AvPilot321
Full Member
Hello folks,
I'm dealing with a rough running engine in my 2007 5.3L LTZ that is driving me up the wall. The truck has no active, pending, or stored codes to give me an idea where to start. The engine runs really rough at idle, almost like a misfire, and seems to subside some when it warms up but is still noticeable. The roughness can be felt at all speeds, and there is even a slight hesitation while accelerating as well as a slight lope and jerking while stopping. There does not appear to be any abnormal noise or ticking coming from the engine as far as I can tell.
I have replaced the spark plugs, less than 400 miles on them. The ignition system was tested with a spark plug tester at the end of each wire and appears to work well. I also checked the engine with a spark plug wire removed from each cylinder one at a time, and the roughness got considerably worse each time, leading me to believe that it is not a true/full misfire. The fact that a cooler engine produces a worse result led me to think I have a vacuum leak. I have inspected all vacuum hoses and gone around the manifold with a stethoscope with no sign of a leak. MAF sensor and MAP sensor were replaced. I have yet to have a smoke test done, but the Manifold pressure readings and MAF readings do not seem to fluctuate as I would expect with a leak.
The fuel pressure has been tested, and injectors sound like they are working properly, and all test 13.6-13.9 ohms resistance. One thing I can't seem to find an answer on is the fuel pressure spec with a running engine. The GM service manual lists 50-60psi with engine off and ignition on, but does not have a range for engine running. When testing my fuel pressure I read 58psi with the engine off, ignition on, and passes a leak down test. When the engine is running the fuel pressure remains at 58psi through all ranges. The old spark plugs did have a light coating of black soot at the base, which I believe usually indicates a rich mixture. From what I can tell on other vehicles I should expect the fuel pressure to drop, but there is no mention of this in the service manual. Newer models with the FPCM require something in the range of 45psi when running at idle, but this model truck does not have FPCM. Would this psi range apply to my truck anyway?
To recap what I've checked already:
Spark plugs replaced
Ignition system tested
Two cans of Berryman's B12 ran through fuel system (this stuff works wonders, usually)
Fuel pressure tested: 58psi off AND idle
Injectors clicking and all test 13.6-13.9 ohms
MAF and MAP replaced
Replaced PCV return line because of a cracked boot
No sign of coolant/oil mixing
No Check Engine Light/Codes
I am at my wits end with this roughness, any help is greatly appreciated.
I'm dealing with a rough running engine in my 2007 5.3L LTZ that is driving me up the wall. The truck has no active, pending, or stored codes to give me an idea where to start. The engine runs really rough at idle, almost like a misfire, and seems to subside some when it warms up but is still noticeable. The roughness can be felt at all speeds, and there is even a slight hesitation while accelerating as well as a slight lope and jerking while stopping. There does not appear to be any abnormal noise or ticking coming from the engine as far as I can tell.
I have replaced the spark plugs, less than 400 miles on them. The ignition system was tested with a spark plug tester at the end of each wire and appears to work well. I also checked the engine with a spark plug wire removed from each cylinder one at a time, and the roughness got considerably worse each time, leading me to believe that it is not a true/full misfire. The fact that a cooler engine produces a worse result led me to think I have a vacuum leak. I have inspected all vacuum hoses and gone around the manifold with a stethoscope with no sign of a leak. MAF sensor and MAP sensor were replaced. I have yet to have a smoke test done, but the Manifold pressure readings and MAF readings do not seem to fluctuate as I would expect with a leak.
The fuel pressure has been tested, and injectors sound like they are working properly, and all test 13.6-13.9 ohms resistance. One thing I can't seem to find an answer on is the fuel pressure spec with a running engine. The GM service manual lists 50-60psi with engine off and ignition on, but does not have a range for engine running. When testing my fuel pressure I read 58psi with the engine off, ignition on, and passes a leak down test. When the engine is running the fuel pressure remains at 58psi through all ranges. The old spark plugs did have a light coating of black soot at the base, which I believe usually indicates a rich mixture. From what I can tell on other vehicles I should expect the fuel pressure to drop, but there is no mention of this in the service manual. Newer models with the FPCM require something in the range of 45psi when running at idle, but this model truck does not have FPCM. Would this psi range apply to my truck anyway?
To recap what I've checked already:
Spark plugs replaced
Ignition system tested
Two cans of Berryman's B12 ran through fuel system (this stuff works wonders, usually)
Fuel pressure tested: 58psi off AND idle
Injectors clicking and all test 13.6-13.9 ohms
MAF and MAP replaced
Replaced PCV return line because of a cracked boot
No sign of coolant/oil mixing
No Check Engine Light/Codes
I am at my wits end with this roughness, any help is greatly appreciated.