Questions about gasoline.
April 26, 2006 10:47 AM   Subscribe

All about gasoline.

(Apologies in advance on the two-fer.)

1) This morning, mrs. grateful accidentally filled up the tank with diesel instead of regular gas. The car stopped running, and she had it towed back to the station. They emptied the tank and flushed the system. Are there any long-term deleterious effects that we need to be concerned about?

2) In discussing the previous event, my father-in-law insisted that I could adjust my car so that it would no longer require premium gas. What adjustments is he talking about, and would they harm the engine in any way?
posted by grateful to Travel & Transportation (26 answers total)
In discussing the previous event, my father-in-law insisted that I could adjust my car so that it would no longer require premium gas

I'm not sure what he meant by "adjust" your car without more context. Most cars don't need premium gas. It will help in answering your question if we know what kind of car you drive.
posted by anastasiav at 10:50 AM on April 26, 2006

Response by poster: I drive a 97 Buick Park Avenue Ultra - not my first choice by any stretch, but it was a free hand-me-down from a very generous relative.
posted by grateful at 10:51 AM on April 26, 2006

The Straight Dope about Regular vs. Premium Gas

"The majority of cars are designed to run on regular gas, and that's what the manuals tell the owners to use. Higher-performance cars often require midgrade or premium gas because their engines are designed for higher compression (higher compression = more power), and regular gas may cause knock. If your car needs high-octane gas, the manual will say so."
posted by anastasiav at 10:52 AM on April 26, 2006

This morning, mrs. grateful accidentally filled up the tank with diesel instead of regular gas.

Not to derail, but how does that happen at all? I thought the nozzles were two different sizes precisely to prevent mistakes like these...?
posted by frogan at 10:54 AM on April 26, 2006

If you start using regular gas, your high compression engine will likely start being a lot less efficient and knocking, leading to using more gas. You don't get something for nothing..
posted by kcm at 10:57 AM on April 26, 2006

I don't believe diesel fuel will damage your vehicle (though a car designed for gasoline will not run, at all, on diesel). Once that purging is done and the car starts again, you should be fine.

I guess you've got the version with a supercharger that wants premium unleaded. I have no idea what your father-in-law is talking about - there's no such "adjustment". On the other hand, you could simply try regular unleaded. The car will run with regular unleaded, but if you hear knocking or pinging noises from the engine, you'll have to go back to premium unleaded. (Constant knocking or pinging, in addition to being annoying to the driver, can cause damage to the engine.)
posted by jellicle at 11:01 AM on April 26, 2006

frogan: one is bigger than the other, so you can put diesel in a petrol car, but not petrol in a diesel car.
posted by bonaldi at 11:06 AM on April 26, 2006

Regarding (2):

Look at your owner's manual. It will tell you how the engine responds to regular. Some engines use their electronics to compensate, losing some HP and mileage but avoiding pinging etc. Your book of words should tell you whether your Buick engine is like that or not.
posted by ROU_Xenophobe at 11:09 AM on April 26, 2006

A gas car won't run on diesel because the flash point is too high. Siphon the tank and fill it with gas. It's far worse the other way around. If you fill a diesel car with gas, it will run until the engine fries. If you run regular gas in a high compression vehicle that requires premium gas, the pre-ignition is very hard on your engine. It sounds like a light metallic tapping as you accelerate.
posted by weapons-grade pandemonium at 11:17 AM on April 26, 2006

Putting diesel in a petrol engine and draining it is usually less catastrophic than putting petrol in a diesel engine, as mentioned in this Google Answer, with beaucoup des links, in part because you break down much sooner: the fuel won't ignite.

There are alternative (wrong) opinions in what appears to be the canonical 'I put petrol in my diesel car' blog thread, and it's certainly worth checking the catalytic converter, but as long as you flush they system, it should be fine. (Diesel-in-petrol is much rarer because of nozzle sizes.)
posted by holgate at 11:18 AM on April 26, 2006

Response by poster: Is it possible to "reduce the compression", or is that hard-wired into my engine?
posted by grateful at 11:19 AM on April 26, 2006

frogan: one is bigger than the other, so you can put diesel in a petrol car, but not petrol in a diesel car.

