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RE: [T3] Blow by

On 1 Nov 2001, at 10:53, jesse wrote:

> As far as the hard hot starting goes, have you checked that your 
> check valve is maintaining fuel pressure after the car is turned off? 
> Maybe you are getting vapor lock due to lack of residual fuel 
> pressure.

While this is a vapor/boiling problem, so it's similar, vapor lock is really an 
entirely different animal.

Vapor lock happens on the suction side of a pump, when the pumped fluid 
gets hot so that the suction on it causes it to boil rather than get sucked into 
the pump. It is (was) common on older cars with the pumps on the engine 
mounted ABOVE the gas tank. Mounted this way, the pump had to suck 
gas uphill so that the pressure on the inlet side of the pump was always 
slightly less than atmospheric.This reduces the boiling point of the gas in 
that region. With hot underhood temps, the gas there would boil and the 
pump would only draw vapor, which was not enough to run the engine.

On our, and all modern FI cars, the pump is located under (or in) the gas 
tank. The pump inlet pressure is always slightly positive, and the ambient 
temp is generally cool. The only way these pumps can suffer from vapor lock 
is if the screen or filter preceeding the pump is blocked, so in general this is 
not a problem that we ever have.

Jim Adney
Madison, WI 53711-3054

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