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Engine surging only when it is at operating temperature

Tiny
JAMES SNIVELY
  • MEMBER
  • 2006 MINI COOPER S
  • 1.6L
  • 4 CYL
  • 2WD
  • AUTOMATIC
  • 98,000 MILES
Hello, I am having a problem that I cannot figure out. I have noticed a decrease in mileage also. My engine surges when the car is up to operating temperature. Mainly at lower speeds. It causes the car to "buck". I have replaced the air filter, fuel filter, and spark plugs and it still has not fixed the problem. It doesn't do it when cold. It seems the longer you drive the car the worse it gets. Stumped!
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Saturday, June 25th, 2022 AT 7:09 PM

13 Replies

Tiny
JACOBANDNICKOLAS
  • MECHANIC
  • 109,885 POSTS
Hi,

When this happens, have you ever noticed any dark gray or black smoke from the exhaust? I ask because the coolant temperature sensor may not be working properly. Info from the sensor is used by the computer to determine the air/fuel mixture needed to run the vehicle.

It's only a theory, but a possibility. Let me know if you have a scan tool that provides live data. If so, check two things. First, confirm the ECT (engine coolant temperature sensor) signal seems correct. If so, let me know what the short-term fuel trims are.

Let me know.

Take care,

Joe
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Sunday, June 26th, 2022 AT 10:05 PM
Tiny
JAMES SNIVELY
  • MEMBER
  • 7 POSTS
Thanks for your thoughts! I don't have a scan tool. I am going to replace the coolant temperature sensor within the next day or so when I get time. What you are saying makes sense. I have noticed the tailpipes are pretty black. I have not noticed smoke though. I will let you know how things turn out. Thanks much.
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Monday, June 27th, 2022 AT 3:31 PM
Tiny
JACOBANDNICKOLAS
  • MECHANIC
  • 109,885 POSTS
Hi,

James, please remember that there is a theory at this point. I don't like having people spend their money if it doesn't fix the issue. I hope you understand. However, it certainly could be the issue.

Regardless, let me know how things turn out for you. I'm interested in knowing.

Take care,

Joe
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Monday, June 27th, 2022 AT 8:29 PM
Tiny
JAMES SNIVELY
  • MEMBER
  • 7 POSTS
Okay, I understand, no worries about if it doesn't fix it. I am running out of ideas as to what could be causing the problem and your theory makes sense enough to try. I have talked to my mechanic, and he agrees that it's possible. I am getting the sensor put it Wednesday morning. I'll let you know how it goes. Thank you
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Tuesday, June 28th, 2022 AT 6:01 PM
Tiny
JACOBANDNICKOLAS
  • MECHANIC
  • 109,885 POSTS
Thanks for understanding. I look forward to hearing what is found. I'm interested in knowing.

Take care,

Joe
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Tuesday, June 28th, 2022 AT 7:19 PM
Tiny
JAMES SNIVELY
  • MEMBER
  • 7 POSTS
Hello, well, I had the temperature sensor replaced and the problem still exists. I have noticed that it surges between 30 and 40 mph. Transmission issue maybe? I had the car scanned but no codes came up from the computer.
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Wednesday, June 29th, 2022 AT 5:55 PM
Tiny
JACOBANDNICKOLAS
  • MECHANIC
  • 109,885 POSTS
Hi,

If it only happens at that speed, manually shift it past that speed to see if it still surges. If it doesn't, upshift to the next gear and see what happens. I don't feel it's the transmission, but anything is possible.

I know there was a technical service bulletin related to internal carbon build-up in the engine causing different drivability issues, but nothing at a specific speed.

Let me know.

Joe
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Wednesday, June 29th, 2022 AT 11:15 PM
Tiny
JAMES SNIVELY
  • MEMBER
  • 7 POSTS
The problem is intermittent which makes it harder to figure out. I'll let you know how things go, thanks again.
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Thursday, June 30th, 2022 AT 8:25 AM
Tiny
JACOBANDNICKOLAS
  • MECHANIC
  • 109,885 POSTS
James, you are very welcome, but I don't feel very helpful. If this issue was at any speed, there are several things to check, but since it is specific to a speed, that limits things.

As you said, since it's intermittent, it is more difficult. Actually, it can become frustrating.

Regardless, let me know and if there is anything I can do to help, let me know that as well.

Take care,

Joe
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Thursday, June 30th, 2022 AT 7:39 PM
Tiny
JAMES SNIVELY
  • MEMBER
  • 7 POSTS
Hello Joe, I have taken the car to a person who specializes in foreign cars. He is going to check for a vacuum leak and any codes which come up from the computer which did not at O'Reilly's. I will let you know what is found out.
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Tuesday, July 5th, 2022 AT 6:05 PM
Tiny
JACOBANDNICKOLAS
  • MECHANIC
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Hi,

Thank you for the update. I look forward to hearing what is found. I am interested in knowing.

Take care,

Joe
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Tuesday, July 5th, 2022 AT 10:12 PM
Tiny
JAMES SNIVELY
  • MEMBER
  • 7 POSTS
Apparently a transmission issue. The trans fluid was replaced and it is much better. It was not flushed but replaced. It may need one more time. But it is so much better now.
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Friday, July 8th, 2022 AT 4:56 PM
Tiny
JACOBANDNICKOLAS
  • MECHANIC
  • 109,885 POSTS
Hi,

That's great news. Hopefully, one more time will make it perfect. Thank you for the update. If things change, let me know.

Take care,

Joe
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Friday, July 8th, 2022 AT 8:18 PM

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