 K5 Blazer | Truck | Suburban' started by bigerik, Mar 18, 2001.

  1. bigerik

    bigerik 1/2 ton status

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    Why isn\'t my TH350 putting the power to the ground?

    Help! I got a TH350 from a reputable shop installed by a reputable dealer. I have major modifications on my carbureted Blazer (see at <A target="_blank" HREF=http://www.bigerik.coloradoK5.com>http://www.bigerik.coloradoK5.com</A>), but basically after I put on the 39.5X18 Boggers I have the following major problem: when I am, say, on the freeway going up a hill in Drive, I can floor it in order to accelerate and the tach revs up to like 3800-4000 rpm for awhile and I just start to accelerate when it all the sudden goes back to, like 2600 rpm and won't accelerate. It's like I'm towing something very heavy. In fact, I almost hold up traffic in the slow lane because it can't just accelerate up to 65 mph like a normal car. My suspicion is that the output at the rear driveshaft is good, but the big Boggers won't spin or get going fast enough so that the tranny is "getting back" the torque and is having to absorb it in the form of burning up the clutch bands or something else in the tranny. Drivetrain is a mildly built 350, TH350 rebuilt 2 yrs ago and a NP203 w/ parttime kit. All running gear is 1 ton w/ grade 8 hardware (very well maintained). All fluid levels are fine. I checked for vacuum leaks and only found a low quality Q-jet gasket. I have a huge K&N air filter w/ stock fuel pump, but fuel filters are new. Vehicle has a body lift so maybe tranny linkage may be tweeked. Tranny vacuum modulator is connected. ANY ideas you have would be appreciated. I think that these big Boggers are the culprit, but I put in 5.13 gears to accomodate for that. Do you think it's the tranny? HELP!
     
  2. bigerik

    bigerik 1/2 ton status

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    Re: Why isn\'t my TH350 putting the power to the ground?

    Anybody? I would hate to put in a new tranny (like I could afford it) and have it do the same thing. Do you think it might be some vacuum problem? Any suggestions for where else to look?
     
  3. bigerik

    bigerik 1/2 ton status

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    Re: Why isn\'t my TH350 putting the power to the ground?

    I'm really stumped. I have 2 almost identical trucks with auto trannies. If I am launching from a dead stop, the one with a TH400 will just accelerate and go like a bat out of Hell. The Blazer with a TH350 will start to launch hard, but cannot stay revved up and I can see the tach drop down from 4000 to 3000 rpm and then my fun launch is over. It is refusing to stay revved up for some reason. It has the stock-type torque converter. It doesn't seem to do this in the dirt, so maybe tire spin helps. Maybe no one is responding because you guys are all out wrenching or wheeling. I look forward to hearing feedback. I didn't put in a new torque stall converter when I put in the rebuilt TH350.
     
  4. BadDog

    BadDog SOL Staff Member Super Moderator Author

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    Re: Why isn\'t my TH350 putting the power to the ground?

    Wish I could help, but I'm not a tranny expert by any stretch. Insufficient pressure, stall too high, worn clutches, who knows without digging in. Only thing I can say is that with a rig built up like yours and used in the bogs with 39" tires, I wouldn't even start with anything less than a TH400 and a 205 (which is what I’m building for the rocks). The gears should take most of the strain off but that’s still allot of Mass to move. TH350s can be built to handle just about anything but the TH400 in stock form is already well along the path. Only benefit I see to the TH350 is weight which is usually not an issue for us (and of course it is much more common).

    Good luck...

    Bad Dog

    85 K30 CUCV, 350 TBI, TH400, NP205, D60/C14, 4.56
    Coming soon: 4" lift, 40" tires, massive cutting, shorter wb and rear overhang.
     
  5. tRustyK5

    tRustyK5 Big meanie Staff Member Super Moderator GMOTM Winner Author

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    Re: Why isn\'t my TH350 putting the power to the ground?

    Well I'm no tranny expert either..but to me it sounds like you have a bad convertor. Or possibly when the tranny was installed the convertor wasn't properly lined up with the tangs for the front pump...

    Rene

    [​IMG]<A target="_blank" HREF=http://jules.coloradok5.com>http://jules.coloradok5.com</A>
     
  6. bigerik

    bigerik 1/2 ton status

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    Re: Why isn\'t my TH350 putting the power to the ground?

    Rusty, I appreciate the tip. I have more mud bogs in six days so I need all the tips I can get. Thanks again--I'll be there when you need me.
     
