# Fuel tank pressure...



## WingedBeast1968 (Sep 18, 2009)

Posted in the tech section, but not many scan that far down the screen 

My car runs badly. Last night I removed the fuel filler cap and a lot of air rushed out and spat petrol at me.

Tonight, I left the car for an hour, took the cap off, and same thing without the petrol spitting.

Fuel tanks run a slight vacuum normally?

Cheers,
W


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## saucyboy (Nov 1, 2009)

Don't get the running rough bit mate but I've certainly had the woosh as pressure escapes upon opening the filler cap.


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## snuffy (Mar 26, 2014)

I think the owner's manual says you can expect some pressure so take care when removing the petrol cap. 

Mine has maybe a bit, but not so it belches petrol.


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## WingedBeast1968 (Sep 18, 2009)

Yeah, sucking air in is fine. Blowing fuel out the filler neck is not good.
I think the pressure is causing rich idle and cruise, and surge when changing down from 4th to 3rd on overrun.

Looking for some info, experience, help?
I'm thinking EVAP fault? But no MIL.


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## speedr33per (Apr 19, 2007)

leave the cap off or loose and drive it and see if the fault happens again quickest and easiest way to confirm or debunk it


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## vxrcymru (Sep 29, 2009)

I find that the harder the car has been driven the more vacum and even splutters of fuel come out when you open the fuel cap.

After hard driving i open it very slowly....


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## Grimblin Gibbon (Jul 16, 2009)

There have been cases of kinked or twisted filler pipes which have caused similar problems!

Do you have trouble filling the car with fuel, excessively long time to fill?


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## Papa Smurf (Sep 25, 2008)

I have experienced the same problem when doing track days and sprints. The lower the level of fuel the bigger the problem, and it will eventually cause the car to misfire or stall completely. 

Iain Litchfield recommends keeping the tank over 3/4 full when on track or driving enthusiastically, as the petrol tank is very close to the rear diff and exhaust, and that creates heat which can boil the petrol and give severe blow back when opened.

This may not be your problem but it certainly has been mine recently, and combined with fuel surge on long right hand corners, the problem becomes dangerous with the car cutting out and a possible spin.


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## WingedBeast1968 (Sep 18, 2009)

Thanks for all the input guys 

Filling up at the Shell garage is no problem. Fuel temperature in the data was 25 degrees. I'd just trundled home from work when it spat fuel at me.

EVAP systems are for preventing petrol vapour venting to atmosphere. If positive tank pressure is common with the GT-R, don't they all have a serious problem? The tank should run a small vaccuum? All my other cars have.

Given a pressurised fuel tank is a common experience, where is the pressure coming from? I don't mean a bit of contents expansion pressure, I mean the sort of pressure that rushes out the filler neck, with fuel vapour and droplets in it.

This should worry people, I think.

Cheers,
W

P.S. The MOT guy said it was a bit rich at tick over, and so did the emissions at fast idle. This has been a problem since 11k miles, and it's killing my motor.


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## Oski (Mar 8, 2014)

I was going to start a similar thread a while ago.
I noticed mine makes a noticeable hiss when i open the cap and a fuel smell hits you straight away, definitely pressure on mine?


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## WingedBeast1968 (Sep 18, 2009)

Yup, the hiss is supposed to be sucking not blowing. 

I'm contemplating getting it up in the air to see if anything is trapped or kinked or disconnected.

The service manual says the computer monitors the vaccuum drop after shut off, to determine the integrity of the EVAP system. To detect a leak. Says nothing about detecting enormous pressure build up. The valving is for allowing air in, and sometimes out under extreme circs.

I bet the time attack cars at the ring didn't have EVAP on them :chuckle:


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## barry P. (May 9, 2010)

nurburgringgtr said:


> I have experienced the same problem when doing track days and sprints. The lower the level of fuel the bigger the problem, and it will eventually cause the car to misfire or stall completely.
> 
> Iain Litchfield recommends keeping the tank over 3/4 full when on track or driving enthusiastically, as the petrol tank is very close to the rear diff and exhaust, and that creates heat which can boil the petrol and give severe blow back when opened.
> 
> This may not be your problem but it certainly has been mine recently, and combined with fuel surge on long right hand corners, the problem becomes dangerous with the car cutting out and a possible spin.


