Please help me make a decision

[Snoots]

It cant be too hard to find a driveline shop to inspect the ujoints..I got mine done at the transmission shop.

That 'clank' does sound like a U-Joint.

 

[ali_c20]

I don't know if it is related but when I put it in reverse it clunks so hard from somewhere in the driveline that it sounds like something is going to break apart. I can't tell from the drivers seat but a guy at a 4 x 4 shop said it sounded like it's coming from the rearend.

The np203 clunks a lot when the chain is stretched and a heavy vehicle is not helping chain life. All the play in the driveline adds up and it sound really bad. I had this on my k5 and fixed it with a np205 and a limited slip rear end.

 

[Hunter79764]

I don't know if it is right or not, but I have a 14 SF rear in mine and was all ready to replace the U-Joints to get rid of the clunk from reverse to drive. Then I started checking to see if one end was worse than the other, turned out both were nice and tight, no play. Did more looking on this site and others and found that the 14 bolt with a locker can produce quite a bit of slop without it really being a problem, so I'm learning to live with it.

On a side note, you said you have a small block 400 with a Turbo 350 trans. Are you sure? The factory application would have been a Turbo 400 three speed auto from what I can tell, and I don't know why anyone would swap a TH400 for a TH350, given the amount of things that would have to change with it and the fact that you would be dropping down in strength.
If it's the case that you have a TH400, going to a 700-R4 would be a fair amount of work, NOT a direct swap. First of all, your transfer case would have different number of input splines that would not match the 700, and I believe the length is quite a bit different, meaning both driveshafts would need to change and your existing crossmember would need to move or possibly be replaced. Add on the TV brackets you would need and I think you would have more money in the install and modifications than you do in the transmission itself.
A 4L80 would be a closer swap as far as input splines and match the strength rating, but would also move the crossmember and change driveshafts, I believe. Either would be a lot of work for an eventual MPG gain that might or might not show up.


If I were you, I'd start with a good Sea Foam treatment in the fuel system and directly to the intake (you can watch videos on YouTube, there especially a good one from Project Farm on Sea Foam treatments on a lawnmower engine). Do a tune up with plugs, wires, etc., and consider an electric fan if you want to improve the fuel consumption. I'd also warn against using the fuel gauge to determine MPG's, they are not linear at all, and should be considered a fuel "Indicator" instead of a "gauge"...

 

[Rusty Nail]

Had a new idea.

I recall that the store "4 wheel parts"? Offers wicked inspections - amirite?

I also thought that a Chevrolet/GMC DEALER could provide a detailed inspection also, and I think either of those ideas is a good way FOR YOU to spend a little bit of money and be provided real answers.

If you have to buy a whole shop hour I think i would be worth it.

 

[topper]

I don't know if it is right or not, but I have a 14 SF rear in mine and was all ready to replace the U-Joints to get rid of the clunk from reverse to drive. Then I started checking to see if one end was worse than the other, turned out both were nice and tight, no play. Did more looking on this site and others and found that the 14 bolt with a locker can produce quite a bit of slop without it really being a problem, so I'm learning to live with it.

On a side note, you said you have a small block 400 with a Turbo 350 trans. Are you sure? The factory application would have been a Turbo 400 three speed auto from what I can tell, and I don't know why anyone would swap a TH400 for a TH350, given the amount of things that would have to change with it and the fact that you would be dropping down in strength.
If it's the case that you have a TH400, going to a 700-R4 would be a fair amount of work, NOT a direct swap. First of all, your transfer case would have different number of input splines that would not match the 700, and I believe the length is quite a bit different, meaning both driveshafts would need to change and your existing crossmember would need to move or possibly be replaced. Add on the TV brackets you would need and I think you would have more money in the install and modifications than you do in the transmission itself.
A 4L80 would be a closer swap as far as input splines and match the strength rating, but would also move the crossmember and change driveshafts, I believe. Either would be a lot of work for an eventual MPG gain that might or might not show up.


