Fuel Characteristics and Vapour Locks

[Klaus Schlueter]

Hi all,
In a recent thread Chris (Vmad) wrote:"Vapour lock seems to be getting more common with modern volatile fuel; wether Ethanol contributes to this I am not really sure. What do you think Klaus?"

Well, here are my thoughts and experiences:
Yes Chris, I have the strong feeling that your assumption is correct and fuel characteristics have dramatically changed since some years --at least in Germany.

Since approx. three years I have problems with unreliability of my Stag concerning fuel management. The fifteen years before faults were always based on mechanical/electrical problems. But now it is different - let me try to explain:

When I run the Stag in early spring or autumn with moderate outside temperatures no problem at all. But when it gets warmer like now with 20 to 25 C the following happens
- start of the cold engine: perfect
- driving let's say downtown Munich and parking for ten to fifteen minutes with the warm/hot engine the next start is again perfect. But then it is not possible to spontaneously accelerate. She runs with constant speed (slow) for around 100 metres and then, all of a sudden acceleration works again without problems.
(BTW: I have perfectly refurbished my Strombergs with really everything thinkable).
So I decided to no longer play around with float and vent valves and complain but try to find the reason. And I am meanwhile convinced that it can well be the fuel.

At first I measured the temperature of the fuel hoses in the warm/hot engine area (ambient temperature approx. 23 C):
- At the filter ca. 50 C
- above the LH rocker cover ca. 65 C and after 10 min. (engine switched off) 70 C
- fuel inlet port of the LH carburettor ca. 70 C

Secondly I intalled a pressure gauge in the fuel line in front of the filter (see pics. ) which is now located below the dashboard. Very interesting:
- my Hardi fuel pump ( No. 14412, very reliable) feeds between 0,22 and 0,3 bar.
- stopping the warm/hot engine the pressure stays at around 0,25 and 0,3 bar
- and now: watching the gauge during the next minutes I can see the needle slowly moving up and the maximum pressure I measured was 0,52 bar
That means the standard float needle valves are really closed to hold 0,5 bar.
Question: Is the pressure increase based on vaporisation of the fuel in the hot hoses or carbs?

Then I studied the literature - interesting: In former times the fuel was called in Germany "Vergaserkraftstoff" - translated "carburettor fuel" - with vaporisation starting at around 100 C. As all vehicles nowadays have fuel injection working with higher pressure the vaporisation temperature was not important and dramatically reduced. The first fractions start now to vaporize at approx. 60 to 70 C. Is that due to cheaper fuel production?

Verdict: At the moment I am convinced that - given the engine management (fuel/electric) is in first class condition - most of our Stag problems with starting or running hot is related to the above mentioned facts. If all this is correct what can we do? My thoughts:
- get rid of the increasing pressure when the engine is stopped. I am working on that and will report my experiences during the summer
- get temperatures above the engine down (louvres, insulated fuel hoses,running the electric fan for a while when the engine has been switched off or what else?)
- try whether a "Gasblasen Abscheider" works sufficiently (I do not know the correct word, somethíng like "gas bubble separator" - Drew please tell us the correct terminus)
- find out more technical specs about vaporisation of E0, E5, E10, Superplus etc.
- Do other carbs have the same problems? (Weber, SU, Holley etc.) and if not why not?
- is fuel injection or TBI the only solution?
- Are there differences from country to country?
- I am very interested to hear your thoughts and comments and of course ideas to overcome these problems. Perhaps I am on a totally wrong track - please let me know.

Klaus


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[jpyke]

Hi Klaus,

I agree totally with your findings. My experience is like yours - a little worse I think because I dont have an engine driven fan (only an electric one) so I have even more problems with the carburettors getting too hot. I have recently changed the Strombergs to a Weber and although it is better it can still be a problem to start when really hot and the car has been left for a while, so I don't think changing the carb is the solution. Here are a few ideas:

1. Get a suitable pressure relief valve with a return line to the fuel tank. This would prevent the pressure soaring when the engine is switched off hot.
2. Fit a 'T' piece in the fuel hose with an elecric solenoid in it that when off, connects the fuel hose back to the tank. This would mean that the fuel system was de-pressurised whenever the engine was switched off.

[dasadrew]

some more reading:

http://en.wikipedia.org/wiki/Vapor_lock

[V Mad]

The Weber 38DGMS has a return union which we normally block off; it would be worth someone connecting this to a tank return pipe to see if that helps.

[Giles]

Do not know if the SU pump likes running all the time, but on the SU carbs on my old off road racer on a Rover V8 there was a return for the fuel, it just used a connector after the feed to the carbs with a much smaller hole in it to keep enough pressure in the fuel line to feed the carbs but also allow the fuel to circulate. I assume it came off a Range Rover. Never had any vapourisation problems but it was a few years ago now so maybe the fuel was better? but under bonnet temps were very high at times.

On my Stag I used to have hot start problems in hot weather but fitting a electric fan in addition to the mechanical fan has cured it. It only comes on when temps are high but is wired with a permanent feed and on a hot day will come on a couple of times after the engine has been turned off.

[Peter]

Klaus,

Fantastic information but have you brought the fuel line into the car to supply the guage? This could be very dangerous especially in an accident... If you want to read fuel pressure in the car an electronic remote sensor feeding a guage would be the only safe way to go (I think they make them for aircraft). Do you get much pressure variation with RPM when driving? The SU pump is designed to run to a pressure loading, you would probably need a pump from a vehicle with a fuel return to use a return pipe, otherwise you could be changing fuel pumps fairly often.

