Misfiring?

SBMISRY

MEMBER
2003 PONTIAC GRAND AM
3.4L
V6
2WD
AUTOMATIC
225,575 MILES

Recently changed fuel pump. Drove fine then I got a p0455 for small emission leak. A couple of days went by then got a p0456 Evap vent. I am now getting random misfire in cylinders 3, 4, and 5. Got new double iridium plugs and new wires. Any ideas? I do have a Launch Creader VII+ code scanner.

6 Replies

JACOBANDNICKOLAS

MECHANIC
109,755 POSTS

Hi,

For some reason, both of the codes you supplied are not listed for this vehicle. I know a P0455 is a "gross EVAP leak. These are usually easy to find. If the fuel cap is off, the evap canister is damaged or disconnected, or there are hoses disconnected or broken between the fuel tank and the charcoal canister.

Now, the P0456 is identified as a "Very Small EVAP leak." If we have a large leak, I'm not sure why it is showing a small leak too. Are you sure the codes you indicated are correct?

As far as the misfire, chances are you are drawing fresh air into the combustion chamber causing a very leak fuel mixture. Does your scan tool provide live data? If it does, can you tell me what the short-term fuel trims are at operating temperature?

Next, inspect the charcoal canister and hoses to it. It is located near the fuel tank. See pic below.

Let me know what you find.

Joe

See pic below.

Image (Click to make bigger)

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Friday, January 6th, 2023 AT 10:51 PM

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I may not have listed the codes correctly. I do know the first code was for a small leak. The second code was for evap vent valve. When I go into diagnostics it says excessive vacuum. I have to drive 40 miles to work so I will list the short- and long-term fuel specs in an hour or so.

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Saturday, January 7th, 2023 AT 3:10 AM

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In park with engine at idle short term fuel trim is going from -2.3% up to 2.3%. It doesn't stay steady on any given number. Long term fuel trim is going from 16.4 % up to 18.8%. Most of the time it stays on 17.2%. Engine rpms are all over the place from 690 up to 744. Ignition timing advance is going from 18.5 deg up to 20 deg. Also, the evap system leak test failed. I think the codes were p0442 and p0446. They occurred separately.

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Saturday, January 7th, 2023 AT 4:16 AM

JACOBANDNICKOLAS

MECHANIC
109,755 POSTS

Hi,

Okay, it makes sense with those codes. The LTFT are indicating the comp is trying to compensate the air/fuel mixture by adding more fuel. You really don't want that number to be above 10%.

I have a feeling there is a leak at the EVAP vapor canister purge solenoid. That is where I would start.

If you look below, I attached a pic to help locate the part. Confirm the vacuum hoses are connected, not leaking, and not dry rotted.

Let me know what you find.

Take care,

Joe

See pic below.

Image (Click to make bigger)