Good morning everyone,

This is my post on here, I have been reading this forum for a while to help me with my project. I have now come to post because I am in need of some direction.

I have a 4.8L from a truck that I had built last spring(2025) after I had damaged the #7 piston in a the previous engine. Last month the #7 piston burnt up again, I had thought everything was up to par.

So now Ive been told by another tuner it possibly could of been my ignition coils(d585's) auto firing or the #7 cylinder was leaned out.

I have gapped the rings to .028" top and .030" bottom, a walbro 450lph fuel pump, 700cc Injectors, boost referenced fuel regulator, base fuel pressure 60psi, br7 spark plugs gapped at .020", btr .660" lift valve springs, btr stage 2 turbo cam( intake lift .607 and exhaust lift .618), 7875 turbo, wastegate set at 15psi, melling 10295 oil pump( oil pressure at idle is around 45 hot).

Peak spark advance is 12 degrees on the high octane table and 9 degrees on the low octane table.

I hope I didnt over do it, Any help would be appreciated because i dont want this to happen to a 3rd engine engine I get it rebuilt.

 

IIRC, #7 is the hottest running cylinder in an LS. Not saying that alone caused this. But it probably means #7 is a bit more on the fussy side where fuel and spark are concerned. Has the injector in that cylinder been checked for proper fuel flow. How old, how big, and how many miles on the injectors?

 

Thats what ive found out reading through all the forums. Ive read that people will get the injectors flow tested and put the richer of the injectors in the #7 and #8 cylinders.

I plan on getting a new set of larger injectors, but to answer you I had installed these injectors brand new after the first engine failure, they are 700cc at 43.5psi fuel pressure from canadian performance injection, about 20k km on them( Its my daily). I havent pulled the engine apart yet, used a scope to see a small hole in the piston. Didnt bother compression testing the driver side.

My apologies for not giving all the info but i also compression tested the entire passenger side cylinders and they all seem to be at 100 psi on 2 different testers. bore scoped them and I dont see any cylinder wall damage or piston damage.

 

What exactly would make your 585s auto fire? Are you increasing the dwell in the tune? What ECU are you running?

 

That all seems to be in order, stock ecm with hp tuners ? Knock sensors still functional? Actual timing been verified with piston stop and timing light? Did injectors come with individual flow data? I put the richest 2 in 7 and 8 . What AFR are you running at 15lbs of boost ? I'm guessing you're not running W/M ? I've heard of the coils auto firing when the dwell has been increased past a certain point, I've not touched my dwell settings even at 17lbs and. 025 gap on br7s

 

Might be time to have the injectors flowed if nothing else is found

 

Im not saying for certain thats the reason of my issue, Im trying to eliminate any of the possibilities for it to happen again. My dwell time is stock on the P59 ecu.

Knock sensors are still functional, I havent touched any settings for them. My actual timing hasnt been verified, sadly I was going to a few months ago but I was leaving it until it got warmer. Since it had been running good all summer and fall i figured id be fine. When I get it rebuilt ill confirm it.

The injectors didnt come with individual flow data, I recieved a spreadsheet with flow data for each pai rating, offset voltage, short pulse adder, min injector pulse and short pulse limit.

My afr at 15 psi is at 10.3, I am not familiar with what W/M is? Ive left my dwell at the stock settings, so this could still be something else. Im not set on any one issue but trying to get some other ideas what could be causing it.

I plan on getting new injectors from a trusted company like DeatschWerks or F.I.C.

 

W/M is what I type referring to water and methanol injection. It might be the reason I haven't destroyed my lq4 yet , helps cool the intake charge and water can absorb a lot of heat in the chamber and reduce the risk of detonation . Do you think the damage is being done during boosted times or possibly somewhere else in the tune it leans out or has too much timing? 91+ octane ? What are your coolant temps? When you log it Do you see any KR ?

 

I dont use W/M injection at this point in time, it has crossed my mind though.

It definitely is happening under boost, I dont always keep a close eye on the AFR gauge after ive had it tuned. Every time ive seen the gauge on boost it is on point. This last time I wasn't watching it since I was passing someone on the highway when it happened.

I am using 93 octane, and coolant temps are always around 180f. I have seen some slight knock(1 or 2 degrees) but its usually on decel under no load but it doesnt happen often. Ive never seen knock on acceleration.

 

What kind of rpms are you hitting? Some guys here have had the alternator quit charging at over 6000 engine rpm with stock pulleys, the result is lower voltage to the fuel pump and if it's near max capacity fuel pressure drops and it leans out . Of course that's when you are likely to be looking at the road or tach and miss it on the guage

 

The max rpm I go to is 6500, That is definitely i tough one to track since i have a mechanical gauge for fuel pressure. I can go back in some of my logs and see if there is a voltage dip in the higher rpm ranges. I think ill replace the alternator and change the pulley for peace of mind. Will look into a fuel pressure sensor I can monitor on hptuners.

Thank you for pointing out some things I never would of thought of.

