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Rough running engine 2007 LTZ

AvPilot321

Full Member
Joined
Oct 16, 2020
Messages
12
Location
Florida
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 did not see a spec on the running engine either but 58psi during all ranges sounds good.
Any voltage variations or is it constant 14 volts.
Any suspension or brake warning lights on
 
Which voltage, battery voltage? The battery voltage does fluctuate as the battery is charged, but that is a function of the GCU. I just had the battery tested last week, and the alternator is new this year after the GCU failed. I recently had a battery drain overnight but I found that to be related to the Bose stereo amp connection.
No warning lights like many of the other drivers have reported in relation to faults in the electrical system.
 
Given this is a 2007 and you have tested almost all of the usual suspects, I wonder if you are seeing the beginning of an AFM lifter failure.

Maybe the AFM intake lifter that is beginning to fail is still opening, but not as much as it should.

Perhaps a compression test would yield a clue.
 
Finally got a chance to run the compression test today after having to travel for work. Looks pretty good to my untrained eye, all with 5% of each other.

1 - 190psi
2 - 189
3 - 196
4 - 195
5 - 199
6 - 190
7 - 189
8 - 195

These numbers were achieved on a slightly warmed engine on a. On a side note, I initially followed the Haynes manual procedure to do a compression test. In that procedure it tells you to block the throttle in the wide open position. I do not recommend this unless you want to get the reduced engine power and stabilitrak messages with a no start condition.

I guess my next step is to find a shop that can perform a smoke test...
 
What do your fuel trims look like? Pretty close to zero? Misfire counter show any one cylinder higher than others despite no CEL?

In case you feel like trying to perform a smoke test yourself, the handmade kits on ebay are reasonably priced
 
What do your fuel trims look like? Pretty close to zero? Misfire counter show any one cylinder higher than others despite no CEL?
Interesting you ask, didn’t know what I was looking for with this new scan tool.

Misfire data for last/current drive cycle (idling for 22 minutes in my driveway):

Cylinder 1: 25
Cylinder 3: 1
Cylinder 8: 3
All other counts Zero

What constitutes a counted misfire? I have run the engine before with each cylinder deactivated (true misfire) and the rough idle is not quite as bad as a full missing cylinder.

Fuel trims (again, don’t know what I’m looking for) at idle:

Short Term 1: -3.1 to 2.3
Short Term 2: -3.1 to 1.6

Long term 1: -3.1 to 1.6
Long term 2: -5.5 to -3.1
 
I don't like that misfire count on cylinder 1... You might consider swapping components one at a time with another cylinder and then look for misfires again.. Coil, plug and injector.. Or shotgun it and change each part with a different cylinder, but that is a lot of moving pieces at once.

To read a bit on fuel trims, this might be helpful. https://www.mechanic.com.au/news/understanding-short-term-and-long-term-fuel-trims1
 
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Oh and also, cylinders 1, 4, 6, 7 are AFM.. When testing swap with a non-AFM cylinder, use 2, 3, 5, or 8 so the AFM lifters are not a variable. I wonder if EXT4ME called this one early and it's a lifter issue...
 
I don't like that misfire count on cylinder 1... You might consider swapping components one at a time with another cylinder and then look for misfires again.. Coil, plug and injector.. Or shotgun it and change each part with a different cylinder, but that is a lot of moving pieces at once.

Ok, so I went out to do some swapping. I got as far as the coils. I swapped the #1 coil with the #5 coil on a cold engine. Fired it up, engine still runs rough but I got a confusing result on the misfire counts, as seen below. I did not change anything on cylinder #3. These numbers are misfire for current run after the swap and an 11 minute idle:

Cylinder 1: 0
Cylinder 2: 1
Cylinder 3: 29
Cylinder 8: 3
All others zero.

That's a weird result, I thought, so I swapped coils #3 and #5 and idled again. These numbers below are misfires for current run after the second swap idling for 10 minutes at operating temperature:

Cylinder 1: 1
Cylinder 2: 1
Cylinder 3: 0
Cylinder 6: 1
Cylinder 7: 1
All others zero.

These numbers are irritating me, there are such drastic changes with little to no modifications being made, so I decided to take it for a test drive. These numbers are misfires for current run after a 20 min test drive through all speed and acceleration ranges.

Cylinder 1: 5
Cylinder 2: 2
All others zero.

