Porsche 911UK Forum

Welcome to the @Porsche911UK website. Register a free account today to become a member! Sign up is quick and easy, then you can view, participate in topics and posts across the site that covers all things Porsche.

Already registered and looking to recovery your account, select 'login in' and then the 'forget your password' option.

Broken down, cut out. Fuel tank collapsed, bent sender

deMort said:
If the fuel cap had some sort of pressure release then it would be that .. but it doesnt and the fuel filler tube is sealed by the fuel cap .

Funny you should say that, when I was drilling the fuel cap a spring came out of it, (which I thought was strange) I wiggled the inner and outer of the cap and it does seem to be in two parts, although the movement was very minimal. :?:


deMort said:
Sometimes i do sit here and realise .. im telling you guys to do this and replace that .. i am in effect spending your money ..

I don't think anyone would worry about spending money on parts from your advice, the cost of any labour they would incur would soon outweigh that.
 
The gray valve you've dis assembled, that reveals a lot. In your previous posts you say this valve only flows in the direction of the arrow allowing air to flow from the tank to the canister but not letting anything flow in the other direction.

Looking at your pics it looks to me like there actually is a route for air to go in the opposite direction. I think this is what the two groups of four small holes are for. They allow a small flow in the other direction by lifting the small orange diaphragms. If you blow through the valve it may seem like it's only one way because in one direction (from tank to canister) it flows a lot quite easily. I bet if you blow in the other direction there will be a flow but only a small one, not much but enough to equalise the pressure in the tank as the fuel level slowly drops!!!

I think this is the answer to the mystery of how the pressure is equalised as the fuel level drops

Also looking at the design of the valve it will flow in the direction of the arrow when there's more pressure in the tank than in the canister allowing vapours the flow to the canister eg on a hot day when the fuel tank heats up.

What's not so obvious is that if there's a vacuum applied to the canister the valve will shut and prevent this vacuum from reaching the fuel tank even though that flow would be in the direction of the arrow too, so in this respect it behaves differently to a normal one way valve.

What do you reckon any of this make sense?!!

Mac
 
I think that is the missing piece of the jigsaw. The small holes must let air back the other way back to the tank. The 2 x 4 holes did have little diaphragms on them but I couldn't work that out at the time because they got damaged when I cut it apart but what you have said makes sense now.

So deMort is correct that this is the route the air must take. I will try to do a diagram later of how it flows. It must pull the air from the open ended pipe at the top of the Canister but as it has to go through the large foam filter and carbon particles it doesn't draw any air unless it does actually need it and the two bottom pipes that go from tank to engine have little restriction between them so that is the easiest direction of flow until the vacuum takes over.

Fingers crossed when the new valve/ pipe arrives and is fitted that it cures this. I'll still attach the vac gauge to see what is happening though.

That arrow has been a red herring all along, I don't see the point in that being there :x
 
I set my rig up that deMort suggested, using a vac/pressure gauge connected to an old fuel filler cap and sealed to act like a normal cap would by sealing the filler tube.

45838894955_2c15418c18_c.jpg


46754204291_704f65c165_c.jpg


I had the gauge taped to the windscreen so that I could see what was happening while driving.

I also set my Gopro up as I couldn't look 100% of the time while driving.

45838904865_e94ca98c0d_c.jpg


I covered 40 miles on all kinds of roads, motorway, dual carriageway and local roads. I also used around 10 litres of fuel so this would have made a decent space in the tank to have to fill with air again.

Unfortunately after all this set up, the gauge didn't move at all during the whole 40 mile journey. I think this is a good thing and hopefully suggests everything is working as it should?

I will monitor the fuel cap when opening it to see if air is sucked in at all, this will only show up on a long run though.
 
'the gauge did move at all"

should that be Didn't?

Sounds like good news after a challenging journey, congratulations.
 
Sorry for the delay .. not been very well this week so not been online since Monday ..

Just make sure the fuel cap / vacuum gauge is properly sealed .. other than that you seem to have no vacuum buildup and hence you dont have a fault anymore.
 
deMort said:
Sorry for the delay .. not been very well this week so not been online since Monday ..

Just make sure the fuel cap / vacuum gauge is properly sealed .. other than that you seem to have no vacuum buildup and hence you dont have a fault anymore.

No problem at all kind sir.

I made sure the seal on the cap was tight and the pipe to the gauge was too.

I did half expect to see the gauge move in either direction, sucking or blowing when the valve opened either way but it didn't budge so hopefully that is the end of it? Need to take it out for a long run at some point soon just to confirm.

Get well soon.
 
logically there should be no vacuum buildup hence the gauge would remain at zero...

Considering just how strong the fuel tank is then i would guess at a negative 5 bar or more to collapse one ..

I did a conversion to a vacuum for this .. it made no sense to me ! so i stuck with negative bar .. you know what i mean :)
 
I'm just coming back to this tread as I had a gauge issue that I think is cured now but when I was at it I found out some info that may be of use.

