The Answer...

[Beav]

I found a TSB that explains a lot about the current rash of driveability problems. I had forgotten about this because I've been living in SW Missouri where this wasn't an issue (that and I was driving a diesel for the most part.)

This is kinda lengthy so hang in there. If you have any questions I can answer them, it's really simpler than it looks.

VOLATILITY RELATED DRIVEABILITY CONCERNS SERVICE TIP

ISSUE

Some vehicles using winter blend fuels may exhibit a stall on start up and a no restart, hard start condition or a no start condition in unseasonably hot weather (greater than 27C / 80F). These conditions may be related to fuel volatility from winter blend fuels during winter-spring and autumn-winter transitions and may appear to be a fuel pump failure but may be caused by winter blend fuel.

ACTION

Refer to the following text for further information.

NOTE: Fuel tank additives will not resolve these conditions because vapor forms from cavitation within the fuel pump. Filling the tank to full may be effective (I've always said they run better full than empty. -beav) Advise customer to use the recommended octane grade per owner manual and not a higher grade.

Explain to the customer that this concern may be due to a low tank level of winter blend fuel combined with unseasonably warm weather. Advise the customer that filling the fuel tank to full may be effective in resolving this concern. Also, advise customer to use the recommended octane grade per their Owner's Manual and not a higher grade.

BACKGROUND

Gasolines are seasonally adjusted, meaning that they have higher volatility (vaporize easier) in the winter and lower volatility in the summer. Government mandates to improve air quality have resulted in significant changes to gasolines, such as the mandatory use of oxygenates (etahnol, methyl tertiary butyl ether (MTBE), and others) in the winter or the reduction of Reid Vapor Pressure (RVP) in the summer. The addition of oxygenates (especially ethanol) increases volatility while the reduction of RVP reduces volatility.

Gasoline distribution practices often do not allow branded marketers to have much control over their gasoline's volatility other than RVP. Oxygenate (e.g., ethanol, MTBE) use is more dependant on local gasoline markets, rather than specific marketers practice. Therefore, it is difficult to recommend specific brands to avoid volatility related complaints.

(end)

What this all means is that when the weather warms up the increased volatility of winter blend gasoline allows the fuel pump to cavitate (I can explain cavitation if anyone is interested, basically it means the pump is spinning too fast for the fluid it's trying to displace) and the fuel pressure drops as a result. Low fuel pressure usually manifests itself similar to a vacuum leak - a lean condition. Low power, uneven idle and surging, et al.

The reason they advise to keep the tank full is the increased pressure (weight of the gasoline) at the pump impeller may be enough to thwart cavitation. Also the usage of recommendeed octane grade vs. higher grade is to avoid increasing the volatility (which is, afterall, the underlying problem here.)

Take heart, in a week or two most gas stations will have replaced their winter blends with summer blends and the problem(s) will disappear until fall or next spring. Some markets are driven by temperature, some by legislation. In Colorado I believe there's a certain date on the front range that separates the winter/summer blend seasons. Others may be similar.

[svx_commuter]

I have a friend that works at a refinery and they can't wait for winter to come. That is when they get rid of the all the "light ends" left over from the summer time. This would be butane and other similar gases from the breaking down of crude oil. These are extra light gases that can't be sold. The light gases can't be added in the summer because they vaporize to easily. the cold weather is needed to keep them in the tank longer.

Like you said, Beav, the fluid turns to vapor, cavitates, in the fuel pump. The fuel pumpt unloads and spins faster. Also not good for the fuel pump.

I had no idea that a TSB was written on this. I did not realize it could be a big problem. You may also want to think about WHERE you buy gas from. The cheap gas is cheap for a reason and usually starts out with the major gasoline refiners. However they would not sell it in there stations.