1987 454 TBI down on power, runs good otherwise

[beady]

Probably a TH400 but OP didn’t say.
@beady, so fresh rebuild didn’t slip a couple weeks ago and now does?
Did the seller give you the rebuild receipt? Since it was just done?
Likely there’s warranty, although maybe you can get the seller to take it in? Little gippo shops a lot of times only honor warranty to the “current owner.” Great way to reduce warranty claims….
I’d ask the shop anonymously and go from there.

TH400, sorry. Odo reads 80K.

The guy I bought it from (guy has a shop, all the cool toys, etc., so works on stuff) said he had bought it for his son and went to preemptively service the trans (since he didn’t know what shape the truck was in, even tho it’s super clean) and dropped the pan and saw black residue. So he dropped the trans and took it to his friend with a trans shop (country area, so I think is legit) who rebuilt it. Then the guy put it back in. His son ended up with a job who gave him a 4x4 work truck, and they had other toys, so decided to sell the Suburban. As I said above, trans is pristine externally, as is fluid, and I believe the guy. I live 5 hrs from where I bought it tho, so it’s not a small deal to get down there and back, let alone if I had to leave it there and make the round trip twice. And I seriously doubt it’s a chain place, lol.

I’m just not an auto box guy, so haven’t had to mess with them much before to know symptoms. I’m pretty sure if I take it to a shop they will just say “it needs to be rebuilt” to make money and not say “sounds like low line pressure from a poorly installed seal on the A solenoid in the valve body” (I made that with my limited knowledge, lol).

No one has touched a vehicle of mine since 1999 except for mounting tires and alignment. Clutches, suspension, engine pulls, head gaskets, everything by me. I refuse to break that streak for a fully mechanical auto trans that I suspect has a rebuild error (leaking by seal, clutch packs too loose, etc.) which I should be able to diagnose and repair, lol. I’m even willing to pull it apart and rebuild it myself if I can be fairly confident of the issue.

My long term plan for the truck was to manual 5-spd swap it anyway, so if I can’t resolve this issue for under $500 in parts I might as well suck it up and go for the manual swap now. It’s our backup tow vehicle for trailering, so I’d prefer to get it working as-is now and figure the swap out before I dive into it.

Thanks,

Mike

 

[Ricko1966]

Here's my gut feeling on it,something was assembled wrong on transmission assembly. Reason 1 it pulls good at low rpm, reason 2. If I took out a transmission with no problems and just preemptively decided to rebuild it. I reinstall it and it doesn't work properly,my first guess is I did something wrong. Not wow what a coincidence I just rebuilt this transmission and just by dumb luck,the converter failed. Not that it's not possible it's just if I had to bet money. My money would be the freshly rebuilt part has a problem. That being said, you can pull it disassembly it inspect it and reassemble it,maybe you'll get lucky and find a cut oring,or the valve body is loose,etc. and have little or no money in it.But it sounds like transmission inspection is probably beyond your skill set. So best thing might be start your manual swap now. My .02

 

[beady]

Here's my gut feeling on it,something was assembled wrong on transmission assembly. Reason 1 it pulls good at low rpm, reason 2. If I took out a transmission with no problems and just preemptively decided to rebuild it. I reinstall it and it doesn't work properly,my first guess is I did something wrong. Not wow what a coincidence I just rebuilt this transmission and just by dumb luck,the converter failed. Not that it's not possible it's just if I had to bet money. My money would be the freshly rebuilt part has a problem. That being said, you can pull it disassembly it inspect it and reassemble it,maybe you'll get lucky and find a cut oring,or the valve body is loose,etc. and have little or no money in it.But it sounds like transmission inspection is probably beyond your skill set. So best thing might be start your manual swap now. My .02

I think it’s because if you drive it almost normally you don’t notice the issue. You have to give it some gas. So maybe the guy put it back in and puttered around and was like “seems good”. I mean it is nice and solid pulling away, and shifts really nicely up the gears. Not harsh, zero flare. Reverse, too.