  7. bigerik

    bigerik 1/2 ton status

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    Re: Why isn\'t my TH350 putting the power to the ground?

    Anybody else got any ideas on this?
     
  8. Blazer_Boy

    Blazer_Boy 1/2 ton status

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    Re: Why isn\'t my TH350 putting the power to the ground?

    What size tires were you running before this?
     
  9. Ditchdigger

    Ditchdigger Registered Member

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    Re: Why isn\'t my TH350 putting the power to the ground?

    I'm no Tranny expert either although I have rebuilt a TH-350
    Hydramatic. I didn't take the time to study what every part does, Basically, know how to tear it apart
    and rebuild it.
    But the one Important thing and probably the most overlooked
    part of a vehicle when making modifications in Torque and HP
    is the converter!

    The more Torque you have in the converters RPM range the better it will perform
    It all depends on how much Torque your engine is puttin out
    and what RPM most of it is. you may need a 100+ RPM stall converter But, their are many other reasons why a converter isn't working correctly like long duration Cams can have an impact on performance and many other things you also might be experiencing early shifts (in most cases) or even late shifts, you may have done engine modifications and drivetrain and yes even larger tires have an Impact. that is all the basics I can remember but there is more to it than that, I would suggest finding out more info online about converters no matter what trans you have (or even calling TCI or B&M)
    they all do the same, Its one of those forgotten things that you rarely have trouble with but, when you've done many (lets say) performance add-ons you might think your vehicle is performing better and it is, but as soon as you get the right converter your even happier thats what I found out with my 81 Trans AM and was also somewhat doing what your talking about but not as dramatic, and don't want to give you any more info because the last I dug into transmissions was over a year ago and can't remember for sure all the variables!
    But maybe your trans was rebuilt incorrectly and they always go bad many miles later, thinking it wasn't the shops fault!
    well this it turning into a Novel
    hope this helps!

    If it ain't Intimidating / It ain't a Chevy!
     
  10. bigerik

    bigerik 1/2 ton status

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    Re: Why isn\'t my TH350 putting the power to the ground?

    When I first put on the 10" lift 2 yrs. ago, I had the 39.5X18 Boggers on. It worked OK, but then I put on major serious performance parts (all I could find), and at some point put the big Boggers on my other truck. At that point, I ran 38.5X14.5 TSL/SXs. After I put on the Edelbrock RV cam and aluminum intake, I put the Boggers back on for the mud race. Maybe the vacuum got changed when the mechanic put in the new RV camshaft. These Boggers are very heavy; the Blazer could spin the 38.5X14.5s, but it still exhibited these awful symptoms with the slightly lighter tires.
     
  11. bigerik

    bigerik 1/2 ton status

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    Re: Why isn\'t my TH350 putting the power to the ground?

    Hey Ken from the "Who has big tires" post! Read this and let me if you think I just flat out need more motor! It sounds like you got some massive trucks!!
     
  12. CaptCrunch

    CaptCrunch 1/2 ton status

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    Re: Why isn\'t my TH350 putting the power to the ground?

    Once you get up to speed does it hold a speed without having it floored? I would guess converter also from this info... didn't look to see what kinda mods your made to the motor, but this could be an issue too... man those 39x14.5 have gotta weigh like 130 each mounted... then if you have 1 ton stuff and the big heavy drums out back... you are gonna need some power... not to mention TORQUE! BTW... IMO you always should replace the converter w/ a tranny rebuild... especially if there was a tranny faliure. Good luck Bigerik.

    [​IMG]
    1987 Chevy K5 Blazer- 350 TBI
     
  13. muddin4fun

    muddin4fun 3/4 ton status

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    Re: Why isn\'t my TH350 putting the power to the ground?

    I would also want to know what kind of torque converter you're running. If you have a lot of torque and HP then it will tear a stock one up real fast. Just runnin hard on big tires will do it also. What color is your tranny fluid?

    [​IMG]


    <A target="_blank" HREF=http://muddin4fun.coloradok5.com>http://muddin4fun.coloradok5.com</A>
     
  14. bigerik

    bigerik 1/2 ton status

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    Re: Why isn\'t my TH350 putting the power to the ground?

