Hey everyone, just got back from the dealer for some fixes & one of them was the infamous cold start sputter.

Here's what they told me--use 89 octane instead of 91. Why? There's less ethanol content.

From a chemical engineering perspective (my background) I could see how that might be the case--ethanol creates less vapor pressure cold & is a water carrier.

Of course this brings up a whole host of questions, I don't think the ethanol is any less 'cold' whether the engine is warm or not.

Wouldn't it be a constant probelm during normal running, vs just cold start-up? I guess the ECU maps differently when cold...

And, is the ethanol content that much different in 89 that 91? Well, gas is different from state-to-state & region-to-region, so no way to nail that down.

Lastly, in a JCW engine that makes 133 hp/L, wouldn't the car de-tune itself to work with the lower octane, lowering the output hp? I know that's the case w/ super high compression turbo & sc motors, but does the JCW engine fall into that category?

I know this is a widely discussed topic, so let the comments begin!

Thanks guys.


ZM

 

i had a very bad cold atart issue with a customers car and after a lot of work done to the car we decided to drain the gastank and put in 87 gas.(we had no other ideas) that cured the problem. so you may be in the right track!

 

I'm not sure why the ethanol content would be different. Ethanol, if added to gas at all is usually listed as a percentage. Such as E85 wihich is 85 percent ethanol. I get gas from two BP stations, one station lists up to 10 percent ethanol, the other has no ethanol at all. Either way I get the stumble. I have not tried lower octane however so who knows.

In any case, a small rev of the engine and the stumble is gone.

 

E85 (85% ethanol blend) only works in cars built to run ethanol fuel. Ethanol , up to 10% can be safely and effectively used as a component for standard gasoline to increase octane. Typically, due to the cost of ethanol, it is used only in higher octane blends (91-93 octane). Ethanol contains oxygen (petroleum distillates do not). Therefore a blend of gasoline containing 10% ethanol will cause the engine to run much leaner than straight gasoline blends, resulting in a lean misfire under certain mapping conditions.

 

Hey lhoboy--I see you also have a JCW. So, is there an HP loss w/ 89 b/c the engine has to 'detune' (if it does)?

What's the consensus on mpg loss w/ ethanol blends? 8-10%?


ZM

 

Seems like an odd explanation. 'Round here the blend is 10% regardless of the octane. What is probably changing is the mapping of the lower octane and has nothing to do with the ethanol content.
Some gas stations here don't have ethanol blends at all. All my cars run a little smoother, better, whatever on those fuels.
The good news is that your fix is working, I just don't buy into the ethanol theory too much.

 

I'm holding out on changing to 89. The sputter rarely happens for me & really doesn't bother me--just was curious about it.

I'm pretty sure all the octanes around here are 10% ethanol, as well.

I'd def change to 89 if someone had info that there's no performance hit from 91 in a JCW. How much would that perf hit be anyways??


ZM

 

JCW paperwork says very clearly (NOT LESS THAN 91 OCTANE). I certainly wouldn't go lower for fear of detonation. The timing adjustment on any modern vehicle has a limited range.

 

I tend to agree, but the dealer said this info was coming from MINIUSA. Who knows, though.


ZM

 

MPG loss will be proportional to the decrease in the heat of combustion of the blend. The heat of combustion of ethanol is about 80% of that for straight gasoline. Therefore a 10% ethanol blend should yield 98% as much as straight gasoline, and hence a 2% decline in mpg. This assumes of course, that the engine is able to optimally adjust its timing and a/f mixture to optimize for the leaner burning fuel.

 

Depends on where " 'round here" is. Some areas around the country are mandated by the U.S.EPA to sell only oygenated fuels (meaning 10% ethanol blends or MTBE blends).

 

The same place that couldn't fix a storage box rattle or know how to sell touch up paint.......

 

LHO, that's how I figured the mpg loss (after I posted the 8-10% figure that I'd heard).

My numbers show HcEt is 63% that of gas. Giving HcFuel of 96%--more in line w/ the 1-1.5 mpg loss I see with the 'winter gas' ethanol blends around here.

Either way, not that significant unless you're driving 25k+ miles a yr.

 

From what I understand of octane ratings, the higher the octane rating, the more combustion retardant that is in the mix (from a simplistic viewpoint). Therefore, with the lower octane, you would be better off at cold start, but once the engine is up to temp, you have the potential for knock and other early combustion problems on the MCS under boost - plenty of other threads around here arguing this issue, so I won't even start.

So, I guess, take your pick. Personally, I stay away from any fuel with an ethanol additive @92 octane, and haven't had any problems.

