Stumbled upon the problem accidentally. Ourother car is an '06 Outback 3.0 that prefers 91 octane (which is hard to find so she gets the 93 treatment). As a result of being used to tapping the 93 button, I accidentally through 93 octane in my '97 Outback.
Started immediately having cold start issues after the car had sat for an extended period of time. The exact issue was that the motor just would not fire up. It would crank away for 20+ seconds while I pushed the gas pedal before she would finally stumble to life. The idle would be extremely rough for the first 45 seconds or so and then it would smooth out. The colder it was overnight or while I was at work, the harder the start would be. Above 40 degrees overnight, it would have no issue with the start. Coolant temp gauge showed a few ticks higher than it does when on 87 at full operating temps. If the engine was the slightest bit warm, it ran fine and would start up with no issues.
Next fill was back to 87 octane last night. This morning at 10 degrees air temperature, the car started right up after 2 seconds max cranking. No struggle, no missing, no stumbling to life. If the premium was in there, I question if it even would have started.
So my question is, without running out and running a compression test when it’s single digits with no garage, is this a sign of a bigger problem or is this somewhat reasonable to expect on an older, tired engine?
I admittedly don’t know a ton about the intricate mechanics of gasoline combustion but do know (and so does everyone else) that higher octane fuels are more resistant to ignition.
My speculation would be more towards a bad batch of gas, but it’s entirely possible that your motor’s compression is so low that it has trouble increasing the air-fuel mixture’s density to a point that ignition will occur. This is obviously a temperature dependent relationship, so it’s more likely to start once warm.
Since the “flash point” (unsure if that’s the correct term, I don’t think it is) of 87 is lower, it’s more likely to start because you’re on the ragged edge.
I always knew there was no advantage in running premium in a car that specified 87 but I had never heard of it causing problems like this.
I’m guessing Newman is on the right track. Your ignition system is on the edge of being not good enough and the slightly higher resistance of premium to detonate pushed it over that edge.
If it’s running fine on 87 again I’d probably just ignore it until it becomes an issue on 87.
This would be a problem in a diesel as it uses the compression and heating of the air/fuel mixture to self ignite. A spark based ignition system does not require the high compression ratios to compress and heat the fuel. This would cause knock in gasoline engines due to the higher volatility of gasoline vs. diesel. Fuel air density while dependent on temperature would not be affected by octane rating. Octane is the resistance to knock, in order for it to knock it would have to first ignite via the spark plug and then have multiple flame fronts due to other factors (hot spot, oil vapor, poor fuel quality, improper octane rating) which led to improper flame fronts colliding in the cylinder. I see what you’re saying about compression affecting the density of the air/fuel mixture approaching TDC but if this were the case he would have problems on all fuels all the time and I think you are thinking about this in terms of a diesel.
93 octane prices are high and 93 is not required as much as 87 octane. This tank of fuel is less likely to be emptied and refilled as much as the 87 octane.
So one of two things is most likely.
Due to the lack of refilling, the tank is more likely to have condensation pool up and you could have got a tank of fuel with some water in it.
Due to the lack of refilling, the tank could have had a summer blend of fuel. The reed vapor pressure is different between summer and winter blends and thus volatility is different.
good information given in this thread.Unlike mine since im having the same issue there telling me to say fuk my car and walk just beacause i dont use propar grammar.
Welcome to life. You get out what you put in, and when you ask for help with something that looks like it was written by a 12 year old texting don’t expect to get much back. Even more so when you come into other threads crying about how you got called out.
im gonna say that the octane is a bit too high for the car. probably not quite enough compression on the colder days. try a hotter plug and it may fire right up.
in the drag sled we have a remapped computer and high octane fuel, we have to run a hotter plug with that. doesnt like to run right with the stock plug. before we freshened up the top end it wouldnt even fire up unless we stuck a hotter plug in.
different application, but still similar.
i never understood why people would run a high octane in a car when not necessary. high mileage car = lower compression.
i agree with not running high octane when not required, but 9/10 times in a healthy high mileage motor the compression is higher than normal due to carbon build up on the pistons
run the lowest octane as possible for best gas mileage/start up
As for the other car running fine there are a few factors to consider. 1) the other car is newer, 2) other car has a different engine 3) I seriously doubt that the fuel mapping of your car, and the “newer car” are the same (basically comparing oranges and apples). As carnut put it, octane rating effects the fuel’s ability to resist detonation by anything else aside from the spark plug, not ignition. Im willing to lean towards bad gas. Just because the newer car ran fine on the 93 the day before doesnt mean that the tanks wasnt just filled with fresh gas the day before, and had finally had a chance to settle when you were filling up (this is why i always go to high volume gas stations).
As for the other car running fine there are a few factors to consider. 1) the other car is newer, 2) other car has a different engine 3) I seriously doubt that the fuel mapping of your car, and the “newer car” are the same (basically comparing oranges and apples). As carnut put it, octane rating effects the fuel’s ability to resist detonation by anything else aside from the spark plug, not ignition. Im willing to lean towards bad gas. Just because the newer car ran fine on the 93 the day before doesnt mean that the tanks wasnt just filled with fresh gas the day before, and had finally had a chance to settle when you were filling up (this is why i always go to high volume gas stations).