    I can get up to speed and it does hold speed, but it acts like it's towing something. People told me it would always run like it was towing something after I put on the one ton stuff, 5.13s, lifts and big tires (all at the same time). I drove it with these heavy tires for 6 months and then put 38.5s on it, and I guess I had forgotten what a slug it was with the 39.5 Boggers. The mods are huge, open K&N, Performer intake, RV cam and timing chain, MSD 6T box and 8.5 mm wires, coated Hedman headers, 3" dual exhaust with H-pipe and Flows (also RV/towing shift kit in tranny).
    I think I figured out the problem! When I accelerate at freeway speeds (especially uphill), the kickdown on the tranny is giving me 2nd gear until I hit 3900 rpm, and then it's shifting into 3rd, but I don't continue to accelerate when it's in 3rd because it can't rev high enough (i'm talkin' about an uphill, freeway climb). I will adjust the vacuum modulator on the TH350 in order to move up the shift points and see what happens. Yes, CapnCrunch, I wish the mechanic would have put in a new torque converter when he put in the tranny;it still has a stock type convertor. I should get a good B&M one. My tranny fluid happens to be new from the shift kit install, but when the shop drained the old fluid they said it had small metal chunks in it-- it was definitely not burnt, though. I should power flush the tranny (for $125).
    I broke the last Turbo 350 upon landing from a crazy jump (with my kids in the truck). I love it!!
     
  15. Ditchdigger

    Ditchdigger Registered Member

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    Re: Why isn\'t my TH350 putting the power to the ground?

    Hey, bigerick
    In my Trans Am I went from a 301ci(stock) to a 455ci with some mods after the 455 was dropped in i did nothing to Tranny and drove it for awhile and here was the deal!

    with my high end cam (far opposite than yours) and larger carb and headers, I was going on a level grade at about 10mph in second gear and I would accelerate and it would shift into third way too early, I just couldn't get any Rpms in the powerband for those early shifts so i switched to a 300+Rpm converter to get some descent acceleration and also had to get an adjustable modulator to help with shift points, although I might have been happier with a one size lower converter because she is just like a screamer before shifting which I guess is ok but can get sickening doing leisure driving
    just make sure you get the one you want, for instance, through B&M and most other Trans companies you can explain your situation and the drivetrain you have and engine mods (basically anything that relates too a converter witch is about everything and they can tell you what you should have
    and it makes things alot easier!

    heres a few things in converters from my old B&M catalog
    A torque converter reacts to engine torque; the more torque you feed a converter the better it will perform , for a converter to operate correctly, you need to build your engine to make as much low and midrange torque as possible in the same RPM range as the converter is rated for, in street engines this means limiting camshaft size to no more than 230degrees duration at .050 lift, then you don't really need a performance converter,
    But, perhaps the most misunderstood thing is Stall speed, it is directly related to the amount of torque your engine produces- the more torque the higher the stall speed.
    here are some variables in how a torque converter isn't working properly; small cubic inch engine; low compression ratio; Tunnel ram manifold; large carb throttle bores; long duration camshafts and retarded cam timing!

    you said you had metal chunks (possibly gear teeth from 3rd)
    I do think a TH-350 is rather wimpy from looking at your trucks
    tires and engine mods even if you've got a kit but that doesn't necessarily strengthen everything else.

    and should have definitely had a converter replacement after rebuild all that junk gets stuck in converter and chunks of metal like you said will eat away at the vulnurable fins inside the converter and those old guys that say you can flush out converters YOU CANT trust me you may get some of it
    but not all and the damage has already been done

    all of this information is the worst case scenario next to burnt clutch bands and stripped gears or geared to high in 3rd depending on what the trans came out of originally even a stock Blazer trans pulling those meats even with a gear change you could experience under load characteristics
    but,and I say BUT, it can be as simple as a faulty modulator or kickdown and gear shifter linkages or a faulty trans (and they almost always go out many miles after the rebuild when in fact it was an improperly rebuilt trans
    I hate to scare you like this because you rarely have trouble with transmissions (go through 2 engine rebuilds versus 1 trans rebuild is the usual case in stock normal driving,

    glad to help I know how frustrating it can be but, your not the only one!!!!

    If it ain't Intimid ahh! F*#k it!!
     
  16. ColAdo82K5

    ColAdo82K5 1/2 ton status

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    Re: Why isn\'t my TH350 putting the power to the ground?

    Hey Eric,
    After that torque convertor change out, how about a manual valve body ? I havn't heard of many off roaders use these compared to drag racers but you would be about to control all of the shifting in your tranny. Also, do you have a big tranny cooler ? Any big trucks out there with manual valve bodies ? It may help you out in the mud bogs.
    Blake
     

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