454 TBI lack of power

[shanegtr]

Ok, just putting it out there to see if anyone has any ideas here. As the title suggests, 454 TBI. Starts and idles fine and accelerates smoothly under light throttle, but get 1/2 throttle or more and it seems to bog down and not deliver as much power as it should(had more power available previously).
Things replaced so far (not because of this issue - just what Ive done as its a new vehicle to me), spark plugs, air filter, fuel filter, engine oil & filter(theres a nice oil leak around the filter but thats another story), coolant and thermostat, trans fluid changed and new pan filter and new fuel pump (old one died - which I thought may have been the issue). Timing was checked and at idle it was spot on 4deg as per the manual.
Any suggestions on what else to check?

 

[bucket]

Even with the new pump, I'd double check the fuel pressure. If it's a few psi low, a TBI engine will still run well, just lack power.

Does it still have the original pellet type catalytic converter?

 

[shanegtr]

Not sure what the original pellet type cat looks like, but there is a cat way back down near the rear diff. I can post up a pic and you can let me know if its an original one

 

[legopnuematic]

I second @bucket on checking the fuel pressure and catalytic converter. I am not sure on TBI 454's but on 70's small blocks they have a butterfly valve on the passenger side exhaust manifold that when cold closes to redirect exhaust through the crossover in the intake manifold to warm it up and if it sticks close then it will heavily restrict exhaust flow making it idle ok but gutless anywhere else couldn't hurt to check when looking at the cat. If it has one,checking the EGR valve for proper function too can't hurt.

 

[shanegtr]

Heres the only cat in my exhaust. Funny as one of my original thoughts was that there may have been a restriction in the exhaust. There is a small pin jole leak around on of the welds up near the front - i previously noticed it seemed to be leaking with more pressure (didnt notice it last night however)

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[bucket]

Well that's an odd cat to see under there. Looks like it might be an original cat from a late model truck?

If there's anywhere to easily disconnect the exhaust ahead of the cat, you can test drive it unhooked to see if there's more power.

 

[shanegtr]

Unfortanatly its all welded so no easy way to disconnect

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[Craig 85]

If your truck was like my '87 C30 and my '88 C20 Suburban, that is the second of 2 cats the 454 trucks had. There was a big pancake (pellet) style one near the transmission (photo) then your style cat near the rear axle. The cat you pictured is the honeycomb brick style.

On my '87, the front cat failed and was throwing pellets into the 2nd cat, causing it to clog.

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[shanegtr]

That one then is definatly not there. The front section of pipework has been replaced at some point and still looks newish

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[bucket]

If your truck was like my '87 C30 and my '88 C20 Suburban, that is the second of 2 cats the 454 trucks had. There was a big pancake (pellet) style one near the transmission (photo) then your style cat near the rear axle. The cat you pictured is the honeycomb brick style.

On my '87, the front cat failed and was throwing pellets into the 2nd cat, causing it to clog.

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Wow, I never realized that. My big block Burbs have been pre-87's with duals and no cats.

But that makes complete sense, if the pellet converter had failed recently, there could be a mess inside the rear cat.

 

[WHEELMAN]

Did you ever pull the TBI off and clean it up and put a kit in it.when I pulled mind.the base gasket was messed up.broke up some.you can get the rebuild kit.from different places.rockauto.maybe eBay.

 

[4WDKC]

1. Any check engine lights? if so get code
2. Just becasue its a new fuel pump doesnt mean its still good, the factory pumps were low pressure and not known for putting out the same psi. Swapping to a TPI camaro or 96+ Vortec truck pump is a direct swap in and much more reliable.
3. Factory exhaust is a restriction and cats from those days were horrible at best, if you need a cat for emissions replace it with a new high flow unit.
4. How many miles are on the engine it maybe tired and have a couple of issues causing this.

 

[shanegtr]

I second @bucket on checking the fuel pressure and catalytic converter. I am not sure on TBI 454's but on 70's small blocks they have a butterfly valve on the passenger side exhaust manifold that when cold closes to redirect exhaust through the crossover in the intake manifold to warm it up and if it sticks close then it will heavily restrict exhaust flow making it idle ok but gutless anywhere else couldn't hurt to check when looking at the cat. If it has one,checking the EGR valve for proper function too can't hurt.

Interestingly that you mention the EGR valve. Today it's been throwing up DTC 32.

 

[Rusty Nail]

My first thought was "fuel pressure" but my second thought involved the check engine lightbulb...

 

[shanegtr]

I'm reasonably sure it its that rear cat causing most of the issues. I've been having a **** drive today where it seems to block up (can hear a lot of exhust trying to escape from a pin hole leak) and then I loose a heap of power and then she overheats. Dosent help when the weather has been hot either. I'm going to cut the cat off when I get home and see how it goes.
On a side note, I cannot figure out how the "egr" does anything as its not hooked up to the exhaust in anyway that I can see