I noticed a while ago when I filled up the gauge was very slow to rise and I thought there may be a problem in the tank, it took about an hour to get to a true reading of what fuel was in the tank, the next time I filled up it was fine. It wasn't until I had a run over to Whitby that my fuel light came on with 30 miles range and and I filled up with fuel that the gauge didn't move upwards at all and just kept going down to zero, I knew I had around £60 worth of fuel in so I just drove home knowing I had enough.

I stripped the battery and battery tray off to get to the top of the tank and sender unit, I had thoughts of the tank being collapsed again but when I pulled everything out the tank was fine and I started to swap parts over from my other fuel senders, I used a cleaner actuator arm and refitted everything. Nothing was binding so I think it was just a bad reading. I refitted and have filled up twice since, both times getting a true reading.

Now this is the reason I am updating, I found the 3.6 cars do actually have a fuel filter and I also know the reason when the tank collapses the sender fails at the same time other than just the stand bending.

This is the fuel sender scale and after switching an old one over, as I thought this may be my gauge problem and after installing I found out it didn't work and then realised why, there was a hairline crack in the board and any time the tank collapses it'll damage this as its very brittle a little like ceramic.

yt1ceAj.jpg


When I removed another one it fell to pieces, it was already broken in it mounting. I refit the one that was already fitted.

2Mkc4lR.jpg


I also decided to change the fuel pressure regulator as I suspected it may be this causing some running issues, this didn't make any difference to the running but this does show you how to replace it if you ever need to.

eqtkdad.jpg


And I also found whilst stripping the sender unit down a fuel filter, I'm not sure if it available separately but this is good news if you are chasing a fuelling problem. This one looked fine and with the screen filter at the base of the pump I suspect they don't get dirty very often.

I can easily be got to in less than an hour.

oLxmu2C.jpg


I did eventually changed the whole float assembly as the one fitted that connects the sender scale was a little dirty and I think this did cure the dodgy gauge, or at least I hope it has?
 
In this type of application you can't have any more than 1bar pressure differential. Even if you pump a hard vacuum in the tank it is the external air pressure that collapses the structure - ie no more than 1 bar. So if the tank did collapse because of a pressure differential it is not as strong as you might think. 1bar is still a good deal of force when you look at the psi x area...
 
Can i just say .. neither i nor anyone else at work had any idea there was actually a filter inside of the unit .. i very much doubt its available as a seperate part due to not even us knowing there is one there but thank you ..

i've learnt something new today :)
 
The post was mainly for you as I knew you would be interested.

I assume it is available somewhere and I will update with a part as soon as I find a number or ID of some kind. The pump screen protects it to some extent but as with all filters they will get dirty and most pumps probably have a screen before they get to the main fuel filter so it may be also of use to others chasing a problem.
 
Thank you .. i have to say i've studied all the pictures in this thread ... we never split things apart these days i'm afraid .. its all replace the unit even though i could often fix things .. did it for many years pre Porsche days.

Bit of info re carbon canister ..

I changed one on a 986 the other week , the new canister is modified , it doesn't Have the top pipe , vent to atmosphere one .. it seems to have an inbuilt vent in the same place .. it also feels different inside .. when i tap it i can feel springs vibrating at the base .. some sort of internal valve i guess ..

Its also the standard 987 part .. they have just stopped making the 996 ones and use the 987 ones instead .. cost cutting .

The valve on the filler neck . it doesn't fully shut when the filler cap is removed , you can still blow through it but with a much greater resistance than when the fuel cap is fitted pressing on the switch , cant remember if you already said that though :)
 

Attachments

  • canister_1_789.jpg
    canister_1_789.jpg
    61.4 KB · Views: 2,604
  • canister_2_464.jpg
    canister_2_464.jpg
    118.8 KB · Views: 2,604
Well it seems this saga is not yet over, I was out enjoying the car around the Peaks on Sunday but when a mile from home the car conked out, I'd ran out of fuel with 80 mile range left. Got a lift to get fuel and it fired up and drove on.

I'll admit I did think the gauge was actually reading truer than it has for a while but it seems not, I was getting decent MPG similar to before the tank problems and the amount I put in seemed to show correctly on the gauge although I didn't make a note of exactly how much I put in.

I haven't taken the sender out again yet and I also was wondering if there was a calibration procedure for the C2 tanks, I know we have said not but I can't work out why the gauge is reading wrong still. The tank does seem to be venting fine and there are no problems with the tank misshaping again.

I have been really careful to make sure I haven't had anything bind the float when installing the sender and I checked the float assembly to make sure it is not damaged or bent by comparing it to the other two floats I have.

I'm thinking of using the tank capacity of 64L (Googled capacity but also saw 76L but I think incorrect) and putting in increments of 16L, 32L and 48L of fuel to measure the gauge is reading 1/4, 1/2 and 3/4 of a tank correctly but other than that I can't think of a more accurate test?
The petrol station is about 1/4 mile away, if I put 2 or 3 litre of fuel in it to get there then put in the required amount, obviously I need to drain the tank fully first.
 

New Threads

Forum statistics

Threads
124,350
Messages
1,439,432
Members
48,707
Latest member
race911turbo
Back
Top