The drive was not satisfactory, there was hesitation and a rhythmic misfire feel, but not near enough misfires registered.
I'm not quite sure how to interpret this. I understand the the AFM failure can start to set in without having full misfires, but if this is the case why did cylinder #3 have such a hard time on startup? Is there a correlation between AFM misfires and operating temperature? I did not notice any abnormal oil pressure readings at any point. There is no audible ticking from the running engine. However, there may be a very slight tick at cylinder #1 and #8 with the stethoscope on the valve cover, but it is so faint that I'm not convinced its a problem, as these engines do tend to tick sometimes. I'm scratching for any way to confirm that this is or is not and AFM lifter issue before digging into this thing. I've never torn down a motor, and am willing to, but don't want to do it just to chase something...

Another question, when the AFM lifters go bad, is replacing the lifters and the VLOM generally a good fix, or are people seeing problems with the camshaft as well?
 
1 misfire is not a big deal if you read up on it.

Takes a certai amount to trip misfire light and I think it is in the 100s

Misfire works as follows from memory of a long time ago:
Computer watches crank sensor or something nd when it triggers a spark for a cylinder it expects the crank sensor and rpm to increase a bit.
if rpms do not increase ( we are talking minor amounts) then it says MISFIRE and starts a count..

See if it misfire then engine rpms drop a bit until next cylinder fires since no power. or reduce power from misfire.

PRetty neat and this is how it knows which cylinder.

Now if more then 1 cylinder trigger misfires then you may get the flashing light and generic all misfire code...

SO to have 1 misfire is not a big deal.

But when light comes on for misfire then you have issues.
 
I've been reading this post, when you swapped the coils and didn't have any misfires seemed weird, I would check your grounds on the engine harness and make sure they aren't corroded, I've also run into running problems where the computer connections are also corroded, you may want to pull the computer connections and check for corrosion on them, it's been a huge problem with these and cleaning them has corrected running problems, also dash light problems and guage problems. I hope this helps.
 
UPDATE

Been doing a good amount of traveling for work, so the truck had been sitting in the driveway. Broke down and took it to a shop(s). The first shop’s diagnostic capabilities seemed to be limited to reading codes, which there were none. Their advice was to take it to the dealer, thankfully they didn’t charge me for this advice.

Got in contact with an acquaintance who owns a shop in another part of town, and had them do some diagnostics. Again they couldn’t figure out what was causing the misfires, but found the engine mounts to be worn. Not broken, just worn.

Had the engine mounts replaced and the truck rides like it’s brand new. Zero misfires counted on a nearly 1 hour drive home.

Problem solved; Engine Mounts.

Very happy to have it fixed, and disappointed that I missed it. Not sure that I would have been able to diagnose a worn mount on my own anyway.

EDIT: very temporary crutch, still having problems.
 
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That is bizzare that motor mounts could cause a misfire. I wonder if they disconnected the battery which somehow temporarily cleared the codes. Don't want to be a pessimist but that just baffles me.
 
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.
Based on my experience with my 2007 Z71, I would replace the spark plug wires. I bought Taylor cable wires for my Avalanche and the engine ran much smoother. An Olds Aurora I had was rough off the line, and wires helped it too. Give wires a try; if the wires haven't been replaced since new, it's likely time to do so.
 
That is bizzare that motor mounts could cause a misfire. I wonder if they disconnected the battery which somehow temporarily cleared the codes. Don't want to be a pessimist but that just baffles me.
I appreciate the pessimism...
There never were any codes to begin with. Just a shaking motor/rough idle with some random misfires counted. The thought was maybe the worn motor mounts were causing the misfire counter to trip? I don’t know how it drove so smooth from the shop without counting misfires, but whatever is wrong with it wore those new motor mounts in real quick. After putting nearly 500 miles on it in the last two days it’s shaking more than it was before, visibly now. Started counting misfires again: 1 (40), 2 (22), 3 (4). These counts were on a two hour drive. It doesn’t have a loss in power, and it doesn’t feel like it’s missing when accelerating. I’m now at my wits end...

EDIT: Only one of the three cylinders counting misfires is AFM, I doubt an AFM lifter going bad would cause misfires in non-AFM cylinders. Thoughts?
 
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Based on my experience with my 2007 Z71, I would replace the spark plug wires. I bought Taylor cable wires for my Avalanche and the engine ran much smoother. An Olds Aurora I had was rough off the line, and wires helped it too. Give wires a try; if the wires haven't been replaced since new, it's likely time to do so.
The wires have been replaced since I’ve owned the truck. Maybe I will try investing in some higher quality wires as you said.
 
That is not really a lot of misifres the way the computer calcs a misfire... hence why they have a certain threshold to say there is a MISFIRE and trip a code.

Let it go.... just drive and enjoy
 
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