John, can you not get some additional heat mat stuck between the exhaust and fuel tank, that should solve the problem.


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## WingedBeast1968 (Sep 18, 2009)

speedr33per said:


> leave the cap off or loose and drive it and see if the fault happens again quickest and easiest way to confirm or debunk it


Good idea! Just been out for a drive with the cap loose.
All symptoms cured. Coasting down through the box is smooth and drama free, no clunks at all! Cause confirmed! Yes! I'm so happy I know what the problem is.

Now to fix the damn thing!


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## WingedBeast1968 (Sep 18, 2009)

To all those who said I was imagining it, or they just do that, you know who you are? You know nothing! :chuckle:


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## speedr33per (Apr 19, 2007)

WingedBeast1968 said:


> Good idea! Just been out for a drive with the cap loose.
> All symptoms cured. Coasting down through the box is smooth and drama free, no clunks at all! Cause confirmed! Yes! I'm so happy I know what the problem is.
> 
> Now to fix the damn thing!


quickest way is to use a small 1/8th part drill and assuming the cap is plastic drill a hole in the cap that way air can get in but fuel shouldnt escape


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## WingedBeast1968 (Sep 18, 2009)

Can't do that because the EVAP will detect the leak and put the MIL on. I'll find out which solenoid valve isn't doing its thing and replace it. :smokin:


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## ifty (Jul 27, 2005)

Keep us updated as I think quite a few of us have this issue including me


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## WingedBeast1968 (Sep 18, 2009)

Will do.

Having done some more reading about EVAP systems, the cannister vent valve is supposed to be open while the car is running, and only closes when parked up or when the leak test logic wants to do a test. Mine must be closed all the time, or the pipe work is blocked. More to follow.


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## Steve (Sep 21, 2001)

All cars "vent" air form the fuel tank when the pressure is released, never heard of fuel coming out to though!

As per standard Track or fast road driving, keep the tank full, especially with a GTR old or new, I have tracked my R33 for 16 years and never suffered from fuel surge and also track my 35 for 6 years and never needed fuel swirl pot. 

Just use your head and think what you are doing and how hard you are driving the car etc


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## WingedBeast1968 (Sep 18, 2009)

Took the purge valve off and tested okay. Not that then.
Took, sorry, tried to take the rear undertray off tonight. Somebody's little helper has done the bolts up so tight they're bending the standoff flanges instead of coming undone. Buy a new car, and have it destroyed by the very people who are supposed to keep it in tip top. Bloody fuming!

Where does it say "tighten undertray bolts to the point of yield, so nobody can get them undone, ever again.". F'ing B'stards!!

Anyway, still to check vent valve and cannister itself. This blockage is definitely the problem, as I'm driving the car with the filler cap loose. Running like new.

Still really upset.


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## barry P. (May 9, 2010)

WingedBeast1968 said:


> Took the purge valve off and tested okay. Not that then.
> Took, sorry, tried to take the rear undertray off tonight. Somebody's little helper has done the bolts up so tight they're bending the standoff flanges instead of coming undone. Buy a new car, and have it destroyed by the very people who are supposed to keep it in tip top. Bloody fuming!
> 
> Where does it say "tighten undertray bolts to the point of yield, so nobody can get them undone, ever again.". F'ing B'stards!!
> ...


If you haven't changed the bolts to stainless and covered the threads in copperslip they will have rusted in, nothing to do with how tight, it's a very common problem with the undertray bolts.


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## WingedBeast1968 (Sep 18, 2009)

View accepted. However, I used WD40, and each bolt let go with a massive CRACK! Believe me, they were in way too tight.

Wow. Type 2 cars (incl. euro) don't have a vent valve. Which would account for the few who smell petrol fumes when the car is hot hot hot. And therefore, those who don't (me) have a blocked charcoal cannister. So I guess we don't get any EVAP related DTCs either. Which is nice 

I'm guna get my drill!


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