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Engine losing power at 3/4 throttle?

Tiny
98GTP
  • MEMBER
  • 1998 PONTIAC GRAND PRIX
  • 3.8L
  • 6 CYL
  • 2WD
  • AUTOMATIC
  • 103,000 MILES
Supercharged 3.8 cu. Runs good most of the time. When I give it a good amount of throttle, not WOT, sometimes it bucks hard while losing power. Most offer this will happen at a stop light or while passing on highway. Sometimes it accelerates fine. New plugs and wires are good. New pump, new filter.

Any help would be appreciated. This is my car I bought new as a teen. I am in the process of making like new again.

Thanks
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Saturday, June 17th, 2023 AT 10:01 PM

13 Replies

Tiny
JACOBANDNICKOLAS
  • MECHANIC
  • 109,885 POSTS
Hi,

Have you checked to see if there are any diagnostic trouble codes stored? If not, start with that.

Here is a link that explains how it's done:

https://www.2carpros.com/articles/can-scan-controller-area-network-easy

What you described does sound like a fuel-related issue, but I noted you replaced the pump and filter. By chance, do you have a live data scan tool? If you do, could you let me know what the short-term fuel trims are when the engine is at operating temp?

Also, let me know if there are any rattling sounds from under the vehicle that could indicate a catalytic converter issue.

Take a look at this link and let me know if anything mirrors what you are experiencing:

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

Let me know.

Take care,

Joe

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Sunday, June 18th, 2023 AT 10:03 PM
Tiny
98GTP
  • MEMBER
  • 8 POSTS
Yes, I have an Aeroforce scan gauge.
No codes
I monitor LT fuel not ST.
But I can monitor ST fuel trims. But will fluctuate wildly. Certain RPM or MAF Freq you are looking for?
No rattles. Cat is fine.
I do have an exhaust leak, but do not believe this to be the culprit for losing power like this.
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Wednesday, June 21st, 2023 AT 8:44 PM
Tiny
JACOBANDNICKOLAS
  • MECHANIC
  • 109,885 POSTS
Hi,

Chances are you are correct about the exhaust leak. However, if it is before the catalytic converter, it can cause the downstream sensor to believe it is running lean. As a result, the computer tries to compensate by adding fuel. The short-term fuel trims should fluctuate. However, at idle they should be between -10% and +10%. In a perfect world, it would be 0, but that isn't likely.

Also, the coolant temperature sensor can also cause a problem with the fuel mixture. So, when the engine is cold, check the signal from the ECT and see if is near ambient temperatures.

Let me know.

Joe
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Wednesday, June 21st, 2023 AT 11:32 PM
Tiny
98GTP
  • MEMBER
  • 8 POSTS
Short term fluctuate but hold 0 at mid throttle or greater. Coolant temperatures are correct as well.
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Thursday, June 22nd, 2023 AT 8:25 AM
Tiny
JACOBANDNICKOLAS
  • MECHANIC
  • 109,885 POSTS
Hi,

If it is at 0% when you are a mid-throttle or greater, that doesn't make sense. Does it go positive first and then steady at 0?

Let me know.

Joe
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Thursday, June 22nd, 2023 AT 7:59 PM
Tiny
98GTP
  • MEMBER
  • 8 POSTS
It fluctuates plus or -7, until mid-throttle word flattens out at zero. I will check tomorrow to see at what percent throttle it goes to zero.
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Thursday, June 22nd, 2023 AT 8:01 PM
Tiny
JACOBANDNICKOLAS
  • MECHANIC
  • 109,885 POSTS
Hi,

At idle and at operating temperature, is that when it is between + and - 7%?

Joe
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Thursday, June 22nd, 2023 AT 8:10 PM
Tiny
98GTP
  • MEMBER
  • 8 POSTS
I just went out to the car. Car is at operating temperature. At idle ST fluctuates -2.3 to 3. LT bounces a. It around 3-5. Eventually settles in as it learns the ST. Then ST trims narrow in closer to 0. LT trim holds pretty steady 3.X.

I ran it pretty hard on the way home tonight from work. Still never reached wide-open throttle. I don’t think I even got to 80% throttle. She ran good up to about 110Mph. No hiccups. I can check other things in the morning.

Thanks
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Thursday, June 22nd, 2023 AT 8:18 PM
Tiny
JACOBANDNICKOLAS
  • MECHANIC
  • 109,885 POSTS
Hi,

Good Lord, 110 mph? LOL Did the problem return at that speed? I ask because the fuel pump would have been under a load the entire time and may not be providing what is needed.

Joe
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Thursday, June 22nd, 2023 AT 10:47 PM
Tiny
98GTP
  • MEMBER
  • 8 POSTS
Nope. Ran great. New pump and filter.
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Friday, June 23rd, 2023 AT 5:02 AM
Tiny
98GTP
  • MEMBER
  • 8 POSTS
Once I hit 30% throttle, short term fuel trim locks in at zero.
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Friday, June 23rd, 2023 AT 5:25 AM
Tiny
JACOBANDNICKOLAS
  • MECHANIC
  • 109,885 POSTS
Hi,

Okay, that isn't normal. They should go positive because fuel is being added. So, we need to check the o2 sensors.

With the vehicle at idle and operating temperature, what voltages are the oxygen sensor?

Let me know.

Joe
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Friday, June 23rd, 2023 AT 8:12 PM
Tiny
98GTP
  • MEMBER
  • 8 POSTS
Why would you think fuel is being added? It would only add fuel via st/LT trims if o2 sensors thought it was lean. If I am running rich, then trim levels would full fuel. During this throttle response the o2 sensors are seeing what they want. Which is way st is zero.
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Friday, June 30th, 2023 AT 8:11 PM

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