Fuel degradation Testing?

[andrewh]

Just back from an Autumn run in the 3.8 which was superb. However, I did get a misfire under load. Now I felt this could be stale fuel. In fact it smelt stale on the exhaust fumes. I reckon its not run for about 7 weeks, thats all but probably not done much mileage recently before that . I know fuel goes off very very quickly now, but is there any easy test to check fuel quality. There really should be no other reason for a misfire to have emerged whilst the car has been stood. Of course I will check all the usuals but its the fuel I am concerned with and whether there is a test. Presumably Octane degrades without the stabilisers we used to see?

[cactusman]

Fuel won't degrade in 7 weeks if in a tank...sorry but it won't.. Mis fire under load is usually (but not always) caused by weak spark at one or other plug so I would be looking at loose HT wires, dirty distributor cap, failing rotor arm. Failing plug or dodgy resistor cap......Failing that a carburation issue either due to a carburettor issue e.g. sunk float etc...or fuel feed/starvation issue. Clogged filter, weak pump etc etc...only if the fuel has been there for many months will it have deteriorated to any degree and if it has gone bad the engine will run badly at most throttle openings....

[andrewh]

thanks for the reply. However, the fuel in the car may have been in there much longer than 7 weeks. I know all the other issues, so don't want to get involved in all that. I am interested in fuel . Every time I start a car after a lay up, they all misfire and smell the same, a sweet sickly smell that is the result of the fuel going off. I also have heard that if you have stale fuel in a tank, it will turn the fresh fuel stale more quickly. An Aston Martin specialist I know says that 9 out of 10 cars coming into them with poor running or no running is fuel going off, and its getting worse apparently. They also said, you should store your car with as little fuel in it as possible for the above reasons. Hence, I wanted to try some experiments with fuel in my cars, and stabilisers or preservers to see whether they help. If I can find an easy way to test fuel, it will be a starting point for me.

[cactusman]

The only way to know if the fuel has gone off would be to have it analysed and that would mean taking a sample for (probably) chromatographic analysis to determine the constituents and then comparing with the original specification. This would be very costly. Just smelling it will not really tell you a lot.....Fuel is a mix of iso octane and other C8 hydrocarbons (mostly)....the base fuel. To this is added various detergents etc plus, for std unleaded, ethanol. So if you have std fuel it will contain 5% ethanol. Ethanol absorbs water so, in a vented tank, it will gradually absorb some water from the air... enough to cause corrosion but probably not enough to significantly affect its ability to work in an internal combustion engine. Ethanol is also known to corrode some metal components and more particularly to deteriorate rubber fuel lines, seals etc. Materials leached from the fuel lines etc can gum up carburettors and paper fuel filters will eventually fall apart. Iso octane itself is very stable and will last year's in bottle.
I had cause to attend a friend who had a son who owned an MGB back in the summer. I was told that the MG had sat in the drive for three years due to various family issues. He now wanted to get it started and MOT ed for sale. Certainly it looked as though it had been there for three years judging by the moss on the drive. So I stripped the carbs. Cleaned crud from the float chambers etc. Reset the jets. New rotor arm. Cleaned the plugs....

Then we connected a fresh battery and ran the fuel pump...worked immediately...and allowed a pint of fuel to flush through the pipe. Reconnected the fuel line to the carbs. Filled the floats and checked for leaks....none. Cranked with no plugs for a bit to get oil around again. Fitted the plugs and she fired immediately....and the fuel was allegedly three years old....she ran like a dream...

So my suspicion is that most old fuel issues are really just fuel in the pipe and carbs that has deteriorated rubber etc that then bungs things up and causes poor running. Clean the pipes and carbs first and replace old paper fuel filters and even fuel a year or two old will be fine....remember the xk was introduced in 1948 when I suspect fuel was far less consistent in terms of quality than it is now...all be it that it had no ethanol and contained tetraethy lead....probably. Our engines are really pretty agricultural really so unless the fuel really is ancient and full of crud I would, I am afraid, doubt it is the issue...especially if super unleaded which does not currently contain ethanol.

[andrewh]

Thanks Julian, and an interesting observation on the fuel issues. I had wondered whether there was a home kit fuel tester. Reference the Super Unleaded I was unaware that this did not contain Ethanol. An interesting and useful discovery. Sometimes in rural Kent Super is not available , so its quite possible that the last fill up contained the standard 5% ethanol fuel. I THINK, that most of the new fuel system items, such as SU rebuild kits from Burlen, are being manufactured with Ethanol proof materials, but of course that may be wishful thinking. I know that our local lawnmower service centre send out all their lawnmowers with a special fuel in the tank as they had so many serviced lawnmowers go home and sit for the winter and would not start in the Spring. Some vehicles ( MGB?) are less susceptible to poor fuel than others. I have a number of WW2 Military Vehicles, including a Jeep, a Daimler Scout Car and a Bren Carrier. All are low compression engines and designed to run on anything that became available . So perhaps compression ratio has much to do with poor running on degraded fuel. I think I will leave that one to an Engine designer or Chemist though. Finally, back to the smell, stale fuel does smell before and after combustion, and experience tells me that when it smells stale the cars don't run anywhere near as well as they should.

[ralphr1780]

Andrew, microbial growth in fuel is a common threat which is often underestimated.
It is not limited to one specific type of fuel, but to all aviation-marine-automotive.
Wherever during storage or transport water and fuel come into contact, microbial contamination is likely to occur.
Bacteria growth takes place at the interface water-fuel, in the bottom of the tank, surviving in the water and nourished by the fuel.
At the start, aerobic microorganisms develop and grow consuming the oxygen available in the water. Once the oxygen is depleted, anaerobic colonies take over and these form hydrogen sulphide waste which is corrosive and smelly. Debris and sludge are formed, go to the bottom and get carried into the pumps/pipes/...
Refilling your tank, just dilutes temporarily the contamination and brings in new food.
You are entirely right, contaminated fuel smells different (foul) and burns differently too affecting the performance. Again, too often underestimated.
Pick up a fuel sample from your tank into a glass jar. If it is even so slightly hazy, you clearly have a contamination.

[andrewh]

Thank you Ralph, and again, great information . I hope others will find this useful as well. The fuel at present is clear and correct, so perhaps not the problem as you highlight at the moment. I can smell stale fuel a mile off and came to quite like it!. When I was young, the Jeep I now have was stored in my fathers shed covered with other bits and bobs. I used to creep in there and pretend that one day I would drive it...well now I do. That was over 55 years ago. But the smell of stale fuel always takes me right back there, they say smell is one of the main senses.