Vacuum In Fuel tank Help.

Hello everyone.
I have owned my Micra K11 1.0 2001 for about 6months now. But i have noticed that every time i go to fill up, I loosen the fuel cap and a rush of air forces into the tank like there is a vacuum in the tank. Also i notice a throttle and idle is much smoother after i fill up. Not sure if this is related but ill have to test it later.

Anyone know if this is normal and if it is not how would one fix it?
 
OP
OP
Yaticus
Hello thanks for reply, How would i go about checking this?
I have moved the canister it down to accommodate the N15 Airbox so i hope i didnt not break anything.
 
Air pressure difference when tank is running low from a full fill is normal. Happens to almost every car.

Put it another way, this happens to both my unmodified K11 03 Micra, and N16 05 Almera. Also happened to my old P11 Primera (both 1.6 and 2.0), S14a 200sx and Note company car!
 
OP
OP
Yaticus
Hello thanks for the reply i really appreciate it.

I have been testing this for the past couple of days now but even with the tank full and a 20 min journey there is still heavy Vacuum when i release the cap. So what i have been doing is after every journey i have released the cap to test it and yes still a rush of air. Throttle response has been noticeably livelier and idle is smoother after doing this.

Also about a month ago i had the local garage take a look at it because it was very unstable at idle and lacking on throttle response. I thought it could be a the throttle body playing up but they said no. They also checked the emissions and said that it was below average and the car was running fuel to lean. Could this be related?

So if there is a blocked air inlet somewhere connected to the tank that allows the tank to equalise I would love to know how to unblock or fix it myself rather the garage.
 
I think the "release or air" is actually a positive pressure on the tank side rather than a vacuum. Petrol goes in as a liquid but at normal operating temperature can turn into a gas. As the fuel is bashed around the tank, it helps it vaporise and will also be heated as it passes through the fuel rail from the engine.

This is why its advised not to overfull your tank in summer, the heat increases the vaporisation of the fuel thus increases pressure. The filler cap also acts as a pressure release valve to avoid an over pressurisation (this can sometimes lead to people thinking they have a fuel leak when in fact it’s the fuel tank just venting!).

There are 4 pipes in a basic petrol tank.
1st is pumped to the engine under pressure. This also goes through a fuel filter.
2nd is the return feed (not all fuel pumped to the fuel rail is used, so there is a pressure valve (fuel pressure regulator) which keeps the fuel line pressurised at the correct pressure and releases excess back to the tank).
3rd is the fuel filler pipe
4th is a small vent pipe used for filling otherwise the pump would just click off all the time (although I think there is a fault with the Micra fuel filler pipe as it can be difficult at the best of times unless you hold it in the *perfect* position!)

Some cars have a more complex setup (such as swirl pots, multiple fuel lines, multi stage pumping.....) but I think that’s it for the Micra.

There could be a number of things that causes lean running from a sensor fault, iffy fuel pump, iffy fuel pressure regulator to even just being crap fuel. Personaly I would start with ensuring that all serviceable items are in good order before starting to dig deeper. Spark plugs, fuel filter (the one most people forget about!), oil change, there are no exhaust leaks, no vacuum leaks on any of the pipework (this can cause lean running/bad idle), sensors are working within accepable tollerences (some can be read using a simple multimeter, some may require a machine plugged into the OBD socket). You *MAY* be able to test the fuel pump by checking its resistance with a multimeter. I am not sure of the values or the how to so you may need to dig :)
Did the garage not suggest what the problem was? Do you have the results of the test to hand you could post. My knowlage is mostly academic and basic, somebody may be able to assist further in diagnosing any fault you may have.