As I said, I’m not an auto trans guy and never read auto trans posts, so I don’t have a lot to go by. But, I am a mechanical engineer, and I’m very mechanically inclined. My gut feeling is the hydraulic pressure is too low and generates enough clamp force on the clutches without a lot of power trying to be put through it, but at some point the clutches can’t hold and slip. So as long as you drive it sedately it acts normally, doesn’t slip, doesn’t overheat the fluid. That’s just a guess.

I just don’t know the ways that happens. Seems unlikely seals on each piston for all the gear’s clutches are damaged/not fully seated.

Maybe it’s some pressure relief in the valve body that’s popping off too low. Maybe there’s debris on the seals/seat of the relief (if there is one, but I’d think there would be).

Maybe there’s flow blockage to the torque converter so it can’t transfer enough torque?

Maybe there’s a high stall converter installed accidentally? (Would that cause these symptoms, or the reverse, slipping at low rpms and grabbing high?)

 

[Ricko1966]

Check your pressure,I'll bet it is low,I just don't know why. Pump,leak at the valve body,leak at an oring.

 

[Grit dog]

Wish I could help @beady but I’m not very knowledgeable about auto transmissions. However there’s plenty of folks on here who are, I’d giver a bit and you may get some much more definitive advice.
Sounds like a nice rig though.

 

[Matt69olds]

Start in low gear, accelerate pretty hard. Watch the tach, around 3500-4000rpm put the shifter in 2nd gear, the trans should immediately and firmly shift into 2nd gear. Continue to accelerate, at around the same engine speed put the shifter into drive. It should immediately and firmly shift into high gear. There is no converter clutch on a 400, 3gears is all you get. If it shifts fairly quick without sliding into the next gear I’d be willing to bet the trans is fine. It doesn’t take long for fresh fluid to start to smell like burnt clutch material.

All the TBI engines are wheezing by 4500rpm, there may be nothing wrong. Figure out what’s wrong with the check engine light before going too far with transmission concerns. There is a specific procedure that needs to be followed before setting the timing.

 

[beady]

Start in low gear, accelerate pretty hard. Watch the tach, around 3500-4000rpm put the shifter in 2nd gear, the trans should immediately and firmly shift into 2nd gear. Continue to accelerate, at around the same engine speed put the shifter into drive. It should immediately and firmly shift into high gear. There is no converter clutch on a 400, 3gears is all you get. If it shifts fairly quick without sliding into the next gear I’d be willing to bet the trans is fine. It doesn’t take long for fresh fluid to start to smell like burnt clutch material.

All the TBI engines are wheezing by 4500rpm, there may be nothing wrong. Figure out what’s wrong with the check engine light before going too far with transmission concerns. There is a specific procedure that needs to be followed before setting the timing.

There’s no tach, but you can be doing 55 and floor it and it won’t go anywhere. You can do the same thing in any gear. If it downshifts and revs go up past a certain point (or you are trying to put too much power through trans it seems) it won’t accelerate. At any speed. As long as you’re gentle on the gas it will pull ok. 75 ok, but if you keep trying to accelerate it doesn’t want to do much more (either from lack of power or too much load through trans trying to push large brick shaped object through the air). And definitely won’t do any more of you step on the gas much at all.

I disconnected the brown and white wire when I checked timing.

Mike

 

[beady]

Start in low gear, accelerate pretty hard. Watch the tach, around 3500-4000rpm put the shifter in 2nd gear, the trans should immediately and firmly shift into 2nd gear. Continue to accelerate, at around the same engine speed put the shifter into drive. It should immediately and firmly shift into high gear. There is no converter clutch on a 400, 3gears is all you get. If it shifts fairly quick without sliding into the next gear I’d be willing to bet the trans is fine. It doesn’t take long for fresh fluid to start to smell like burnt clutch material.

