87 Octane not getting it done

floridafitz

Original poster
Member
Jan 2, 2012
151
Winter Springs FL
Been experiencing engine rattling when running 87 octane since I bought this 08 almest 2 years ago. 89 is better. 91-93 best. Weren't these engines designed to run on 87? Is there anything I can do to improve this. Have not run anything like seafoam through the fuel system and plugs are original at 119K.
 

northcreek

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Jan 15, 2012
3,310
WNY
My 07 always ran great on 87 octane, even at 125K miles, you may have other issues.
 

mrrsm

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Oct 22, 2015
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The GM 5.3L Engine is supposed to run "OK" on 87 Octane Fuel... but that fuel has less of an "Energy Punch" than the slower, more evenly burning (Non-Pinging) Higher Octane Fuel can deliver. Your Spark Plugs started to get "Very Long in the Tooth..." beyond 100,000 Miles.

The lower the Octane of the Fuel... combined with the presence of a LOT of Upper Combustion Chamber Carbon Deposits and "Gas-Gum Stuck" Compression rings ... can invite Pre-Ignition events that cause engine stumbling. You could try running with a "One High Octane Top Off Every Third Fill-Up" along with using some Chevron Techron Gas Supplement as an alternative fueling pattern and see if that action smooths things out and peps things up... especially when Towing anything.
 
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Reprise

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plugs are original at 119K.

Start here, as it's time for those plugs to be replaced. Get the AC Delco - both the I6 and V8 run best with them, as documented by many on this site. I prefer the iridium; others have used the dbl platinums, etc.

Actually, I see you have a Denali. Get the iridiums. I think the part # is 41-103 (but validate). Mine runs like a top with them - and I only use 87 octane, as long as it's no worse than E10 (mandated in my area). The ones I pulled out were Bosch - also iridium, and practically new - but the engine runs so much smoother with the Delcos installed. More often than not, there's usually a rebate on them, too

If you have the spend, put some new wires on it, too, since those are original.

Finally, do you tend to favor one brand / location of gas? If it's a 'Top Tier' brand, then you should be good.

The 5.3L has knock sensors in the valley cover, underneath the intake - at least my LM4 does. They're known for going south in that location (one reason why the Gen IV LS motors relocated them to the side of the block). That might be worth checking out.
 

northcreek

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Jan 15, 2012
3,310
WNY
The 5.3L has knock sensors in the valley cover, underneath the intake - at least my LM4 does. They're known for going south in that location (one reason why the Gen IV LS motors relocated them to the side of the block). That might be worth checking out.
The knock sensors do fail ,I know my son had to replace his but, I think a bad knock sensor would throw a code.
 
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Sparky

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Dec 4, 2011
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Are you tuned or stock?

My 5.3 in my Silverado would popcorn some on anything but stock (modified) tune. Any canned tunes from a handheld, even 87 tunes, would cause it to make noise. No idea why, never figuted it out. So stock tune it is except for AFM disabled and tire size adjustment.
 
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Bow_Tied

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Dec 21, 2014
453
London, ON
My '06 Denali is stock and seems to run fine on '87. I will echo reprise on the Delco plugs, change the wires while you're in there. I just had a two speed idle emissions test done and it was running super clean (garage was surprised/impressed) @ 180k km (111k miles)
 
D

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The 5.3 engine you have should be the LH6 GEN 4 engine. It has slightly higher compression than the previous Gen 3 LM4 engine. It should not however have any problems such running 87 octane on a stock tune. My concern would be that your catalytic converters or exhaust system is clogged. The fact that your plugs are original is not helping. You should run NGK TR55'S in it. Other things that come to mind are the MAF sensor could be dirty possibly causing a lean condition, intake manifold leak, dirty combustion chambers, or possibly low fuel pressure. You would benefit from running mid-grade fuel exclusively.

Are you certain you don't have exhaust manifold leaks? LS based v8 are riddled with manifold bolt breaking problems even worse than the 4.2.

Are you tuned or stock?

My 5.3 in my Silverado would popcorn some on anything but stock (modified) tune. Any canned tunes from a handheld, even 87 tunes, would cause it to make noise. No idea why, never figuted it out. So stock tune it is except for AFM disabled and tire size adjustment.

If you dump a tune in it, you have to run a higher grade fuel, when the timing is advanced it cause lower octane fuel to detonate. That's why I hate hand-held tuners, they are cut and paste and not personalized to your setup. HPTUNERS ftw :tongue:
 
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mrrsm

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+1 ...^... and @northcreek 's very clever Thread about the "5.3L Exhaust Manifold Broken Bolt Fix" may solve that problem without having to R&R the Exhaust Manifold(s) if you can spy the exact problem exhaust leak location(s) along the Exhaust Manifold Flanges of both Banks.

Mike's work and contributions like this one is yet another reason why I am going to really miss him ...now that he has moved on to another Vehicle Platform:

https://gmtnation.com/forums/threads/5-3-exhaust-manifold-broken-bolt-fix.17823/
 
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Mike534x

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Apr 9, 2012
894
^ I vote manifold bolts as well. When mine went last year, the ones between #4 and 6 were gone and stuck in the head. I couldn't get them out, but found a nice "shop" in the nice part of town who did it cheap ontop of a six pack. If they couldn't be extracted, I was up for trying out the kit....but also didn't feel like having my hands mangled with all the "generous" room GM left in the engine bay.

When the noise first appeared, I thought mine was pinging since it happened after filling up. That thought went out the window when I heard the little "ting" sound from the bolt heads flinging against the strut and manifold and when they broke free.

Though I'm wondering if maybe the rattle is actually the exhaust note from the engine entering "V4" mode, but thats usually when driving or coasting. Its been awhile since I last heard that "distinct" note. My next though was maybe carbon build up and its just running rough + the need for a tune up.


After replacing the MAP sensor (completely fouled with oil) and having the injectors removed to be cleaned individually, its buttery smooth.
 

northcreek

Member
Jan 15, 2012
3,310
WNY
+1 ...^... and @northcreek

Mike's work and contributions like this one is yet another reason why I am going to really miss him ...now that he has moved on to another Vehicle Platform:
Kind words for sure, thanks... I'm still kinda hang'n roun' on the graces of my wife's Equinox which may barely qualify, saving grace being AWD.
As you know these forums tend to attract people having problems with their vehicles. So I've stepped out of my comfort zone and bought a non-GM Jeep. The Jeep forums (there are many) give me the creeps after plopping down "folding money", reading Jeep owners problems and rants....I feel safe and untouchable here...Mike.:2thumbsup:
 
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floridafitz

Original poster
Member
Jan 2, 2012
151
Winter Springs FL
Appreciate all the replies and suggestions. I'll get to work...probably plugs/wires first.
 

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