 

Where are the pictures? Of this engine and the first if you have them.
I would pull some spring out of the wastegates, 15 psi is about the max I wanna push it on pump gas. Drop it down to 6-8 and use a controller to sneak it up.
Also 10.3 is a little rich for me, probably isn't hurting anything but 10.8 is where I usually settle. Again this is on pump gas. You should be checking the plugs also, wideband isnt always accurate but the plugs see it all.
Other than that i agree with the others. Could be losing voltage, very common when above 6k rpm. Do you have any datalogs?

 

I dont have much for pictures right now since i bore scoped and only took a couple pictures of the damage in cylinder #7.

I agree that 10.3 was on the richer side but after I wrecked a piston the first time because of what I thought to be a lean condition I figured why not give it some more fuel.

I had checked the plugs after the tuning was complete but this is my daily so i dont pull them often, specially since its a real pain to get the back 2 passenger ones out. I do have some data logs on the laptop I can post when I get home from work.


cylinder #7 with hole in piston around 7 O'clock position.

cylinder #7, looks to be dried up coolant that had drained down the wall.

Cylinder #2 with flakey black residue, All of the passenger side cylinders has this.

 

15 lbs. on pump gas is spicy for sure but the stock 4.8 should have a 9.5:1 compression ratio and 12 degrees of timing which is the same I had when I ran 15.5 lbs. on pump 91 so I wouldn't say it's either of those things.
I've heard different on the AFR and have heard that too rich on pump gas can be nearly as bad as too lean. I've always tried to stay around 11.0-11.5 on pump gas. I've heard that too rich and bad things can happen, but you've burned a hole in the piston which wouldn't be from being too rich.
A hole in the piston is going lean or detonation but now here's the kicker, if you've got a couple injectors out of whack dumping too much fuel, the ECU will pull fuel based on all eight which will make the injectors running normally go too lean. I had this exact issue when I bought a set of injectors that were gummed up from E85 unbeknownst to me. The AFR would not get any leaner than 12.0 at idle because I had two that were dumping fuel, had I made a pull it would have been disastrous.
Regarding the power situation, go straight to the Escalade 160-amp truck alternator, it's a bolt in deal and will perform leaps and bounds better than the stock small case unit. I went all the way and got a JS Alternator which provides full power from idle to 9,000+ rpm because I wasn't taking any chances lol.

 

“Im not saying for certain thats the reason of my issue, Im trying to eliminate any of the possibilities for it to happen again. My dwell time is stock on the P59 ecu.”


I just get real skeptical of “Tuners” even muttering the term autofire when it’s a stock ECU that by any stretch of logic they tuned. It’s a pretty narrow set of parameters that makes it happen, and it’s all from a keyboard.

hopefully you get it sorted. Flow your injectors, log your alternator, read plugs.

 

Hard to say that's a hole from the pic. But if anything it looks cracked with a piece blown out. Doesn't look like ring butt, or torched. It looks like some of the cylinders were burning some oil maybe. I bent my first rod by lifting a head and getting water in the cylinder. Strange things happen some times. Get it ripped apart and let us know, but definitely flow the injectors, upgrade the alt and get a bigger pulley. Shitbox supply has them also

 

Lots of great input here.

What are you doing for an oil catch can and how is it plumbed?

Truck about 5000 lbs?

 

I agree that 15 psi is walking a fine line with pump gas, from the reading I have done across the forums I have found many have been successful running 15psi with similar builds. But I guess we all can't be that lucky. I will be getting a known local tuner to do the tuning this time around and I also asked him to go through the old tune to see if there is any signs of a bad tune previously. It disappointing the way the ECU leans out injectors if others are to rich, but i get that its most like fine on a stock engine. I will be sure to go over the fuel system thoroughly this time. I'm guessing to rich of an AFR didn't help my situation, live and learn I suppose. I'm glad I joined here because I have learned a few things so far and don't seem to be getting any negativity like other places.

I will look into getting the Escalade alternator or I was also looking at those JS alternator's last summer but they are pricey and now I have to spend more money rebuilding the engine. This go round I think ill cap my boost at 10 Psi.

Thank you for the help.

I was talking with a local tuner and he said that it could of been one of the possibilities, So I was hoping to get more information like you have provided. Until i get the engine apart, I wont really know what caused the failure.

I plan on replacing my injectors with deatschwerks, Is it common practice to get injectors flowed regardless of them being brand new?

Thank you.

I am sure hoping I didn't bend any rods, It is hard to see what that dark spot is in those picture so I will get the engine apart and get a better look at the damage. Ill post pictures to keep this thread updated.

 

Once you have it apart it might be more clear if the piston cracked due to detonation, or is showing heat related damage.

You might have covered it already, what did the first #7 piston look like?

Your tuner will be able to see in a log if you are experiencing a voltage drop.

Typically my cars have been tuned for ~11:1 AF.

 

I have the data logs, I will go though the last couple logs to see if there is any voltage drops at higher rpms.

Good to know that 11:1 seems to be a safe AFR, I thought a bit more rich would be on the safe side.




These are the pictures of the previous piston from cylinder #7