Horsepower problem

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  #1  
Old 10-18-07, 09:01 AM
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Horsepower problem

I have a 94 Chevy Pick-up 1500 5.7L 350 4 wheel drive. I'm having trouble with the truck picking up to speed quickly. The muffler fell off about a year ago and I ran it with just the cat and pipe after that. Had no HP problems, just loud.
I had tranny problems and had it replaced (rebuilt) less than a year ago. Still no HP problems.
Finally, the cat fell off and had to get a new exhaust system put on (crossbar back to tailpipe). Since the new exhaust, I am having problems getting up to speed quickly. Not normal driving wise but if I want to pass a vehicle or getting on highway and need to pick up speed quickly, it just doesn't want to go. The RPM's don't even rev up like it use to before the exhaust was put on.
Is there possibly an exhaust leak or vacuum leak or possible plug in the new exhaust system? I hear nothing under the hood out of the ordinary with the vehicle in park and trying to rev engine. I've also done a tune up i.e. plugs, wires, rotor and cap, fuel filter, injection cleaned. That was before new exhaust though.
I have to say it's very quiet now but annoying not to be able to get up to speed quickly. I hope I explained it well enough and hope someone has some insite!
 
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  #2  
Old 10-18-07, 11:27 AM
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Hmmmm. Are you sure you have the right exhaust system on? I'm thinking you are having a problem with back-pressure from the exhaust system.
 
  #3  
Old 10-19-07, 06:26 AM
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Originally Posted by daswede View Post
Hmmmm. Are you sure you have the right exhaust system on? I'm thinking you are having a problem with back-pressure from the exhaust system.

I should hope so. Cole Muffler replaced it and it wasn't cheap. It's been about 2 months since they did it. I was hoping it would fix itself or maybe it's just because I'm just use to no exhaust on it. It just doesn't seem right! I just wanted other opinions before I spend the money to take it back to them for diagnosis. Paperwork says exhaust is under warranty but diagnosis and labor, there's a charge.
If they broke it or didn't do it right, I think they should fix it for free!!!
 
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Old 10-19-07, 09:14 AM
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I think back-pressure can be measured via O2 sensor hole; what if you disconnect exhaust before the cat or muffler and see if situation improves; that would help with diagnosis.
 
  #5  
Old 10-19-07, 10:27 AM
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Not sure if you are talking about horsepower or more of a low end (low RPM) torque kind of deal
Sounds more like the latter off-hand
Torque is the stuff that gets you moving...the street use power not the high winding (high RPM) kind

Like when passing, you mean just stepping on the gas right?
Not dropping down a gear or two so it's in the power band (higher RPMs) and it just doesn't take off right?...takes a while to build up passing power?

It's possible the new exhaust system cut out the low end torque...but usually it's the open or too-big-for-stock/combo exhaust system that causes that (bigger isn't always better)
When the system fell off I would expect a drop off in low end torque

Not sure where the "crossbar" is but I assume you needed at least cats back

It's possible after months of driving with relatively unrestricted exhaust you now feel confined with a stock one, it's just that the unrestricted exhaust shouldn't have made that much of a difference in that direction in the low RPM...in fact it really should have hurt low RPM torque

It's possible...it just doesn't make sense off-hand

I'd consider taking it back to them and telling them about your serious lack of power since the new system
 
  #6  
Old 10-19-07, 11:59 AM
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Originally Posted by slickshift View Post
Not sure if you are talking about horsepower or more of a low end (low RPM) torque kind of deal
Sounds more like the latter off-hand
Torque is the stuff that gets you moving...the street use power not the high winding (high RPM) kind

Like when passing, you mean just stepping on the gas right?
Not dropping down a gear or two so it's in the power band (higher RPMs) and it just doesn't take off right?...takes a while to build up passing power?

It's possible the new exhaust system cut out the low end torque...but usually it's the open or too-big-for-stock/combo exhaust system that causes that (bigger isn't always better)
When the system fell off I would expect a drop off in low end torque

Not sure where the "crossbar" is but I assume you needed at least cats back

It's possible after months of driving with relatively unrestricted exhaust you now feel confined with a stock one, it's just that the unrestricted exhaust shouldn't have made that much of a difference in that direction in the low RPM...in fact it really should have hurt low RPM torque

It's possible...it just doesn't make sense off-hand

I'd consider taking it back to them and telling them about your serious lack of power since the new system

Driving normal, it's okay (except going up hills). I have a hard time keeping it up to speed with just pushing gas pedal normal. I have to really stomp on the pedal now to keep constant speed on uphill slopes (forcing it to higher RPMs). If I have it on cruise control, the high (low band) RPM's kick in and it revs up to speed. Around 3300rpms. When passing someone or getting up to speed on a ramp, I have to really push on the gas pedal to get it up to speed but the high (low band) RPM's never rev up like it does with going up a hill with cruise on automatically.
When just the muffler and tailpipe fell off, it was loud but it really had some power. When the cat fell off, it was super loud and not so much power. Sort of how it's running now with pick up speed.
The crossbar is the pipes that come from each side of the engines exhaust manifolds and meets together at the cat. That had to be replaced with the cat because it comes as one piece. The cat is welded right after the joint but after the O2 sensor.
The O2 sensor was replaced too (if that makes a difference).
I know it sounds a little backwards when talking about power with and without the exhaust system. I just want my "get up and go" power back
I just wasn't sure if it was normal.
Is it possible that with the tranny being replaced (rebuilt) between the muffler falling off and the cat falling off (new exhaust) that they aren't working together somehow? It's not an indication that my engine is now on it's way out is it? This poor truck is falling apart piece by piece so it wouldn't surprise me.
 
  #7  
Old 10-19-07, 01:18 PM
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The (stock) mufflers were a restriction
When they fell off you gained (freed up actually) a few horses
When the Y pipe/cat (crossbar) fell off that was too much, you lost low end torque

Now...it's hard to say
Honestly running it wide open like that you could have caused valve train damage, but this is a big American engine, not a little import...you shouldn't have "sucked a valve" out the exhaust or anything
But it's possible

I think a (cylinder) compression test is in order
 
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