Lacks power when warm

Tiny
VERBATIM68
  • MEMBER
  • 2003 CHEVROLET IMPALA
  • 3.8L
  • 6 CYL
  • 2WD
  • AUTOMATIC
  • 150,000 MILES
I'll try to explain the issue as completely as I can.

First of all, no DTC codes at all.

The vehicle starts and idles fine. Upon startup there's a loud sucking noise coming from the airbox before the filter, which I know is normal because of the IAC during cold startup. At any point in neutral or park, cold or hot, the engine sounds fantastic, has great throttle response and revs up nice all the way to the limiter. I've replaced all the plugs, wires, and coils. I did a compression test when the plugs were out and it came out to 167-175 for all cylinders. Every plug looked fantastic, nice tan color, no fouling. Beautiful.

If I start driving right away immediately after I start the vehicle, it accelerates through the gears and shifts smooth with what I would call normal acceleration. Feels like it should and if I could get it to stay that way, I'd be happy. However, after a couple of blocks, (or if I let the engine idle for a couple minutes before driving) the acceleration becomes very poor. There is a more hollow muffled sounding sucking noise (not even sure it's air, but that's the best way I can describe it.) During the poor acceleration. It seems to shift though the gears still, it just takes forever to get up to a speed to get the gears to shift. I'm actually not sure if it's even starting out in the proper gear. It doesn't slip during this, it's just like pressing the gas and it accelerates super slowly, there's no high rpm's as you would see in slipping.

If I press the accelerator maybe 2/3 of the way down so that it drops down a gear, the engine revs higher, the hollow sound goes away and it sounds more like it should. It accelerates much better, but I'm still not sure it's as good as it should be.

If I floor it so it drops down another gear, engine RPM's go way higher, but zero acceleration (transmission is slipping) with a loud rattling like marbles in a tin can. This rattling is worse the more the car is driven (or warms up I guess).

originally, I suspected the cat, but I'm not sure why it would be okay for the first couple blocks before it warms up. Also, in neutral, the engine sounds awesome with great response. Because of this, I"m beginning to think it's not a plugged cat. Even if it was clogging up after coming to temperature, it would be bad at idle too, I'm thinking. I'm also ruling out fuel delivery as it revs up great and very responsive. I haven't checked the rail pressure.

If I stop the vehicle and let it sit for a few minutes, then start it up and drive again, again it's good for a couple blocks and then starts doing the same thing again.

Would doing a transmission fluid and filter change (not flush) fix this? I'm thinking that there's some kind of restriction someplace that clears as the transmission fluid drains and then clogs up soon after starting the vehicle again. The fluid level is correct and the fluid isn't black, but it isn't red either. I'm confused because it seems so great for the first couple of blocks and when it starts having low power, it's not slipping until you floor it, it just bogs down under normal acceleration. If it's the PC solenoid, why does it take a couple blocks to see the problem?

Another question. Will pulling the front O2 sensor allow enough air through the port to allow me to diagnose the catalytic converter? I don't really feel like disconnecting the whole pipe or manifold before the catalytic converter.

I plan to clean the MAF sensor, but looking at the readings on my OBD tool, the numbers don't seem out of line and there is no DTC and the engine seems to be running fine.

Any help would be truly appreciated. Thanks in advance.
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Thursday, July 11th, 2019 AT 2:38 PM

9 Replies

Tiny
JACOBANDNICKOLAS
  • EXPERT
Welcome to 2CarPros.

Honestly, are you sure the transmission is slipping and not the engine just revving without accelerating? I have to be honest, I feel it is the cat. The rattle you mentioned is most likely the insides of the catalytic converter. They can come apart internally and rattle. If it starts to block the air flow, you will lose power.

Here is a link that explains how to test a catalytic converter along with an additional one regarding symptoms:

https://www.2carpros.com/articles/bad-catalytic-converter-symptoms

https://www.2carpros.com/articles/how-to-test-a-catalytic-converter

When you read through the test, you will get to section 4 where it discusses removal of the O2 sensor. It doesn't work as well as removing the cat, but you should notice a difference when driving.

Let me know what you find.

Joe
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Friday, July 12th, 2019 AT 7:20 PM
Tiny
VERBATIM68
  • MEMBER
Hi Joe, thank you for your reply! No I haven t ruled out the catalytic converter at all, I m just really confused as it idles and revs up so nice when it s not in gear. The throttle response is so good when it s not moving.