Sorry!

Peter

[Lingen]

Flooding

Klaus. You have done some interesting research here to identify the rising fuel pressure after engine shut down, and I like the idea of the fuel bypass as a possible cure, as it would be so simple to fit to a Stag. Why not try fitting a tee piece into the plastic delivery pipe from the fuel pump, just next to the fuel tank. This could then be connected to that spare pipe on the top of the tank that is there just for the purpose, as it took the overspill fuel on Triumph's fuel injected engines. All it would need would be some form of restriciton just to allow the smallest amount of fuel to pass through. This would quickly release all the pressure as soon as the engine was turned off, but not over-work the pump when in use. Incidentally, I see there is
E85 petrol available on forecourts in Sweden. What on earth will run on that?

[Klaus Schlueter]

Hi
Thanks for your replies. I will update new findings next week when I am back from the ESM in Cologne.

Klaus

[john lever]

I have a Stag in France and have been experiencing similar problems for 2-3 years: The car runs perfectly in cooler temperatures but when summer arrives (and temperatures of 25C and above), I have a problem after the car has thoroughly warmed up - half an hour. The car runs very roughly and will not pull away and in traffic jams it is a nightmare. I have a colourtune which indicates that the fuel mix is very slightly weak, but the carburettors will not adjust any further. I assume it is running slightly hot due to the weakness. I have a visco fan and an electric fan fitted but to no avail. The temperature gauge reads about half way so it is not running over-hot. I have come to the conclusion that it is petrol vapourisation and have tried insulating the petrol supply line but this has not solved the problem. So I look forward to any solution to this problem.

[martin]

Hiya Klaus, good research sir - but - and forgive me for being so blunt - Peter has it right, running a fuel line into the car is BLOODY DANGEROUS ! ! Martin.

[Klaus Schlueter]

John
just for a short side info: could you please describe the fuel/ethanol/E10 - situation in France? Do you still get fuel w/o ethanol and Super Plus with 98 Octane?
I asked the German ADAC about the situatio in Italy (as we will spend two weeks Stag holidays there). They contacted their colleagues in S. Tirol with the answer that they even haven't officially heard about it!

Klaus

[Bandit1200]

Hi Klaus

Thi9s week I have been experiencing the same symptoms you described in your original thread almost exactly 1 year ago.
Did you ever find a solution?
Best wishes
Steve

[jbuckl]

I have tried a fuel return including a restrictor (1/8" bore). It did work (filter was full for more time)

Currently not fitted though.

Try fitting a rubber fuel hose from front to rear, away from the exhaust silencer, & or a heat shield to the exhaust silencer, as per original.

[Klaus Schlueter]

Hi Klaus

Thi9s week I have been experiencing the same symptoms you described in your original thread almost exactly 1 year ago.
Did you ever find a solution?
Best wishes
Steve

Drossel am Tank - Kopie.JPGDrossel 0,5 mm.JPGFlow sheet.JPG

Hi Steve,
interesting that you digged out this old thread. I stopped posting as I didn't get the expected response to all my questions and thoughts and I thought that this topic wasn't interesting enough for the forum.
Back to your question: did I find a solution? Yes--at least the problem has disapeared. I did 3 things:

1) I picked up Lingens idea (against flooding, not vapor!) of fitting a restriction in a permanent fuel bypas to the unused blanked pipe on top of the tank. First I started with a 1,5 mm restriction -- too large, not much fuel pressure, but enough to drive with 60 km/h). I changed to a 0.5 mm restriction and that really works fine since almost one year now. After stopping the engine the fuel pressure droppes to athmosphere within 5 to 10 sec--no way for floodding any more. There is still my fuel pessure gauge under the dash board (yes I know....) and with the installed ball valve I can always check if the float valves are still tight or not (see flow sheet). This gauge is worth gold!!! (for me). For your info: with tight standard float valves the pressure stays almost constant for many houres.

2) I have insulated all fuel hoses in the engine bay with black insulation foam from the solar water heating industrie. 12mm dia by 9 mm thickness, very flexible and it looks need.

3) This was/is a speciality of my Stag: I wanted to get rid of the periodically leakage of the plastic O-ring plug underneath the float chamber. So I followed Gilbert Rongè's idea (former Belgian club pres.) which he published years ago: cut a 22x1,5 mm thread into the orifice of the float bowl (core dia fits directly) and close it with a metal plug. The plug I choosed had a long hexagon part for the spanner which ended very close to the rocker cover. My speculation is/was that the heat from the rocker cover was directly trasmitted to the float chamber and caused the vapor of the fuel. I changed that to a shorter one so there is an appr. 10 mm gap now.

You see Steve, I did what engineers never should do: I changed 3 things at the same time. And now I dont know the reason for the solution. But I didn't have the time to go step by step---I wanted to drive. Anyhow, I learned a lot and there are many many more details which I found out during this way.

Hope this helps and is of interest for you.

Regards
Klaus

[KOY 23]

Hello Klaus.

Thank you for your research. I believe that relieving the pressure on the float valves is the only way to stop flooding. My Weber has a blanked off return port, so for this and future developments, I will fit a return pipe.

John.