Black Spark Plugs / Stalls in a few secs

[Diablo4]

Hi guys,
In the place where I live only 87 gasoline is available, and it’s a long way to go for Midgrade and Premium. The ambient temperature during this month varies from 23F to 51F. If it’s cold, in the mornings it happens that the car starts and stalls after a few seconds (second attempt is always good). Today it happened that when I started it from the remote key, the engine stalled in 4 minutes (antifreeze 123F).

This winter I have already completely replaced the spark plugs twice. They were always found black, but not as a result of oil consumption, they were not shiny.

I own this car for 3 years and only this year started my issue with black spark plugs . In January 2023 I pulled heads and cleaned combustion chamber and valves. Also I clean injectors every 3 month.

So I have a couple of questions,
Could my problems be caused by low-quality 87 ?
Can Marvel Mystery Oil or Octane Booster solve this problem?
What could be the cause of black spark plugs? MAF sensor is good, injectors are clean.

[redcoats1976]

izzat carbon? i dont think 87 octane is doing your engine any favors,especially if youve already had to pull the heads and clean the chambers.

[morepowerjoe]

Yeah I know it might get expensive but I would at least add an octane booster before you fill up each time because these cars weren't designed for 87 and I would change those plugs right after you use the booster and drive it for a few miles to get all that 87 out there. Now on which one to use because you don't want to go over 93. Something like this will get you to 91 with two bottles.

https://www.amazon.com/Rislone-4747-...ps%2C82&sr=8-8

[chevy pilot]

Runs very rich.How long do you drive every day ?

[Wizard1183]

Originally Posted by morepowerjoe

Yeah I know it might get expensive but I would at least add an octane booster before you fill up each time because these cars weren't designed for 87 and I would change those plugs right after you use the booster and drive it for a few miles to get all that 87 out there. Now on which one to use because you don't want to go over 93. Something like this will get you to 91 with two bottles.

https://www.amazon.com/Rislone-4747-...ps%2C82&sr=8-8

This. Octane booster is definitely needed. The engine isn’t designed for 87….

[FasNuf]

I'd start with checking compression and codes. Not all codes show up on the CEL. Running on a lower octane will cause the engine to knock under load which causes the ECM to lower the spark reducing power. If enough knock happens, damage occurs. It's possible the MAF sensor is dirty or having issues making the car think it's lean and adding more fuel.

[morepowerjoe]

Originally Posted by RoadRunner70

Thank you for reply and link !

Did you try Octane tablets ? I think use 2 tablets is much more cheaper than bottles ))

https://www.ebay.com/itm/175709333796
https://www.ebay.com/itm/114390590368

I haven't ever tried the tablets but I don't think thier any different.

[Diablo4]

Originally Posted by chevy pilot

Runs very rich.How long do you drive every day ?

about 30 miles on highway. Yes runs very rich and I can't find the reason

[Diablo4]

Originally Posted by FasNuf

I'd start with checking compression and codes. Not all codes show up on the CEL. Running on a lower octane will cause the engine to knock under load which causes the ECM to lower the spark reducing power. If enough knock happens, damage occurs. It's possible the MAF sensor is dirty or having issues making the car think it's lean and adding more fuel.

No, MAF is good, fuel trims are excellent, a little lean at idle.
What are the reference compression readings for L99 ? I searched everywhere and can't find anything. I checked compression before this issue 1.5 years ago. I found from 116psi to 203psi (except cylinders 6 and 8 because of bad access).

[Diablo4]

Originally Posted by Wizard1183

This. Octane booster is definitely needed. The engine isn’t designed for 87….

Did you try Octane Booster tablets (as suggested by other member)?

https://www.ebay.com/itm/175709333796
https://www.ebay.com/itm/114390590368

[FasNuf]

Originally Posted by Diablo4

No, MAF is good, fuel trims are excellent, a little lean at idle.
What are the reference compression readings for L99 ? I searched everywhere and can't find anything. I checked compression before this issue 1.5 years ago. I found from 116psi to 203psi (except cylinders 6 and 8 because of bad access).

There is no cylinder pressure spec in the SM that I see, just compression ratio.
L99 is 10.41 compression on a stock engine so I assume yours is.

Make sure all the plugs are out and the TB is wide open to allow air into the engine.
Disable the ignition system.
4 compression strokes are needed for a good test.
Minimum compression shouldnt be less than 70% of the highest reading.

[FasNuf]

I've seen that the atmospheric pressure x compression ratio is a good rule of thumb. so if youre around sea level that puts you around 150psi. 14.7x10.4=154.
The book did use 150psi as an example,, but not a hard spec.

[FasNuf]

It's just weird to me that the car stalls after warming up a little. I doubt its a compression problem,, but you said the LTFT's were good? What were they? It almost sounds like the car doesnt like going into closed loop.

[chevy pilot]

What is sparkpark plug name and type ?