What is shelf life of unleaded petrol in fuel tank?

[Gerard124]

My MX5 has been stood all winter (probably since beginning of November) fuel tank is half full. Just took it for a spin and engine is juddery plus exhaust seems to be noisy - sorry this probably isnt a very good description!

Could this be the old fuel? should I top it up with new petrol, drive it till used?

 

[stevec67]

Stale fuel. Top it up with new stuff. The volatile portion will have evaporated.

Avoid this next winter by storing it nearly empty.

Noisy exhaust may well be that it was nearly rotten when you stored it and 4 months in the damp has finished the job.

 

[steve watkins]

Was it juddery, like only firing on three cylinders or an intermittent misfire?
It has been pretty damp for most of the winter. You may find that with so much moisture in the air, condensation and damp will be all through the vehicle even in places that appear sealed. It is worth giving it a good run for an hour or so to air everything. inside and out.
If you are handy with tools it is worth looking down the spark plug tubes first to see if the plugs and rubbers are damp or sat in water. The heat from the engine will eventually dry them out but it is much quicker if you clean, dry, check for any corrosion in the rubbers, spray with wd40, then go for a run.
If the exhaust is now noisy then it may well have suffered over the winter. Exhausts rot from the inside out and are filled with nasty deposits from the engine. When combined with water this can eat away metal very quickly.
It might also be worth either draining or replacing the fuel filter if the poor running persists as any water will have separated and collected here.

 

[mmterror]

At one point I was doing a lot of short journeys in my car and I was getting though a new mild-steel exhaust what seemed like every 6-8 months. I switched to a stainless steel system and that fixed it.

 

[amazement]

I took a look at our petrol lawm mowers this morning and have a small amount of petrol from last season in a jerry can, I suppose if I top it up with fresh petrol so mixing old with new it should be ok to use?

 

[Dublinia123]

If it were me I would drain to almost empty, drive to the nearest filling station & re-fill the tank - there's usually a drain bung on tanks.

I would have though that condensation will have added a lot of water to the fuel over the past 5 to six months.

If storing again I would either drain the tank and leave both the bung and the filler caps open to atmosphere or fill to the brim.

I know that diesel plant hire guys always re-fill the dumper / compressor / other equipment tanks last thing in the evening to avoid the condensation issue.

 

[demotivated]

It's only ~3 months. Fill it up and drive it around.

 

[stevec67]

I took a look at our petrol lawm mowers this morning and have a small amount of petrol from last season in a jerry can, I suppose if I top it up with fresh petrol so mixing old with new it should be ok to use?

I'd get the mower running on fresh stuff, having drained it off last year. Then any old stuff can be lost into the car (if there's no 2T oil in it of course) or mixed in the mower at 25% once the mower is running OK.

 

[stevec67]

If it were me I would drain to almost empty, drive to the nearest filling station & re-fill the tank - there's usually a drain bung on tanks.

Good luck finding this on a car. I've never seen one. I usually end up draining it by removing the fuel pipe and draining it with the fuel pump, when I have to. I'd avoid it if at all possible, it's messy and unpleasant.

 

[Gerard124]

Thanks for all the help, filled it up with the 97 stuff - given it a good run and engine seems to be running ok now, but lot of noise coming from exhaust area.

 

[Dublinia123]

Good luck finding this on a car. I've never seen one. I usually end up draining it by removing the fuel pipe and draining it with the fuel pump, when I have to. I'd avoid it if at all possible, it's messy and unpleasant.

There's actually a 14mm hex drain bolt in the bottom of the tank, you could just undo it and empty it out.

Just check out any of the MX5 forums for further details.



Please feel free to correct me......

when I'm wrong!

 

[davidsrsb]

Petrol contains very volatile components that will evapourate after a few months, reducing the octane rating.
Another issue is cylinder corrosion. If you are going to lay up a car, make sure the engine oil is fresh as old stuff has acids in it

 

[stevec67]

That's unusual. First time for everything, evidently. On European cars they omit them to stop naughty people stealing fuel.

 

[mudlark]

I had a car stood for two years. I was lucky it went fine. Even tyres and brake disks.

It's possible that a cylinder had an open valve which may have allowed damp in and some corrosion may have occurred. This may be checked to some extent by a compression test. A bore may be damaged or a valve sticking open.

In the old days, to put up a car for winter you would take out the spark plugs, put a small amount of oil in each cylinder and turn it on the starter briefly. Put the plugs back in. In spring take plugs out and turn on the starter to drive any oil out. Clean plugs and off you go.

This may not be appropriate for modern cars so check with owners club or expert. The extra oil about might bugger up the cat or sensor.

 

[JohnW]

My wife has a convertible that she only drives during the summer months - so its left parked up for half a year, no problem with the fuel - but we have had instances of blue smoke from the exhaust for the first 10 seconds or so after first start after sitting half the year - apparently the oil makes it way past the valve seals and collects on the spark plugs which burns off quickly - but if the engine is left idling (without the oil being burned off quickly) it can trip the check engine light due to cylinder misfire... which could damage the CAT / exhaust system - but the missfire would have to be very bad...

 

[Rasher]

I can't see why fuel should go stale in 3 months, but it just needs a blast.
The summers here are as wet as the winters, and the heater in an MX5 is leg-burning, so there is no reason to not use it over winter.
But then I'm biased and my MX5 is my daily car.

 

[tones]

Modern fuels, if stored in sealed containers and kept away from heat, have a shelf-life of at least 6 months. Up to 12 months is possible.

 

[stevec67]

That's true, but a car's petrol tank is not a sealed tank. It has a breather to allow air in as the fuel is consumed. If the car stands, small quantities of fuel evaporate through the breather. The bits that evaporate are the volatile portions.

 

[tones]

That's true, but a car's petrol tank is not a sealed tank. It has a breather to allow air in as the fuel is consumed. If the car stands, small quantities of fuel evaporate through the breather. The bits that evaporate are the volatile portions.

The whole of petrol is volatile, Steve, which is why the stuff works at all. True, it is a blend of C4-C12 hydrocarbons, which will have different evaporation rates, but not enormously different. I can't imagine a few months in a petrol tank making any sort of significant difference to performance.

 

[stevec67]

OK, I mean the *most* volatile portions, if we are going to be pedantic.

The evaporation rates are hugely different. Some blends contain butane, as we all know a gas at room temp. It will coevaporate with the other portions, there's going to be a partition coefficient, various isotropic mixtures, a whole chemistry lesson in there, but some C10-12s will hang around for ever. I saw in a magazine the result of a 2CV left in a barn for 40 years - some fuel was still present, though it had gone brown and oily. Now you're right that a few months in a shed isn't 40 years in a barn but it can and does affect the fuel, I know at first hand. As a kid it was always my job to mow the lawns, getting the mower running in spring was invariably a shit that took all afternoon and often a stripped carb. The fix was to run it until it stopped in October and leave it dry until March. Fresh fuel, a bit of 2T oil, brrrrmmmmm, off it went. If you forgot to do so, oh dear. It's the *most* volatile portions that get a cold engine running.