I know it sounds like a dig and please do not take this the wrong way, but are you 100% sure it's not a placebo effect? Sometimes after doing some work to your car, you think it’s made a difference however it’s made feck all difference. Bit like fitting an "induction kit". In reality, the poor kits do not make much difference apart from removing silencers and make it noisier, this giving the placebo effect of it adding a million and one HP (*slight over exaggeration but I think you can see where I am going with this!) :)
 
OP
OP
Yaticus
Hello SplanK

Thanks for taking the time to write such a detailed reply. it has really given me a better understanding of the fuel tank and how it works. The garage said it was a faulty M.A.F sensor. i do not have the results with me but luckily i live next door to them. It was over a month ago so they might not have the results on them anymore.

As for the car service and general maintenance, service history is perfect and mostly maintained by nissan dealer it self. I changed the spark plugs when i first bought the car in february, the exhaust i have checked and is fine. i have still yet to check the other item that you listed but will do later.
I totally agree with you about the modifications aspect, Im not in to that stuff at all. What i enjoy more is Maintaing a great example of a stock car so they are 100% running to its full potential rather than any modding. I only changed the airbox because i came across it in a local scrappy and thought ill give it a go and see if it makes any difference.(still yet to decided if it has) in the end i couldn't be bothered to put the sock one back in. its only done 34k and is great little runner for what i do It. You really seems like you know your stuff and i wish you could inspect the car personally, but proberly ill have to systematically go though everything you have listed on my own.

Thanks for the advice and its is much appreciated.

If there is anyone else with the same problem please comment
 
A MAF (Mass Air Flow) sensor measures the air intake passing into the engine, and along with a number of other sensors and a base map, it can work as close as possible how much fuel to inject into the engine for a perfect burn depending on what you are doing.

The MAF sensor has a number of designs, but in the K11 Micra, it’s a small wire, not to dissimilar to that of an old style bulb which has a specific current pass through it to heat it. As the air passes over it, it’s cooled. The resistance of the wire (which changes with temperature) is used to calculate the amount of air entering into the engine.

The MAF sensor can sit in different places depending upon the car, but in the K11 Micra, it sits in the throttle body housing – with a torch and a wide open throttle you may be able to see it.

If this fails completely then the ECU will go into a limp mode and will go to a safe known values in the base map and read info from other sensors. It’s a limited map so the car will run like a bag of spanners, over fuel like no tomorrow but will allow you to either limp home, or to a garage. In my experience in having an iffy maf (no reading) which results in limp mode, the engine may not idle and stall.

Even though the air is filtered, a small amount of dirt always gets past no matter what and can contaminate the wire. This can affect the reading and may under-read the air entering the engine thus giving you a lean mix.
You can attempt to clean the wire, and there is many a topic on here about it and various ways of doing the job. If you are not that experienced with tinkering with such sensors and don’t want to blow £150 (well, that’s how much they were last time I looked years ago!) then it would be best to leave it in place, as it can be a pain to remove and they are also quite fragile. With the engine running with some revs spay in some electrical cleaner (you can buy it from Halfords) in short sharp bursts directly into the intake, but from around 20-30cm. This allows the engine to drag the cleaner into the engine rather than it being forced. The engine will bog down but should recover too much spray/not enough revs and the engine could stall. Don’t worry, just give it a minute and restart it.

I would say at the moment that the fuel tank pressure is a bit of a red herring at the moment, especially as your mechanic has suggested that your MAF maybe misreading and the symptoms do match that of a dirty maf (although, it could also match a whole host of other faults….one at a time though!).

A vacuum leak in a hose somewhere can also give similar fault readings as it allows unmetered air to enter the engine, so the engine does not put in enough fuel for the air entering, thus can give you the symptoms you are having so worth having a good look over for split hoses.
And lengthy reply’s – not a problem, just doing some work on servers at work but there is a lot of waiting for stuff to happen so thought I might as well make myself useful!

