Normally I enjoy being the office “car guy” because I can answer the questions people come to me with. Today I got stumped.
A co-worker has a newer VW Passat and a couple weeks ago he had trouble starting it on a cold morning. After the dealer looked at it he said he should run mid grade gas instead of 91 in the winter. The book calls for 91. The dealer gave no futher explaination.
Anyone ever heard of this or have a theory as to why? All I could come up with is maybe the dealer thinks so many less people buy 91 that it sits in the tanks longer, and has a higher chance of getting water mixed in through condensation. I think that’s stretching it though.
I have also heard NY runs boutique blend fuels so maybe they have something wrong with their winter blend. My guess is it was simple dealer BS because they couldn’t find anything wrong.
EDIT: Wasn’t sure if this was gen auto or tech. It could go either way depending on how technical the octane discussion gets.
I guess it could somehow make sense for starting it? But wouldnt you have to worry about pinging once its up to temp if the motor is designed to run on 91?
I dont have a vw, but maybe this will help. My trucks engine knocks on anything under 93 in the summer (when temps are above 65 anyways). In the winter I can go down to 87 with no knock. Someone posted a link once that had some info on air temperature and pinging…Maybe it wont be such a big deal in the cold weather
Well, the other thing to consider is that in winter the air is more dense, so the computer should compensate by adding more fuel, and you’ll get more HP. That doesn’t explain why it would start better with a lower octane though.
Lower octane fuel ignites easier than higher octane… The reason people have to run higher octane is to fight pre-ignition from running high compression or hotter temperatures. For this reason, I also tend to run lower octane in the winter. My car will ping on anything less than 93 in the summer, especially during the hottest weather. But in the winter 89 octane is fine.
10.0:1 compression means I run 93 octane all the time in my VR6, otherwise I have to worry about engine knocks, then the knock sensor picks it up, the ECU retards the timing, and power drops. I believe the power drop from running low octane in a VR6 has been dyno’d at something like 22whp.
To clarify, the dealer suggested using 89 octane instead of 91? If this is the case then the tech was probably thinking that the 91 wasn’t burning properly at this cold of a temperature, especially when the motor and the intake air is below freezing. Since 89 is less stable than 91, it would be more likely to burn completely than 91.
My opinion is that this isn’t a very likely theory, but I suppose it is possible, especially given the above responses that people can run lower octane in the winter than summer in their personal cars. I would guess that the dealer coulnd’t find anything wrong and this theory made the most sense so they went with it.
My guess: There was some moisture in the tank that got into the fuel line and froze up, choking off if not totally blocking fuel flow.
Or I could be totally wrong about the diagnosis. Just my theorizing.
The dealer is treating the symptom instead of the problem.
A healthy VW should start on any pump octane unless the temp is like 40 below.(period)
Edit: I ass-u-me the car is stock.
Haha you just managed to put it a bit more succinctly than I did.
What can I say? I’m a mechanical engineer. I get excited when I get to share some technical knowledge and start to ramble! :retard: I was debating launching into a lecture about what the 4 strokes of a 4 stroke motor are, and how detonation occurs when the air/fuel mix reaches ignition temperature due to heat of compression on stroke 2/4 and then the spark plug also ignites the mix and shockwaves from the 2 flame fronts…
I’d call that pre-ignition rather than detonation, but some people do interchange the two things. Since I tune engine management systems for a living I’m pretty anal about this stuff though. :lol:
So basically, in theory anyway, 89 might fire slightly better than 91, but in no way should he need to run 89 in winter in order to be sure his car will start. Just as I thought, more dealer BS. I’m so glad I do 99% of the work to my own cars.
EDIT: This was Autohaus btw. He’s also had trouble with some shield that has to be removed when they change the oil. I guess after it was done he found 4 bolts to it on the floor on the passengers side and took it back. Even after going back to complain they only bothered putting in 2 of the 4. This surprised me because I’ve heard they are THE VW place to go.
I suppose you would know a thing or two about that sort of stuff.
So what are the differences, if any, between pre-ignition, detonation, pinging, knock, and any other commonly used terms for what I have always thought are the same thing?
if schmitts is the dealership in question here, i have full confidence that they were full of crap when they said it. they just couldnt find a problem.
It’s recommended to use higher octanes during the summer when it’s hotter. Something like that… it’s in my car’s handbook but I don’t remember how it’s worded.
Is it not also true that we get shittier gas during the winter/cold months because of the additives needed to make your car run better in cold ass weather?
If so would buying the better grade gas actually be better off?
here is the short short version:
pre-igntion occurs when some/all of the mix combusts before the spark due to hot spots etc.
detonation occurs when you simply spark too soon. the piston gets forced back before it reaches TDC, and the only way it can do that is by reversing it’s rotation which puts a side load on the rod, shocks the crank, etc. etc.
pre-ignition is doubly bad because you can shock all those parts, plus the burn is uncontrolled since it starts wherever the hot spot(s) were and travels in an unpredictable path
I’d call that pre-ignition rather than detonation, but some people do interchange the two things. Since I tune engine management systems for a living I’m pretty anal about this stuff though. :lol: