4.8 to th350 troubleshooting
 

Got a couple quick questions for you guys out there.

First I swapped a 4.8 for a 305 and reused the 350 trans which ran fine with no issues. After the swap there seems to be some kind of delay when I push the accelerator. It will rev up then begin to move. Kind of like driving one of those go karts at a track. There is nothing electric on the trans so i'm assuming I need to change the converter. The new motor idles and runs fine.

Can anyone confirm the difference is the power band is my problem?

Also can anyone recommend a converter I should use or what stall speed I should used if the is in fact my issue?

Thanks
Jeff

Re: 4.8 to th350 troubleshooting
 

Couple of things: Are you sure the trans has enough fluid when hot? Did you remove the torque converter from the trans at any point during the swap? It may not be fully seated.

How high is it revving before it engages? Does it slip at all when it shifts?

Re: 4.8 to th350 troubleshooting
 

I checked to fluid level first and it has enough. No I left it on there the entire time. Im pretty confident its fully seated but could be wrong. It idles around 500-800 and it revs up to about 1200-1500.

Re: 4.8 to th350 troubleshooting
 

No slip.

Re: 4.8 to th350 troubleshooting
 

1200-1500 is probably the stock stall rpm for that converter if its the stock converter, I think you're probably fine but hopefully someone else will chime in as well. I was thinking it was revving up beyond, say, 2500-3000 with the stock converter.

It could be that the stock converter is getting worn, so its not doing any partial engagement.

Re: 4.8 to th350 troubleshooting
 

Interested as I am swapping a 5.3 with my th400.

Which flex plate spacer did you use?

I read somewhere the flex plate holes don't always match torque converter
holes? You didn't pull yours?

Good luck and please share your solution.

Re: 4.8 to th350 troubleshooting
 

I used the dished flex plate not the flat one that was with my old engine. So I did not need a spacer. I also used the ring to center the engine and trans. The flex plate holes will not line up but that is a easy fix. I simply reamed out the hole about a 1/4 inch.

Re: 4.8 to th350 troubleshooting
 

J,
I have what seems to be the dished fp on my 02 5.3, does that mean I don't need a spacer to mate my th400?
Thanks for confirming the fp holes.
What was your old engine? before the 4.8?

Re: 4.8 to th350 troubleshooting
 

Old engine was small block 305. You shouldn't need a spacer with the dished plate. Is your engine and trans out of the swap truck? If so the easiest way to tell is to set the starter in place and see if the plate will line up on ignition. Then mate the trans and check for a gap in between the flex plate and converter. No gap = your all set

Re: 4.8 to th350 troubleshooting
 

Skyphix
Took the truck out the other day and I was in fact having to rev the engine past 2000 RPM to get moving. Do you think the converter is the cause of my problem? Again it is a mechanical transmission. The shift linkage and kick down and actual shifting are all in working order best I can tell. The only problem seems to be the delay in moving the truck.

Thanks

Re: 4.8 to th350 troubleshooting
 

It sounds to me like the converter is whooped - it probably had no issue with 120hp but 280 is too much for it.

Re: 4.8 to th350 troubleshooting
 

You have any recommendations on a stall speed I should look for? This is a daily driver and not looking to spend a lot on this upgrade.

Re: 4.8 to th350 troubleshooting
 

1500rpm - 1800rpm stall speed should be adequate and if that 4.8 is staying stock, pretty much any converter should work, even the stock one for your vehicle. Something like a 2000 may make it feel a little more peppy but anything over that, in my opinion, wouldn't be worth it. If you think you're going to put a different cam, even a mild one, at some point, go with 2500 now.

I'd almost consider this more a repair than an upgrade, given your use case. I don't really have a budget recommendation for a converter, but myself, I'd probably test out the summit converters like this one https://www.summitracing.com/parts/sum-g2699/overview/ or even the B&M 20400.

Re: 4.8 to th350 troubleshooting
 

Ill start looking into that. Thanks again!

Re: 4.8 to th350 troubleshooting
 

I know this is an old thread but did a torque converter fix your issue ? I’m having same issue. Supposedly the transmission was freshly rebuilt.

Re: 4.8 to th350 troubleshooting
 

You might consider starting a new thread with your problem.. The last time Jstock85 posted was over 4 years ago.. He's probably long gone from here..

Re: 4.8 to th350 troubleshooting
 

Thank you. I guess I should have looked into that. I’ve been searching all over for what my issues are and all the same similarities popped up in this thread hahahahaha. Just no final answer

Re: 4.8 to th350 troubleshooting
 

Do you know how to start a new thread on this site??

Re: 4.8 to th350 troubleshooting
 

Why start a new thread on the same exact subject? It just makes searching for answers with the search feature that much more challenging when having to filter thru so many posts on the same subject.

Simply bumping this one to the top does the same thing as posting a new one.

Re: 4.8 to th350 troubleshooting
 

dirtscoots


What symptoms are you having that you are suspecting are convertor related?

Let us know the engine type, mods etc
the tans type,
if the trans is a Th350, where are you pulling vacuum for the modulator from? What is the vacuum reading at the modulator?
Do you have the kickdown cable in place and adjusted correctly?

What rear axle ratio and tires size?

Thanks

Bill

Re: 4.8 to th350 troubleshooting
 

The more I read and find out the torque converter seems less likely. Motor is 4.8 basically stock. Th350 is supposedly “rebuilt”. I’m pulling vacuum off the side of the throttle body. There are two ports I have it hooked up the higher port. When I first got the truck once fluid levels were correct reverse works normal. Once to start pulling lugging. In gear it doesn’t want to take off until the motor revs to I would guess around 3k. It will go but driving it won’t shift up without flooring it to get it to upshift.
So far I have messed with the kick down cable having it installed and not. And have had vacuum hooked up and not. It considerably does the same thing. I need to get tools to test vacuum that is my next step.

Re: 4.8 to th350 troubleshooting
 

I just took it for a drive. It will go in drive. First gear isn’t terrible. Then it revs up like there is no power. To get it to make a small hill on my road it takes quite a bit of throttle. To get it to shift up I have to hammer the throttle and completely let off. Also for as light of a truck as this is it feels like there should be more power to the wheels. I can spin a tire in gravel. No way will it even come close to getting loose on pavement.

Re: 4.8 to th350 troubleshooting
 

is the snout of your converter mating with the crankshaft? you should be using a spacer sleeve or use the flat flexplate with a spacer. i used the ATP z-270 flex plate. otherwise the converter doesnt interface with the crank pilot hole.

https://www.ictbillet.com/products/l...E1e-kqfpS8P1HC

Re: 4.8 to th350 troubleshooting
 

Do you have the kickdown cable attached and adjusted correctly?

This sounds like the kickdown cable is way too tight holding the kickdown valve fully open.

Remove the cable end from the throttle bracket and try again.

Also triple check the trans fluid level, in Park, running, warm oil, level should be halfway up the cross hatch section.

Let us know the results.

Post a pic of where and how you have the kickdown cable attached to the throttle body and a pic of where the modulator vacuum port is attached to....

Also, where did you find a long kickdown cable ? The normal TH350 kickdown cables are only just long enough to reach the original carb location....

:mm::mm::mm::mm:

Re: 4.8 to th350 troubleshooting
 

Thank you for the replies. It was a long hunting season but I am back to having some time to dabble in my trucks. The kick down cable is definitely not hooked up correctly. I have tried getting it to drive with the kick down cable pushed in and zip tied out then go with manual down shifting.