harveyp6 wrote,...
At the risk of upsetting a few people, I can't see how the vapourisation starts before the fuel reaches the pump, when on all the ones I've done you can see the petrol in the filter boiling off the top. Heat rises, and with no way of releasing that heat from underneath the bonnet, it eventually builds up at the top under the bonnet until it vapourises in the carbs, and in the horizontal pipe across the manifold. The only other high point is over the back of the block, and that doesn't have the same amount of heat coming off it, and where it runs along the side of the block it does get hot, but it does in normal conditions as well. If it was vapourising before the pump and the fuel level was low in the tank then you'd never get the pump to draw it up again, and I've never had that problem.
With the cooling system working to its maximum efficiency, then you can delay the point at which it happens to a point where the car would have already boiled up. As I've said before I've never failed to cure them without resorting to an electric pump, the only thing I would add is that all those were prior to the intro of Unleaded, so that may well change the outcome now, but in most cases, I doubt it.
No it won't, because once it vapourises in the float bowls, then the engine stops, and while cranking on the starter it just continues vapourising as fast as the pump pushes up fresh petrol. You need to cool the carbs, and then it will restart. Although if you were talking about an electric pump, then that will continue pumping regardless of the engine turning over, but it still needs to cool down enough to stop the vapourisation, but the electric pump just enables that point to come earlier.
Hello Harvey,
An interesting way of looking at it, but as I have mentioned on more than one prior occasion, my Rover experienced vaporisation on a cold Winter's day in 1991, temp gauge in the white, carburettor dashops stone cold. It came, it went and in the interim it effected many local Rovers. As I have maintained, the fuel was the problem, and it was leaded at the time.
Why do I believe that the fuel will vaporise before the mechanical pump,...the suction induced by the pump results in a reduction in pressure in the line between the pump inlet and the tank. Exactly how far the pressure drop exists, I cannot say having not measured it.
Here is a link to substantiate my reasoning.
http://www.engineeringtoolbox.com/npsh- ... d_634.html
I recently obtained a book titled “Performance Tuning in Theory and Practice” by A Graham Bell. The author at the time the book was first published in 1981 had 17 years of engine building experience, 14 of them spent tuning and modifying high performance and racing engines.
In the text the author writes,…”The main weakness of using a front mounted pump is that it aggravates vapour lock. A high performance engine requires a lot of fuel so the line from the pump back to the tank will always be subject to suction or a vacuum. This means that the fuel in the line before the pump is going to boil at a much lower temperature than in would at normal atmospheric pressure. The only way around this is to fit a rear-mounted electric pump”
Rover offered a fuel line insulation sheath made from woven asbestos to cover the metal fuel line from the pump back to where the line changes to plastic across the bell housing. The part number is KRC1068. I fitted one to my Rover in April 1990. If Rover did not believe that radiant heat in this area might be a problem, why would they offer such a product?
When my Rover was experiencing vaporisation problems, the fuel in the filter would bubble as you indicated, and when I released the top nut you could hear the pressure being released, then liquid would flow and I would retighten the nut, and away I would go until it happened again. Was this boiling fuel being expelled by the mechanical fuel pump?
A successful solution that I employed was to wrap the fuel filter with a piece of cloth, detach the windscreen washer tubing and run a short separate piece of tubing from the pump and affix it to the cloth, then soak the cloth periodically by pressing the switch. It worked, but I wanted a cure, not a stop gap measure. The fitment of an electric pump beneath the tank running in series with the mechanical pump was the cure for the problem fuel. As I have previously indicated, the fuel was fixed and the problem disappeared.
Ron.
