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Stutters and backfires occassionally

Tiny
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  • MEMBER
  • 2003 CHEVROLET 1500
  • 4.3L
  • V6
  • 2WD
  • AUTOMATIC
  • 300,000 MILES
Parts and Service Done:
New lower and upper intake gaskets, new distributor, new distributor cap and rotor, new spark plugs and wires, new alternator, new fuel spider, new camshaft position sensor, new battery, new catalytic convertors. Fuel pressure within spec.
Problem: Engine had a code P0420 before I replaced catalytic converters. There is no check engine light on. Checked codes and no codes stored. The engine stumbles and will occasionally backfire when I jump on the accelerator. I have checked for vacuum leaks using propane and nothing showed itself. I'm dumbfounded. Any ideas?
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Tuesday, March 30th, 2021 AT 9:38 AM

14 Replies

Tiny
JACOBANDNICKOLAS
  • MECHANIC
  • 108,170 POSTS
Hi,

Oftentimes, a backfire is caused by a rich fuel mixture. You replaced a lot already. Do me a favor. If possible, get your hands on a live data scan tool. I need you to check the engine coolant temp sensor signal to make sure it is accurate. Also, I want to know the short-term fuel trims.

Let me know if that is something you can do. I will watch for your reply.

Joe
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Wednesday, March 31st, 2021 AT 8:19 PM
Tiny
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The P0420 code reappeared 2 days after the cat replacements. I ordered a new scan tool to read the fuel trims and water temp. Although I do not suspect the water temp sensor since the Temp gauge seems to be tracking. The new scan tool should be here tomorrow so I will keep you posted. I'm suspecting that the upstream 02 sensor is the culprit since I have not changed any of them.

Steve
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Thursday, April 1st, 2021 AT 8:36 AM
Tiny
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Hi,

Sounds good. Let me know the results and we'll go from there.

Take care,
Joe
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Thursday, April 1st, 2021 AT 6:21 PM
Tiny
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Joe,

The P0420 went away after I replaced the up-flow O2 sensor on drivers side. There are no codes on the MIL and no pending codes on the scan tool. The truck still will bog down and sometimes backfire when I jump on the accelerator. I also get this noise which sounds like a turbo air let off valve. Not sure where that's coming from. Let me know what readings you want me to take.

Steve
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Thursday, April 8th, 2021 AT 5:27 AM
Tiny
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Engine temperature read from scan tool is between 188-190 F when hot.
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Thursday, April 8th, 2021 AT 9:47 AM
Tiny
JACOBANDNICKOLAS
  • MECHANIC
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Hi,

The temperatures look okay. As far as the backfire, I would first confirm the short-term fuel trims. Have you gotten the scanner yet? Also, the idea that you are hearing a sound like a turbo relief valve, I am questioning if there is a partially clogged converter not throwing a code or there is a vacuum leak that happens under that load. Confirm the throttle body is tight, the gasket is good, and check to make sure there are no rattling sounds from the exhaust.

Let me know.

Joe
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Thursday, April 8th, 2021 AT 7:12 PM
Tiny
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I will read the fuel trims at idle. As far as the cats are concerned, I replaced both of them already. I will also check the throttle body is on tight. I did already buy a new scan tool. I bought a Foxwell NT301. I was thinking that the air noise might be coming from the EVAP purge valve as my scanner is indicating a small EVAP vacuum leak. I would think that I would get a code however.
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Friday, April 9th, 2021 AT 4:02 AM
Tiny
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Hi,

You won't hear a small evap leak. They are actually a pain to find. LOL The easiest thing is to smoke test the EVAP system. Smoke is injected into the evap at very low pressure. If there is a leak, the smoke will come out from it.

Let me know what you find.

Take care,

Joe
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Friday, April 9th, 2021 AT 6:32 PM
Tiny
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Okay so out of spite, I went ahead and replaced the remaining 3 O2 sensors. They were the originals so with 300,000 miles, I thought it would be prudent. The 420 code has all but disappeared but the little initial take of stutter is still there. Both of the STFT's are right around.3 and the LTFT's are.7 and.9 respectively. Any thoughts?
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Tuesday, April 20th, 2021 AT 6:14 AM
Tiny
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The trims look good. As far as a stutter, I always think vacuum leak or fuel pressure. However, with 300,000 miles, it could be compression.

I know you checked for leaks and the fuel system is basically new, DId you check compression?

https://www.2carpros.com/articles/how-to-test-engine-compression

Let me know.

Joe

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Tuesday, April 20th, 2021 AT 6:42 PM
Tiny
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So the P0420 appeared again yesterday. I'm guessing that there is a leak in the exhaust system. So I'm planning to use a homemade smoke machine to introduce smoke to the tail pipe and see if I can detect the leak. My guess is that it either from Bank 1 "donut" or the manifold gasket on that same side.
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Thursday, April 22nd, 2021 AT 6:10 AM
Tiny
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Hi,

Usually, a gasket or donut will be heard. Basically, they usually sound like a ticking when they're not too bad. Have you noticed anything like that? Also, a leak before the catalytic converter will usually cause a lean fuel mixture. As a result, the computer will overcompensate with adding fuel. The fuel trims, however, look good.

Let me know what you find. I'm really interested in knowing.

Take care,

Joe
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Thursday, April 22nd, 2021 AT 7:14 PM
Tiny
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So, I have returned to look for a vacuum leak. While cleaning the throttle body, I sprayed the MAP sensor and the truck stopped. I tried it again and same result. So I removed the sensor and the seal. The seal was "mushy". I placed the seal back into the valve cover, then proceeded to insert the sensor. I started the truck and it was running rough. I went back to spray it again and noticed that the seal had been sucked into the intake. Panic! I've read that the seal will break down with the fuel and not to worry. I ordered a new sensor and seal. Will the seal be okay leaving it where it is or do I need to crack open the upper intake again?
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Tuesday, May 4th, 2021 AT 3:42 AM
Tiny
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Hi,

I have to be honest, it is likely already burnt and out the exhaust. However, like you, I don't like the idea of it in there, but it can't last too long. And, in this case, there is nothing that can be done other than removing the intake. Ugh! I'm sure you don't want to do it again.

If it was mine, I have to say I would likely remove the upper intake and check. But, that is your call.

Let me know if I can help in any way.

Take care,

Joe
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Tuesday, May 4th, 2021 AT 7:18 PM

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