Random misfire

[06MidnightBlue]

For shits and gigs you might try cleaning the MAF sensor.

[catrailrider]

I just cleaned the throttle body and still getting rough idle I dont have a MAF sensor but it has a MAP sensor which cant be cleaned. It has an IAT sensor before the intake. Do those get dirty?

[fishsticks]

This is probably not your issue, but my random misfire is gone.

I finally traced it down to a timing issue with my PCM4Less tune. I reviewed my order sheet and realized I had them tune it for 91 octane. At some point my brain reverted to putting 87 in the truck. So I filled up with super and the miss disappeared. Half tank of regular, miss was back.

How I felt:

[Trail X]

I'd be surprised if that was the root cause Donny... 87 combusts more readily than 91.

Octane requirements are really only dictated by an engine's compression ratio. I'm not sure what PCM4LESS would even change for our engine if tuning for 91. Doesn't really compute to me.

[dvanbramer88]

Ask Kyle. He has his truck tuned for Premium fuel. I think it has to do with the timing and when the PCM sends the spark.

[fishsticks]

I'd be surprised if that was the root cause Donny... 87 combusts more readily than 91.

Octane requirements are really only dictated by an engine's compression ratio.

Octane slows the burns cycle, it doesn't inhibit it. It's used to prevent the flame front of an ignited charge slamming into the face of the piston before the piston has reached TDC. AKA knock.

Reference the multitude of 10:1+ compression ratio engines out there today that run on 87.

I'm not sure what PCM4LESS would even change for our engine if tuning for 91. Doesn't really compute to me.

More aggressive timing curves, including more timing at idle. We added tons of timing at idle when we tuned the LT1 in my Camaro. Why? Throttle response.

My miss never produced a fuel smell or rich O2 code. I believe it's within the realm of possibility that in my case, it was a timing issue.

[v7guy]

I've ran across a stumble recently too at 98k miles. There is no code present.
While sitting there idling it is obviously present. The throttle body has been cleaned, the spark plugs have been replaced, etc etc etc.
All the recommended items are taken care with the commonly recommended brands as commonly suggested. The only maintenance item not done is the O2, but that shouldn't cause a miss.

I'll pull the plugs later to see what they look like despite only having 10k on em

as Donny said, you can advance the timing to up the octane requirement, an old practice from the LT1 days. You can see some decent improvements that way. One of the perks of EFI

[Trail X]

I'd be surprised if that was the root cause Donny... 87 combusts more readily than 91.

Octane requirements are really only dictated by an engine's compression ratio.

Octane slows the burns cycle, it doesn't inhibit it. It's used to prevent the flame front of an ignited charge slamming into the face of the piston before the piston has reached TDC. AKA knock.

Reference the multitude of 10:1+ compression ratio engines out there today that run on 87.

I won't argue with your second point, because I don't know enough about it.

But my understanding is that octane does not necessarily slow the burn cycle... more octane raises the auto-ignition pressure of the gas. Thus why diesel has a low octane level. That's also why you'll only get a knock at higher throttle. You have higher V.E. at high throttle, and therefore higher pressures in the combustion chamber before spark. If you run 87 in a high octane engine, the air/fuel mixture can ignite before the spark fires solely due to it's pressure - at that point, you're right, the pressure wave smacks the piston head before TDC causing the knock... but I do not believe it's due to the combustion traveling too fast. Maybe it goes hand-in-hand? Either way, at idle the cylinder's only getting fed like 4 psi gauge of air, so even in a high compression engine, you don't get enough pressure to auto-ignite the gasoline and get any knock.

[catrailrider]

Well I've used 87-91 octane and the stumbling at idle has been hit/miss with all grades. What's interesting is that the stumble went away when I had a half tank of 91 and a bottle of Tecron

[navigator]

my 07 Suburban has a little oddness at idle as well.
I did a good bit of reading and it seems that many did it from the factory. The GM folks told the owners that they were adjusted a certain way, especially west coast vehicles due to emissions mumbo jumbo.

My Suburban was originally from WA, I haven't cleaned the TB yet on it or anything I wonder if they are related?

[Trail X]

Well I've used 87-91 octane and the stumbling at idle has been hit/miss with all grades. What's interesting is that the stumble went away when I had a half tank of 91 and a bottle of Tecron

I have gone weeks thinking I had found the perfect combination too. I was using 91 and adding Tecron every time I filled up. But inevitably the phantom misfire came back. I'm just glad that my truck is no longer throwing codes like it used to. I used to get a regular P0302 every few months.

[jonbo2002]

my random misfire ended up being a bad coil. autozone said it was either plugs, coil, vacuum or fuel pressure the first time replaced the plugs and it worked fine for a couple weeks then came back but this time it didn't throw a code, but I had a few extra coils sitting in the shed and changed cylinder one and all is great now

[catrailrider]

Why did you change the coil in cylinder 1? Was it throwing a code for #1 cylinder? At $70 a pop I wouldn't want to start guessing which cylinder. Not sure if an ohmmeter would be able to test them but I'd think CEL would sure help

[jonbo2002]

I work in auto salvage so free for me. but like I said the first time I threw a code they said it was cylinder 1 missfire could be plugs, coil, vacume or fuel pressure. so I changed the plugs first not knowing if the orgional owner ever changed them. it worked for a couple weeks then started missing again so I grabbed one of the coils I had sitting in my shed and put it in. no more issues since. I would think it would be hard to test seeing as that it would randomly have problems. I think it is worth it for anyone to venture to their closest salvage yard and pull a couple coils at a couple bucks then buying new.

[djthumper]

Why did you change the coil in cylinder 1? Was it throwing a code for #1 cylinder? At $70 a pop I wouldn't want to start guessing which cylinder. Not sure if an ohmmeter would be able to test them but I'd think CEL would sure help

Where are you shopping that they are $70 each? I replaced 2 of them for under $50 a few months ago.

[Cable810]

I paid $100 for one. Tried to get my money back but they said nope No returns on electrical, I hadn't even opened the box nor installed it.

[jonbo2002]

ouch did they lube you up first???

[Cable810]

The first one I got I ended up breaked the bolt that holds the pack in place and they replaced it thankfully. What do you mean Lube me up???

[v7guy]

The first one I got I ended up breaked the bolt that holds the pack in place and they replaced it thankfully. What do you mean Lube me up???

ummm, before they fu(ked you in the ass during the raping... $100 is rough lol
most the time I forget you were home schooled man, but then stuff like this pops up and it makes me chuckle a bit


I've pretty much given up trying to resolve it, I have no codes, it always runs, wish it would cut it out though

[catrailrider]

Well if you wanna buy the $20 Chinese made coils that's up to you. My TB likes OEM stuff or I'll end up paying more later