 
OP
OP
Yaticus
Hello SplanK

I will try the sensor cleaning thing ago as soon as i can, i have nothing to lose. I was think about buying a MAF of ebay for £30 and changing it my self, but i don't want to risk it. In terms of the split hoses, which hoses should i be checking? A.k.a where, what positions and what types?
 
there are a fair few hoses in the Micra, some are vac, but some do other jobs such as air con (if you have it), coolent or fuel. Have a good look at all of them, and if you are not sure what they are dont disconnect them. Pop up a picture on here if you are not sure! What you are looking for is wear of the hose, such as small/large splits, kinks, or rubbing. Most vac hose's are generaly fine for the life of the car but you never know its worth having a good look around.

MAF - Not sure if the ebay cheap ones are any good. but if you are handy with a spanner/screw driver then they are not difficult if you take your time. Maybe doing a quick search on here may assist.
 

pollyp

Club Member
not forgetting the 5th pipe that leads to the carbon canister is a fuel tank breather pipe.
the carbon canister is a sort of 2-way fume filter/capture device. btw the fuel filler cap is sealed.

when parked:
any expanded fuel vapour from the tank would be allowed to trickle freely out the tank to maintain equal pressure, down the breather pipe to the carbon canister where the active charcoal collects the vapours and the remaining gasses vent off.

when engines running:
now as you consume fuel the fluid level drops.
normally ambient outside air would be able to flow throught the carbon canister, through the breather pipe to the tank to replace the lost volume and maintain an equalised pressure and won't stress the pump or make the cap hiss.
whenever the engines warmed up n under inlet manifold vacuum such as closed throttle, the canister releases the trapped vapours into the inlet to be burnt.

I'd advise you check that the repositioned canister hasn't resulted in the canister-fuel tank breather pipe or canister-ambient air pipe being kinked/restricted. if the breather pipe was restricted or blocked off, it won't allow the tank to breath which creates +ve pressure from expanded fuel vapour during parking and high vacuum during operation which causes the cap to hiss when opened and the vacuum would actually be working against the fuel pressure and could lead to lean afr and hence affect performance.
 
OP
OP
Yaticus
Hello Pollyp Thanks for your reply,

Im a little relieved to read your post as its exactly what i was think the problem could be so thank you for your advice. I have been searching the interweb for about two weeks now and it does mostly point to this conclusion. When I moved the canister down by a fraction I do not remember any hoses being damaged. The engine bay itself is very tidy. But ill check it asap.
I have the coil pack micra so the canister looks a little different from older models. But could you tell me which hose is which? Looking at the canister from front on: Left/Middle/Right hoses. And how do I check them for blockage?
 

pollyp

Club Member
according to my canister:
1 top hose leads back down to the tank = fuel tank breather
1 bottom hose leads straight down to open air = canister breather

at top of canister is this diaphram looking valve feature.
1 thin vacuum actuation hose goes behind the TB = during closed throttle the vacuum pulls on the diaphram opening the canister valve.
1 thicker hose goes behind the inlet manifold (next to the brake booster feed) = when the diaphram opens the canister valve, the collected fumes get sucked out and through this thicker hose into the inlet to be burnt

look for hints of kinking from bending the hose too tightly
 
OP
OP
Yaticus
Thanks polyp, still trying to get my Noob brain to work out everything thats people are suggesting. Im going to take some pictures later in the week and post them up. Almost like a spot the problem game.

Hi R-REG-SR please explain?
 

r-reg-sr

-------
Site Supporter
dont know about the canister cause i aint fiddled,but the fuel system is pressurized
ive aways heard that vacume release noise since 2003
 
according to my canister:
1 top hose leads back down to the tank = fuel tank breather
1 bottom hose leads straight down to open air = canister breather

at top of canister is this diaphram looking valve feature.
1 thin vacuum actuation hose goes behind the TB = during closed throttle the vacuum pulls on the diaphram opening the canister valve.
1 thicker hose goes behind the inlet manifold (next to the brake booster feed) = when the diaphram opens the canister valve, the collected fumes get sucked out and through this thicker hose into the inlet to be burnt

look for hints of kinking from bending the hose too tightly
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