The confusing thing for me is the fact that it drives great for two blocks after startup. Plus the fact that when it is having poor acceleration it doesn t act like the transmission is slipping at all really until you floor it. It s mostly just bogging down.

It s frustrating because half the symptoms seem to be bad catalytic converter and the other half are transmission.

I would have pulled the O2 sensor already but I hurt my back last week so I ve been taking it easy. I hope to get back to it this weekend.

Thanks again for your reply. I m hoping you re right.
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Saturday, July 13th, 2019 AT 6:18 AM
Tiny
VERBATIM68
  • MEMBER
Took the O2 sensor out and went for a test drive. It ran much much better. Accelerated up through the gears just fine. Still not 100% but I m sure it will be once I get the cat replaced.

Definitely the catalytic converter. Pricey but much better than a transmission!
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Saturday, July 13th, 2019 AT 11:13 AM
Tiny
JACOBANDNICKOLAS
  • EXPERT
Welcome back:

I had a feeling that was the issue, and so did you. You're right. Much cheaper than a transmission.

I don't know if you need them, but here are the directions for replacement. The attached pictures correlate with the directions. Also, see if you can find an aftermarket one that is direct fit. It should save you money. Just don't get the cheapest thing you find. I can't tell you how many times I have replaced new cats that end up with cracked welds. Ugh! I feel bad for the people, but remember, you get what you pay for. Stick with a brand name and a company you know will back you if something like that happens.

______________________________________________

CONVERTER
Catalytic Converter Replacement (3.8L)

Removal Procedure

Important: The 3-way catalytic converter is serviced by replacing the entire assembly. Always replace the gaskets when servicing the 3-way catalytic converter assembly. Never install the original gaskets
1. Raise and support the vehicle. Refer to Vehicle Lifting.

Pic 1

. Disconnect the Heated Oxygen Sensor (HO2S) electrical connector.
3. Remove the exhaust manifold pipe stud nuts.

Pic 2

4. Remove the catalytic converter nuts.

Pic 3

5. Remove the catalytic converter hangers.
6. Remove the 3-way catalytic converter.
7. Remove the gasket from the exhaust pipe.
8. Remove the gasket from the exhaust manifold.
9. Clean and inspect the exhaust manifold and the exhaust pipe gasket mating surfaces.

Pic 4

10. Remove the Heated Oxygen Sensor (HO2S).

Installation Procedure

pic 5

1. Install the Heated Oxygen Sensor (HO2S).

Pic 6

2. Install a new gasket to the exhaust pipe studs.
3. Install a new gasket to the exhaust manifold.

Notice: Refer to Catalytic Converter Movement Notice in Service Precautions.

4. Install and support the 3-way catalytic converter.
5. Install the catalytic converter nuts.

Pic 7

Notice: Refer to Fastener Notice in Service Precautions.

6. Install the exhaust manifold pipe stud nuts.
Tighten the exhaust manifold pipe stud nuts to 32 Nm (24 ft. Lbs.).
Tighten the catalytic converter nuts to 60 Nm (44 ft. Lbs.).

Pic 8

7. Install the catalytic converter hangers.
8. Connect the Heated Oxygen Sensor (HO2S) electrical connector.
9. Lower the vehicle.
10. Inspect the exhaust system for leaks.

_____________________________________

When you get it repaired, let me know the final outcome. Also, I hope the back is feeling better. There is nothing worse than back problems.

Take care and come back anytime.

Joe
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Saturday, July 13th, 2019 AT 7:41 PM
Tiny
VERBATIM68
  • MEMBER
Hi Joe, thanks for your help. I replaced it with a decent aftermarket catalytic converter that comes with a one year warranty. Drives beautifully now!
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Monday, July 15th, 2019 AT 1:38 PM
Tiny
JACOBANDNICKOLAS
  • EXPERT
Great! That's good news. Let me know if you need anything in the future.

Take care,
Joe
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Monday, July 15th, 2019 AT 7:48 PM
Tiny
VERBATIM68
  • MEMBER
Hi joe. I m having problems with my ignition switch now. Can you read this thread and tell me what the issue might be?

https://www.2carpros.com/questions/ignition-switch-problem#last
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Saturday, July 20th, 2019 AT 8:24 AM
Tiny
VERBATIM68
  • MEMBER
Actually I figured out the ignition switch.
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Saturday, July 20th, 2019 AT 6:15 PM
Tiny
JACOBANDNICKOLAS
  • EXPERT
Welcome back:

Did you get everything working again?

Joe
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Saturday, July 20th, 2019 AT 9:31 PM

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