If I were you, I'd start with a good Sea Foam treatment in the fuel system and directly to the intake (you can watch videos on YouTube, there especially a good one from Project Farm on Sea Foam treatments on a lawnmower engine). Do a tune up with plugs, wires, etc., and consider an electric fan if you want to improve the fuel consumption. I'd also warn against using the fuel gauge to determine MPG's, they are not linear at all, and should be considered a fuel "Indicator" instead of a "gauge"...

Looking at the vin and the numbers on the burb it came from the factory with a th350 trans and the 400 sb. Like yours all the U joints seem tight. I am going to take it to a trans shop that was recommended next week. Sorry about posting the quote but I don't know where the reply button is. Thank You

 

[topper]

I called the dealer about doing an inspection but they said they don't do anything on cars more than 15 years old.

 

[Blackbeard44]

slow that beast down a little, 70 MPH and shes gotta be screaming down the highway.

I would check tire pressure, and air filter, and try cruising it at 55MPH and see if it improves any. anything without an overdrive trans is going to be using alot of fuel at 70

 

[Rusty Nail]

I called the dealer about doing an inspection but they said they don't do anything on cars more than 15 years old.

This is heartbreaking news I am sad to learn about.

That is EXTRA -y

 

[Grit dog]

This is heartbreaking news I am sad to learn about.

That is EXTRA -y

???
That’s a blessing. Why would one even want to take an old car to a dealer for repair?
Expensive enough if you can’t do your own repairs. Worse if you’re paying for dealer shop time. Find a decent private shop. And realize that an old car that needs to have repairs done at retail shop prices may be a very expensive vehicle.

 

[Grit dog]

The motor seems to run great, no knock, idles great etc. I can't check timing yet, I gave away what tools I had for carbureted engines a long time ago when I didn't think I would be in the situation I'm in. The engine is a sbc 400 with a th350 trans, np203 transfer case and a 14 bolt rearend. It is also full time 4 x 4 but I am having a friend install the kit for that. The gas gauge moves that much that's why Im hoping the sending unit is bad.

Well, sounds like solid running, no excuse for 3mpg if it runs “good”.
Sounds like you haven’t driven it far enough to even know what mpg you’re getting. Forget the gas gauge at this point(but another plus that it works. One less project…). Until you track miles, accurately, IE run at least half a tank from full, verify your Speedo is right or correct for it when calcing mileage, you won’t know what your mpg is.
Back to gearing, what gears does it have? And or what rpm’s is it running at a particular speed in 3rd gear and tire diameter?
Could be geared real low.

 

[squarelyfe]

Try adjusting the vaccum modulator. Check fluid level to start

 

[peats]

I would have the driveline inspected. Change the full time 4x4 and maybe regear it. you didn't mention the ratio but it's probably around 4 give or take. those 400sbc engines were built for ratios in the high 2's. 10mpg would be a best case scenario with those changes. An overdrive tranny is not going to pay for itself in fuel savings. 333 gears might make a small difference.

 

[Bextreme04]

3mpg? I can't even fathom how that would be real. My K20 with 35's and a 350/TH400/NP205 was still getting 6-7mpg with the top ring shattered into a million pieces on one cylinder, carburetor not adjusted properly, and almost all the valves leaking in some way.

 

[Grit dog]

3mpg? I can't even fathom how that would be real. My K20 with 35's and a 350/TH400/NP205 was still getting 6-7mpg with the top ring shattered into a million pieces on one cylinder, carburetor not adjusted properly, and almost all the valves leaking in some way.

To be fair, I did that math based on the OP's vague description of 1/8 tank used in 15 miles. My guess is he hasn't driven it far enough, no calculated his actual fuel mileage accurately. I've no doubt that it doesn't get great mileage, what old truck does... But 3mpg is virtually impossible short of a full pull uphill only with too big of a trailer, or a leak in the gas tank!

 

[SquareRoot]

Hey, I had a similar issue on my 85 a while back. It started shifting late and this was several months after a complete rebuild, including new modulator. Well, what I discovered was, it wasn't getting any vacuum like a good modulator should despite the vacuum lines being good. I had the vacuum advance on the dizzy t'd into the same line and that sum bitz vac cannister went bad and took the sucky air with it. New Vac advance and all was good. Back to my excellent 15 mpg!