1987 454 TBI down on power, runs good otherwise

[beady]

So I bought this 1987 R20 a couple weeks ago, and it seems to run well except it’s down on power as rpms rise. Pulls pretty strongly from a stop, but as revs rise it stops making power. And if you are driving and cause it to downshift, it immediately does nothing. Makes noise and seems to rev, but no power. Won’t accelerate unless you let off the gas and ease into it. Then it will run at 70+ fine, until you dip into the gas too much, and again, nothing. We pulled a horse trailer and it was a chore to keep speed up without it downshifting and going nowhere.

I checked fuel pressure and it’s 12+ psig.

Tried checking timing, but not sure what I’m looking at. Seems like the mark is to the passenger side (to your left when standing in front of truck) of the tow of fixed toothed timing marks on the engine? That’s with the timing wire pulled.

Oh yeah, it drives exactly the same with the timing wire connected or not.

Also, there doesn’t seem to be a check engine light to pull codes. The spot in the bottom of the speedometer is there, I can feel it with my finger, but no bulb. And o don’t feel it dangling around back there either. Is there a way to hook a bulb or meter to sone of the ALDL pins and read flashes that way? Or some reader I can buy or make (made one for my old Benz)?

Thanks for any insight.

 

[beady]

I had a ‘93 gmt400 k2500 350 tbi before this, and it had WAAAAY more power and was super eager to rev out, comparatively.

 

[fast 99]

Plugged exhaust? Drive with a vacuum gauge shouldn't go much lower than 4 inches. Alternate method loosen the header pipe slightly.

Low fuel pressure?

 

[beady]

Plugged exhaust? Drive with a vacuum gauge shouldn't go much lower than 4 inches. Alternate method loosen the header pipe slightly.

Low fuel pressure?

12+ psig fuel pressure.

Recent full dual exhausts from manifolds back.

Can check vacuum, have to find my gauge which is buried somewhere.

 

[Ricko1966]

Find out why your check engine light is missing/inoperable that may shed new light on things,don't rig around it make it work as intended. Bypassing it just may mask an ECM problem.

 

[beady]

So, I’m thinking it’s the trans/torque converter, not engine. It pulls solid and great from a stop, and pulls fine in any gear as long as the rpm’s are low. As soon as they climb, it just slips massively.

Can manually put it in each gear and it’s fine until the rpm’s get higher.

I put it in gear and stood on the brakes and it just breaks the tires free as I give it gas no matter how hard I stand on the brakes (on grass in the yard).

 

[beady]

Anyone know how to tell if it’s the torque converter or clutch packs in the trans slipping? I’m a manual trans guy so haven’t ever delved into autos.

Fluid looks great and doesn’t smell burnt. But I’ve been driving in gently, keeping it out of the rpm ranges it slips. I’ve probably put 1000 miles on it since I bought it.

 

[Grit dog]

Maybe hook up a tach so you can easily see how much it’s revving when it shouldn’t be.
I’m not a trans expert but it should be able to smoke the tires from a Brake stand on pavement too. Tq converter would be bad at all speeds or gears I think. Maybe slipping clutches or? In 3rd or 4th gear?
Does it pull hard at high rpm’s in 1st and 2nd gear?
I don’t have the answer but rambling what I’d likely try to diagnose it.

 

[Ricko1966]

Take off fairly hard in manual low,shift to 2nd,shift to 3rd, when you hit third does it act like it's in neutral? I'll bet it does and you need the transmission rebuilt.

 

[Bounty Hunter]

Perhaps check the on the fuel pressure? could need to be someone switched out the TBI to the '94 - '95 units which ran a lot higher...like 30psi

 

[beady]

Maybe hook up a tach so you can easily see how much it’s revving when it shouldn’t be.
I’m not a trans expert but it should be able to smoke the tires from a Brake stand on pavement too. Tq converter would be bad at all speeds or gears I think. Maybe slipping clutches or? In 3rd or 4th gear?
Does it pull hard at high rpm’s in 1st and 2nd gear?
I don’t have the answer but rambling what I’d likely try to diagnose it.

Only 3 speed. Acts the same in every gear, fine at part throttle, slips past that point.

 

[beady]

Take off fairly hard in manual low,shift to 2nd,shift to 3rd, when you hit third does it act like it's in neutral? I'll bet it does and you need the transmission rebuilt.

Does the same thing in each gear. Fine at low/part throttle until revs get higher, then slips.

Trans was supposed rebuilt only a few dozen miles before I bought it. Trans is spotless/freshly painted. Fluid is perfect, not a hint of discoloration and zero burnt smell at all. I have not been letting it slip, I just keep throttle below a certain point and it shifts and drives just like it should. Zero flare btwn shifts.

 

[beady]

Perhaps check the on the fuel pressure? could need to be someone switched out the TBI to the '94 - '95 units which ran a lot higher...like 30psi

12+ psig

 

[Ricko1966]

Does the same thing in each gear. Fine at low/part throttle until revs get higher, then slips.

Trans was supposed rebuilt only a few dozen miles before I bought it. Trans is spotless/freshly painted. Fluid is perfect, not a hint of discoloration and zero burnt smell at all. I have not been letting it slip, I just keep throttle below a certain point and it shifts and drives just like it should. Zero flare btwn shifts.

What transmission is in it?

 

[Grit dog]

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.