Isn't it the other way around? Or is it one way in the UK, and the other in the US? That would explain the various petrol-in-diesel messups in past series of The Amazing Race.
posted by holgate at 11:19 AM on April 26, 2006

Putting petrol in a gas engine is catastrophic.
The engine gets this weird accent, then explodes.
posted by weapons-grade pandemonium at 11:22 AM on April 26, 2006

Most cars that require higher octane gas can use regular and the onboard control system will adjust the engine timing (and in the case of a supercharger, the boost) to compensate and reduce knocking. I wouldn't rely on these changes for more than an occasional tank of regular gas though.
posted by caddis at 11:23 AM on April 26, 2006

grateful, no, the compression ratio is inherent to the mechanical design of the engine. You could decrease it by swapping in shorter piston rods, but that would be quite a task.
posted by zsazsa at 11:23 AM on April 26, 2006

Running regular in an engine designed for premium, especially one with a supercharger, is a Very Bad Idea, even if it does have a knock sensor. The possibility of expensive engine damage is too high.

And why? Back when premium cost 20% more than regular, I can see the temptation. But with current gas prices, the percentage increase in the cost of high test over regular is at an all-time low. We're all getting screwed with our pants on, what's an extra stroke or two?

On preview: assuming your engine is supercharged, it might just be possible to reduce the amount of boost and lower your compression enough to run on regular, but it would take a lot of driving to save enough in fuel costs to make back the cost of the modifications. Leave it alone.
posted by mojohand at 11:25 AM on April 26, 2006

What model car is this that doesn't have an incompatibly-sized gashole?
posted by rxrfrx at 11:25 AM on April 26, 2006

Response by poster: The diesel/regular mixup occurred on a Toyota Sienna.
posted by grateful at 11:30 AM on April 26, 2006

grateful, your engine is supercharged. I would highly advise your continued use of premium fuel, if you like your car working.
posted by knave at 11:58 AM on April 26, 2006

The only "adjustment" I can figure is either reducing the boost from the supercharger, or eliminating it completely. Either method is going to reduce the performance of the car. It may be safe, but you have to determine what level of boost is safe on low octane gas, which may not be easy.
posted by knave at 12:01 PM on April 26, 2006

Even if it runs without knocking, you'll probably get decreased mileage if you use regular in an engine designed for premium. This will probably destroy any potential immediate savings.

Lower octane isn't as big of a problem in average winter weather, but in the summer, it's extra-important to use the correct fuel.

Theoretically speaking, you could swap out the ECU, and get one programmed to run your engine on regular... but the expense to do so would be absurd. It would probably have a 100,000 mile payback.

Your father-in-law probably thinks you can just adjust the timing a bit, and be fine, but this isn't particularly true anymore.
posted by I Love Tacos at 12:58 PM on April 26, 2006

In older cars, adjusting the timing to reduce spark knock was fairly simple. You could adjust it at the distributor by rotating the entire distributor, or by simply adjusting the mechanical or vacuum advance. Retarding the timing by a few degrees (the spark comes *later*) reduces spark knock. It also reduces engine power.

In newer cars with high energy electronic ignitions, this is difficult or impossible to adjust. I'd guess your 97 Buick falls into this category.
posted by ikkyu2 at 1:30 PM on April 26, 2006

Question 1: long term effects

Answer 1: none likely, but I would play it safe and, beyond the flush/empty, replace the fuel filter.

Question 2: reducing your car's need for higher-octane gas

Answer 2: if your car can use lower-octane gas, it'll say so in the manual. It is strongly advised that you do not run lower than this; the engine will retard the timing to use lower-octane gas within range, but may not be able to go below that limit, and pinging can damage the engine over time. Reducing the compression is a major undertaking; you'll essentially be rebuilding the engine, and your computer likely won't be able to deal with the new environment. If you want to save gas money, sell the car and buy a smaller one with the proceeds.
posted by davejay at 2:00 PM on April 26, 2006

Although it's been answered numerous times, i'll throw in my 2 cents here....

if you car is indded supercharged, then you MUST keep running high octane if you want it to last you. Higher octane fuels can take more pressure before they self ignite.

What happens if you use 87 octane, on a High compression or turbo/supercharged application is called detonation. As the piston is moving up and compressing the gas/air mixture, the lower octane gas is less stable, and the heat generated from the pressure is enough to ignite the mix before the spark plug does. So the piston is still moving up, and it self ignites wanting to push the piston down, but it can't becasue the momentum is forcing it up to complete the storke....VERY BAD....on hotter days this is exasperated becasue everything is just hotter to start with.

if you care at all about the car, and it is supercharged, stay with premium (91 is probably enough)
posted by TheDude at 4:02 PM on April 26, 2006

If you wanted, you could probably make your gas even more unstable by popping in a few of these :^)
posted by flabdablet at 4:59 PM on April 26, 2006

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