Looking for purchase advice. My 02, which sits in the garage quite a bit, is having random misfires with the flashing 'service engine' light. I know things can go bad just from sitting. My mobile mechanic wants to replace all the coil packs first. I can't find a set of ACDelco; only singles here & there. Without going too crazy with high performance, is there a good quality brand that anyone can recommend? Thanks in advance

 

Coil packs don't go bad just from sitting. In fact, LS1 coils are one of the most reliable stock parts you will find; even at 200k+ miles they are usually still fine and they would be equally fine sitting on a shelf in dry storage for 25 years. Mine are still original on my '98; at only 20k miles this car has sat in the garage for most of its life.

My first advice is to find a new mechanic. Replacing all the coil packs is a horrible first step and I'm shocked any professional would even recommend this. So let's get on to some proper diagnostics:

When do these misfires happen? Idle? Under acceleration? Engine hot or cold? All of the above?

You mentioned having a "flashing" service engine light. This indicates a serious misfire, which means you should have more than just a P0300 Random Multiple Misfire code. Additionally, you should see at least one P030x (X = 1-8, indicating impacted cylinder) code that points to a specific offending cylinder (or cylinders, if more than one P030x). This is where diagnostics should begin, with the specific cylinders indicated.

If you have only a P0300 code, then the SES light shouldn't be flashing. But in any event, with an advanced scanner (something any mechanic should have) you can view cylinder specific misfire data to see which ones show more misfire events than others, even if there is no P030x code. So that would be another thing to check.

I'd also want to look at fuel trim data (Long Term Fuel Trims, LTFT) on both banks for any suspect readings.

It could be a simple matter of bad gas, fouled plugs, deteriorated plug wire(s), or several other issues that can't be determined without doing some of the basic diagnostics listed above. The coils would be my very last guess based on what is known thus far.

 

I had a severe misfire on my Silverado and it ended up being a cracked spark plug. A good scanner should tell you what cylinders are having an issue.

 

This is a garage kept car with 9000+ miles. 2002 WS6. I take it out for short drives.

Misfires happen: All of the above.

He did say it was likely bad gas, and I do have a can of Liqui-Moly, but how do I run it through if the car shouldn’t be driven? And I have a new fuel filter ordered.

He scanned it once before, and #3 was identified, and we changed the coil pack. It was okay for one drive, but the next time I took it out, it happened again to a different cyl. Changed that one & it still misfires.

 

At 9k miles, those coils probably have another 30-50 years of life in them.

So what do you do for fuel maintenance? Mine is also a rarely driven car, usually no more than 1 tank of gas per year. I manage this by adding Stabil and Red Line SI-1 to every tank of fuel to keep it fresh and keep the injectors clean - this has prevented any problems with mine for over 20 years.

How old is the fuel currently in your tank? And how full is the tank? If the fuel is old and untreated, you can siphon out as much as possible (or pump it out with the fuel filter removed) if the tank is mostly full, then add a fuel system cleaner (such as Red Line or Techron) as well as several gallons of fresh fuel. But it's possible that the injectors might need additional bench cleaning if they have become severely dirty.

In any event, I don't think the problem has anything to do with the coils based on what you've posted. This is most likely a fuel problem based on mileage and limited use (especially if you don't keep the fuel system strictly maintained for said limited use). Spark plugs might also be fouled, and/or a plug wire might have been damaged or poorly seated when those two coils were changed.

 

That’s something I’ve had to learn the hard way. I haven’t done any fuel maintenance. I really try not to let the car sit too long, but between work & weather, it's not as much as I'd like.

There is about half a tank in there. It’s probably 4-6 months old, because that’s when I started having the problem.

I changed the coils myself, because they were in the front and easy to access. I don’t think it’s those two plug wires, but I’ll have them checked.

So, jumping ahead to draining the gas, putting in some fresh, and the cleaner, do I just start the car and let it run for a while? Is it safe to drive, or just let it idle for x amount of time?

 

It would/will take some time for the cleaner to do it's thing, so yes you'll have to drive/run the engine a bit before it would get any better. Just don't drive it hard or at high rpm. I would use a concentrated batch of whatever cleaner you pick (Red Line & Techron are both good choices, as well as Seafoam), so I wouldn't fill the tank more than half way at most - probably less than that even, until/unless it starts to run better.

But if the injectors are especially fouled, they may need to be removed and bench cleaned. Before doing that, I'd want to look at scanner data for fuel trim information and also to see when and where the misfires are happening.

Before doing any of this though, you'll want to check those plug wires just to make sure it's not something super simple.

If fresh gas and a good fuel system cleaner makes it better but not perfect, you might need fresh spark plugs and/or still need to bench clean the injectors for optimal operation.

Going forward, regarding the fuel system, here is what I do with mine: Every tank of gas gets Stabil with a ratio of 1 ounce per 2 gallons of fuel, and Red Line SI-1 with a ratio of 1 ounce per 4-5 gallons of fuel. These products are safe if used in stronger concentrations, but those are the minimum ratios that I use for routine fill-ups. I've owned this car since it was almost new, and it hasn't seen more than 1 (or rarely 2) tank(s) of gas per year for the last 22 years. Zero issues with the fuel or ignition system this way.