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Engine Surges after long run?

Tiny
DMHUGGINS26
  • MEMBER
  • 2008 CHEVROLET IMPALA
  • 5.3L
  • V8
  • 2WD
  • AUTOMATIC
  • 121,690 MILES
This issue has been going on for about 4 years now and only seems to surface when I get to stop-and-go traffic on the interstate after a long run (over an hour at least). Idk if the cruise makes it worse, but I think it does. Now the issue: When I get to stop and go traffic from say, a wreck on the Interstate, the engine surges when I release the brake to roll and keep up with the traffic. It goes away after 5-10 minutes usually, but it is a bit uncomfortable feeling the car surge ahead on its own, albeit not really fast. Once again, this really only happens after a long interstate run and appears when I get into a situation where I come to a stop and then release the brake to let the car roll. Thats when this surge appears. I live in southern AL, so idk if the heat comes into play, but I feel like this has been more apparent in hotter weather too. Car has 119, 00 miles and has had the transmission rebuilt.
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Saturday, February 17th, 2024 AT 11:24 AM

19 Replies

Tiny
JACOBANDNICKOLAS
  • MECHANIC
  • 109,896 POSTS
Hi,

By chance, have you tried physically cleaning the throttle body/throttle plate? Also, when you experience this, have you ever noticed a rich fuel smell from the exhaust? Has the check engine light ever stayed on while the engine is running?

Let me know that. Also, let me know if you have or have access to a live data scan tool. If you do have one, let me know what the short-term fuel trims are. Also, if you can, check them when the problem exists when the engine is warm, the vehicle is parked, and the engine is at idle.

If you haven't already, try cleaning the throttle body.

Here is a link that explains how that is done:

https://www.2carpros.com/articles/throttle-actuator-service

Let me know.

Take care,

Joe

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Saturday, February 17th, 2024 AT 6:57 PM
Tiny
DMHUGGINS26
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Sorry for the late response. I just had the throttle body cleaned at 100,000 miles. It continued right after. I had plugs and wires changed at 90,000. The plugs were Champion iridium and wires were AC Delco. The car DOES emit a bad sulfur smell: especially after I push the throttle like anywhere from 40-100 percent down. I have no engine light on, and the surge does not happen when the car is parked. It will when rolling idle from a long run. I thought about the issue maybe being the cat converter? I have a Magnaflow cat-back exhaust and idk if the cat sensor was disengaged when the aftermarket unit was put on.
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Tuesday, February 20th, 2024 AT 12:33 PM
Tiny
JACOBANDNICKOLAS
  • MECHANIC
  • 109,896 POSTS
Hi,

The converter, at this point, makes the most sense. By chance, do you ever notice any rattling from the exhaust? There could be something loose internally causing the surge. Also, by chance do you have the scan tool I mentioned above?

Keep in mind that inexpensive fuel can sometimes have higher levels of sulfur and cause an odor. Additionally, a failing catalytic converter can do the same.

Let me know.

Joe
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Tuesday, February 20th, 2024 AT 7:48 PM
Tiny
DMHUGGINS26
  • MEMBER
  • 24 POSTS
No rattling. I also don't have the scan tool. How easy is that system to use? Can a cat converter fail with no check engine light? I forgot to mention that I did have the fuel system "cleaned" like 1,000 miles ago. Lastly, one more piece of information: About 4-5 years ago my car went through water that got up to the windshield. I had no indication the part of a parking lot I was driving through had water that deep. Anyways, the car sputtered for like 2 mins or so. I have always wondered if this issue could be related.
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Wednesday, February 21st, 2024 AT 6:16 PM
Tiny
JACOBANDNICKOLAS
  • MECHANIC
  • 109,896 POSTS
Hi,

The OBD2 system is easy to use if you have a live data scanner. As far as the issue, without checking a few things with a scan tool, it will become a guessing game.

As far as the issue, it could be something as simple as a vacuum leak. On the other hand, there could be different sensors that could cause it. For example, a throttle position sensor, an oxygen sensor, a mass air flow sensor, and so on. By any chance, do you think you could get your hands on a scanner?

Joe
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Wednesday, February 21st, 2024 AT 6:58 PM
Tiny
DMHUGGINS26
  • MEMBER
  • 24 POSTS
I probably could. I'm leaning more now towards taking it to my mechanic and leaving it for like a week so they can log data on it. I need to have 2 gaskets replaced anyways. If they can't find it, then I will look into using an OBD2 myself.
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Thursday, February 22nd, 2024 AT 11:17 AM
Tiny
JACOBANDNICKOLAS
  • MECHANIC
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Hi,

I understand. If you change your mind, let me know and I can walk you through the process. As far as the gaskets, which ones are leaking? That is something we can take care of as well.

Let me know if I can help.

Take care,

Joe
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Thursday, February 22nd, 2024 AT 8:07 PM
Tiny
DMHUGGINS26
  • MEMBER
  • 24 POSTS
The rear valve cover and the oil pan gasket. The oil pan gasket just came in from eBay. It was bent and I am trying to contact the seller about it. While it has been brought up, I'm guessing it is not okay to be using a bent oil pan gasket?
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Friday, February 23rd, 2024 AT 12:25 PM
Tiny
JACOBANDNICKOLAS
  • MECHANIC
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Hi,

If that is the heavy gauge rubber gasket, it may be damaged if it is bent too far. It will flex but isn't meant to be bent excessively.

Replacing the gasket isn't a fun job, so like you, I would want to do everything possible to make sure it is done right the first time.