 

That's one way to explain it. Another is like LHO mentioned--higher octane fuels have higher oxygen content, keeping combustion temps lower.

Lower octane = less oxygen = higher temps = higher chance of pre-detonation.

I'd stay away from ethanol if I could, but I'm pretty sure everything here is blended 10%. I'll look around this weekend.

Hey LHO, I'm moving to DC in a month. Are there non-ethanol blends around there?


ZM

 

I heard this same thing from my service advisor about two years ago, tried the switch, saw no change, went back to 91. At this point, I am skeptical that there will ever be a fix for the cold start sputter. It is my opinion that the lower octane recommendation is a line somebody thought up to get customers to leave them alone for a while about the issue. It's a minor issue and I have learned to live with it.

 

It's been awhile since I was formulating gasoline so I wasn't certain about the relative heat of combustion for ethanol, and your calculations could be more correct. I can check my books when I get home tonight.

 

I think you are confusing octane with vapor pressure. Octane is a measure that indicates a fuel's propensity to autodetonate (without the presence of spark) under temperature and pressure. The higher the octane, the less likely to detonate. Detonation (an uncontrolled and damaging explosion) is totally different that combustion (a timed burn through the expansion cycle).

Vapor pressure will indicate how easily a fuel is ignited under various temoperatures. You want high vapor pressure in the winter to make cold weather starts easier, and low vapor pressure in the summer to avoid vapor locking in the fuel system. Hence, the refiners produce gasoline to different specs based on the season (it's the law as well).

Fuels can be blended for any combination of octane and vapor pressure.

 

Yeah, listen to LHO. I'm just an armchair fuel blender. He's the ex-professional.

 

To clarify, very high octane fuels don't need to have ethanol in them. (Example: 108 octane racing fuel - no ethanol). Ethanol just happens to be a high octane blend component that can be used to blend the higher octane fuels.

Your equation should read:
Lower octane + higher temps = higher chance of pre-detonation. (oxygen bearing components generally mean higher octane , but not always)

During the winter here in DC, the EPA mandates oxygenated fuel, so you're pretty much stuck. I'll keep an eye open for straight petro gas and will let you know if I see it. I'm not certain about the local labeling laws, so there may be no way of telling without doing a quick water test.

 

i think the root of the issue with 10% ethanol fuel is how it reacts with water in the holding tanks of the gas station.

If the gas station has become lax with flushing and cleaning their tanks it becomes a big problem. Gas station tanks let water in... it's just the nature of the beast. It's no problem with gas with no ethanol because it will not mix. When you add 10% ethanol to the fuel it will mix with water and therfore make it into your gas tank.

Technically gas stations are supposed to flush their tanks 1-2 times a year if they use 10% ethanol fuel at their own expense. It's pretty clear why they wouldn't.

i could be wrong but that's one of the bigger issue's with 10% ethanol fuel as i see it.

 

You are not wrong! 10% ethanol is easily miscible with gasoline if water remains out of the equation. However, any water that gets into the fuel will attach itself to the ethanol and when ethanol get saturated with about 4-5% water it is no longer miscible and separates out of the blend. That means that less than a half percent of water in the gasohol will cause a 95/5 ethanol/water blend equal to 10+% of the gasohol volume to sink to the bottom of the station tank where the gas is pulled from. Gasoline engines don't run on that.

In a straight gasoline tank, a couple percent of water contaminated will lay harmlessly at the bottom of a station tank , well below the draw tube.

A good station selling gasohol will flush its tanks with gasoline a couple times a year and will test the tank daily for water at the bottom. They use a white paste on the end of a stick that turns pink in the presence of water.

This is potentially a big problem where it rains alot and water is likely to get into fill pipes, or where the temperature fluctuates and can cause excessive condensation both at the station and in the vehicle fuel tanks.

Just don't let your tanks sit empty for too long.

 

so the answer to the question then is that no, switching to 89 octane won't cure cure the problem?

mine does this occassionally, and now that summer is here, seems to do it a little more frequently than before. to me it's more of an annoyance than a big problem, unless it's signifying some other problem, which doesn't seem to be the case from what i've read about this

 

Orrrrr.... you can live in a rural area which sells fuel with less or no ethanol in it. Lucky me!

Another thing to consider lho is that as far as my knowledge is, the ethanol is added to the gasoline in a rather inconsistant manner (added into the truck full of gasoline) which could lead to a bit of variance in your calculations vs real world mpg.

This said, the official advice is to use non-ethanol blended gas in your mini