All the TBI engines are wheezing by 4500rpm, there may be nothing wrong. Figure out what’s wrong with the check engine light before going too far with transmission concerns. There is a specific procedure that needs to be followed before setting the timing.

I can manually run through the gears like that and it shifts perfectly fine. Can’t tell you rpms though. You can manually downshift fine too, as long as it doesn’t spike the rpm’s/you don’t mash the gas after downshifting.

 

[Matt69olds]

Put a temporary tach on it.

I’d be curious what the fuel pressure is doing at highway speed. I know you said the pressure is ok at idle, but you can make 12 psi thru a straw, the engine won’t be happy under a load. Same thing with exhaust. While a TBI 454 isn’t a powerhouse, I would think it should accelerate with some authority at 75.

 

[beady]

Put a temporary tach on it.

I’d be curious what the fuel pressure is doing at highway speed. I know you said the pressure is ok at idle, but you can make 12 psi thru a straw, the engine won’t be happy under a load. Same thing with exhaust. While a TBI 454 isn’t a powerhouse, I would think it should accelerate with some authority at 75.

Yeah, I’ll try to put the gauge back on and read at speed. Probably can’t this weekend as I won’t have a second person to help me.

I just drove it to work and back (110 mile round trip) and I may be back on it being an engine issue vs trans. It pulls away great from a stop at about half throttle (the way a normal person would drive). Not a hint of softness in the drivetrain, etc.

I manually downshifted on the back roads and it downshifts smartly and engine brakes well.

Another thing to note, even with true dual exhausts and 50 series flowmasters, there is no “bellow” when you romp it. (Like the engine isn’t pushing a lot of air/making a lot of power, my ‘93 suburban with 350 tbi and true duals really let you and the ppl around know when you stomped it/loaded the engine at WOT)

I had a thought on the way home about the injectors. How are they set up on the TBIs? Is there one for low/part throttle and another for high power, and could it be running on only the smaller injector (if that’s how TBI is set up)? Because it seems to behave very well at low power throttle positions.

 

[YakkoWarner]

From what I can tell on mine the injectors aren't a low-high configuration. both spray more with increasing throttle when you observe them - but that question does bring up a possibility. If only ONE of them is working (or one working well one marginal), that could explain the low-throttle performance being acceptable, while not getting enough to really feed the fuel under higher demand. The condition may not persist long enough to trigger a lean run condition on the ECU since you back off when it occurs.

Fuel pressure under load is still an important question to resolve also - I'm not sure how it can be measured while driving since usually the instrument has to be connected to a T fitting at the filter or the throttle body itself, neither of which lend themselves well to monitoring while driving. I'm assuming you've already eliminated the fuel filter itself as a possible flow restriction?

 

[Ricko1966]

Before speculation on this goes any further, TBI injectors are synchronous, left and right and spraying the same amount at the same time.

 

[Snoots]

Question. Do you have the stock exhaust manifolds?

 

[beady]

From what I can tell on mine the injectors aren't a low-high configuration. both spray more with increasing throttle when you observe them - but that question does bring up a possibility. If only ONE of them is working (or one working well one marginal), that could explain the low-throttle performance being acceptable, while not getting enough to really feed the fuel under higher demand. The condition may not persist long enough to trigger a lean run condition on the ECU since you back off when it occurs.

Fuel pressure under load is still an important question to resolve also - I'm not sure how it can be measured while driving since usually the instrument has to be connected to a T fitting at the filter or the throttle body itself, neither of which lend themselves well to monitoring while driving. I'm assuming you've already eliminated the fuel filter itself as a possible flow restriction?

Yeah, my fuel pressure test kit is just long enough to reach in to the passenger floor (when hooked to filter location) is why I need an extra person to read while driving.

Fuel filter looks new, and threads clean, but I didn’t replace it.

 

[beady]

Question. Do you have the stock exhaust manifolds?

Yes, looks like stock manifolds with all new true dual piping back to tail pipes.