Let me know if I can help.

Joe
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Friday, February 23rd, 2024 AT 8:25 PM
Tiny
DMHUGGINS26
  • MEMBER
  • 24 POSTS
Joe,

I don't know if the feed is still active, but I had virtually all of the gaskets replaced on the Imp and had a new direct-fit cat converter installed last month. Update: The sulfur smell is gone, but I seemed to have retained the surge. I am noticing it may be even more noticeable now on short runs. I also have seemed to narrow down that it is happening after the car downshifts into first gear. When it does, the engine acts almost as if extra air got into the engine right after the downshift. I broke down and bought a Bluetooth OBD2 and recorded fuel trims from 2 short runs. I will upload them here so you can see if something is awry there. I know you thought last time we talked that something may be going on in the fuel trims.
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Monday, July 8th, 2024 AT 10:28 AM
Tiny
JACOBANDNICKOLAS
  • MECHANIC
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In a perfect world, the STFT would be 0%. However, we accept trims that are between +10% and -10%. The trims are well beyond the -10%. That tells me the engine is getting too much fuel, and the PCM is trying to compensate for it by closing the injectors faster. An air inlet restriction would cause this to happen as well. For example, a plugged air filter.

If I had the vehicle in my shop, the first things I would be checking are fuel pressure and then a pressure drop off to see if there is an injector leaking.

If there was a vacuum leak, the trims would be high on the + end of the scale.

https://www.2carpros.com/articles/how-to-check-fuel-system-pressure-and-regulator

The link above explains how to test fuel pressure. You will need a fuel pressure gauge. If you don't have one, most parts stores will lend one to you.

I attached the fuel pressure specifications below. Try this and let me know what you find.

Also, before starting the engine, check the signal from the coolant temperature sensor. It should indicate a temperature close to ambient temperatures.

Let me know.

Joe

See pic below.
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Monday, July 8th, 2024 AT 7:10 PM
Tiny
DMHUGGINS26
  • MEMBER
  • 24 POSTS
Joe,

Thanks and will do. One more item I forgot to mention in the last message: the MAP sensor and a P0420 code triggered the check engine light just about a week after I had all of the afore-mentioned work done to it. Is this normal after having the cat replaced?
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Monday, July 8th, 2024 AT 7:19 PM
Tiny
JACOBANDNICKOLAS
  • MECHANIC
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Hi,

The P0420 indicates the catalytic converter is operating below threshold. In most cases, that is caused by a faulty converter.

If you have or have access to an infrared thermometer, check the temperature at the converter inlet and compare it to the outlet. The outlet should be somewhat hotter. If it isn't, the converter isn't doing its job.

Take a look through this link:

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

Let me know.

joe
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Monday, July 8th, 2024 AT 7:29 PM
Tiny
DMHUGGINS26
  • MEMBER
  • 24 POSTS
Joe,
My coolant temperature pre-igntion was 82.4 Fahrenheit. Ambient temperature was 88-89 degrees. The shop that fixed my gasket leaks and put the cat converter on is going to check the cat converter next Monday and swap it out if another is needed. I'm going to have them check the fuel pressure since I checked my Haynes manual and the fuel check procedure seems a bit out of my comfort zone. If it is not the fuel pump or regulator, what other items could I have the check to find the culprit of the rich mixture? Thanks.
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Sunday, July 14th, 2024 AT 10:44 AM
Tiny
JACOBANDNICKOLAS
  • MECHANIC
  • 109,896 POSTS
Hi,

It could be a leaking fuel injector. When they do the fuel pressure test, ask them to do a leak down test at the same time. There shouldn't be any added cost with doing it. Basically, all they do is watch how quickly the fuel pressure drops when the pump is turned off.

Let me know.

Joe
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Sunday, July 14th, 2024 AT 7:16 PM
Tiny
DMHUGGINS26
  • MEMBER
  • 24 POSTS
Joe,

Okay thanks. I also logged data on 2 more short runs pictured below. Is there a reason the long-term trim would be lower than the short term? They way I understood it, ltft was collected like every few seconds from stft data? As pictured in the first image, the ltft was closer to -25 than the stft.
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Thursday, July 18th, 2024 AT 11:07 AM
Tiny
JACOBANDNICKOLAS
  • MECHANIC
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Hi,

The LTFT's are an average of the STFT's. What you are seeing makes sense. However, we are still getting too much fuel. Were you able to have the pressure tested and a leak down test?

Joe
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Thursday, July 18th, 2024 AT 7:31 PM
Tiny
DMHUGGINS26
  • MEMBER
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Joe,

The place I took it to drive it for a test and found the fuel trims I found. They said they traced it to a bad MAP sensor and replaced that. They said the car was running better after. I don't know if they actually did a manual fuel test now. I just drove the car today for 2 short stints and the trims are back in the negative. On top of this, the car is now throwing more codes, many of which are related to the MAP sensor. I will share those here.
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Friday, July 26th, 2024 AT 10:22 AM
Tiny
JACOBANDNICKOLAS
  • MECHANIC
  • 109,896 POSTS
Hi,

We need to recheck the MAP sensor voltages. If they are too high or low, it will cause a few of these codes to set. If the sensor is good, then we likely have a wiring or connector issue. However, even a new sensor can be faulty.

Take a look at the test below. This is where I would recommend starting. This test is based on the P0121 code.

Let me know.

Joe

See pics below.
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Saturday, July 27th, 2024 AT 8:45 PM

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