hi

 

I´m still trying to sort out the problem I’m having with the petrol tank.

 

I know all about the petrol tank causing a slight vacuum through pumping the fuel and the air not being replaced entirely until the vacuum pressure opens the valve on the petrol cap to allow air in but mine is doing the opposite, when I open the fuel cap air isn´t being sucked in it’s actually being blown out, so much so that if I jam the small valve on the petrol cap open it starts to spit petrol out. How can the petrol tank be under pressure like that? :confused: I have to open the petrol cap every 20-30 km to release the pressure otherwise the pump starts to fail.

 

Any ideas would be greatly appreciated.

 

Maz

i´m not sure topless...the OEM fuel pump worked fine for 4 years and then it died....baring in mind the car has done over 250 000 km i think it was time the OEM pump died. had no issues with the petrol tank in that time , wiring and fuel system worked fine and did for about 6 months on the Walbro mate.

 

My first port of call would be to replace the warlbro with a oem one complete on cradle, and if the same thing happens look at wiring/fuel system.

 

Have you done a fuel pressure check?

Andy

 

Looking at the diagram it states that when blowing through the check valve in the direction of the carbon canister there should be resistance with only little going through. blowing in the direction of the fuel tank from the carbon canister it is free flowing.so in theory, if air is coming from the direction of the carbon canister it would find it more difficut to return the same way, or at least I would have thought so.

 

so are you saying you think the problem could be either the check valve or the pressure release valve in the filler cap ?

 

 

Topless

 

I was thinking the same thing about getting a new OEM pump, the only thing stopping me is the fact that the car ran fine for about 6 months with the Walbro..the car still runs fine...just the pressure in the tank now. And no, i haven´t checked fuel pressure , but like i said, engine runs well..no issues with fuel pressure to the injectors etc.

There isn't a pressure release valve in the filler cap, it is a vacuum release, you said the car never ran as well with the warlbro, why persevere with it when a used oem one is pennies?

 

The car can very well with the fuel pressure being out of sync, also you can run it on the fly so you can see what is happening with the fuel pressure as you drive.

Topless

 

The fuel cap does have a pressure release vavle in it..if you take the cap apart you will see that if you stick a pin in to one of the holes around the centre hole you can lift a small plate which is kept down by a spring..this is to release preassure...in the centre of that plate is a small plastic pin which you can see when the top part of the cap is off...you can push the pin in...that is the valve that allows air into the tank ..it´s like a double valve.

 

I´d need to get the equipment to test the fuel pressure first :(

 

and as for a used OEM one being pennies...i wish I could find a TT pump for "pennies" I´d have snapped it up long ago mate.

try changing the cap first its going to be cheeper and quicker than changing the pump.

I thought the going rate for used oem pumps was about £40?

Eveything seems to point towards what topless has said about the new pump just highlighting the problem, if i were you id try a new fuel pressure regulator, or borrow one of a zed you know to be working fine, if the fuel pressure reg goes dicky it can make the pump work too hard, thus killing your old one and making the new one cut out.

Personally I would try changing the fuel cap first.

 

As the pump has run for 6 months with no problem it dosen't, in my opinion, suggest that that is not the problem.

 

I think it will be one of the one way valves gone wrong and allowing air the wrong way or not allowing it out as it is supposed to.

 

Just my 2p's worth.

 

Darrell

Have never heard of fuel cap going wrong but hey it is the cheapest and easiest option so give it a go!

 

One thing to consider is the warlbro's did have a reputation for going faulty, also not calling you workmanship into judgement but if the retaiining straps for the pump and or hose has come loose this could account for the excessive tank pressure?

I believe the problem will be a blockage in the Carbon Canister/ and or Purge control valve.

 

The design with the canister allows vapor to build up until purged by the running engine.

 

Since the tank is sealed to prevent vapors escaping to the atmosphere, (and the only pressure relief through the cap is inward to counter vacuum caused by cold fuel/ fuel drop) if the canister cannot purge pressure will build up. The only pressure relief available to the tank to the atmosphere is via the canister and thence intake manifold and engine!

 

Remember as fuel warms and expands it achieves greater volume, + vapor pressure will have the same effect as pumping up the garden sprayer.

 

Look at andys pic to see system.

 

Dont want to be rude but cant see how the pump can pressurise system since only pumping same volume of fuel round and round, that is takes from tank and returns it less that burnt- to pressurise it would need to be pumping fuel from another source into a tank with no vent- think about it- thats how an air compressor works!!

 

Regards

 

Dave

I am just trying to work out how excessive pressure in the fuel tank would stop the engine, the only way I can see is the fuel cannot return via the fpr and then it floods the engine?

There should be neglible pressure on the return side of the fuel pressure regulator- if it does rise there is a possibility that it will react on the diaphram and increase pressure on the injector side- more fuel will be injected at each pulse even tho the length of each pulse remains the same (same principle as adjustable regulators on performance engines) and could effect the engine through overenrichment.

 

Regards

 

Dave

Sounds like FP test will be the best start to finding the source of your problems then Maz?

Honestly think fuel pressure reg symptom, not cause.

 

Money on Purge valve/ Carbon canister! :)

 

Regards

 

Dave

easy way to find out, just do the carbon canister removal!

Thanks for all the replies, I really appreciate it :)

 

Topless:

 

getting a new fuel cap isn´t that easy either mate, according to courtesy Nissan the 300zx from 89 to 06/90 is different to the later models.perhaps that´s where the double valve system with pressure release is. the cap is cheap enough, but $75 just for mailing it.

 

I´ll have to get hold of a fuel pressure guage to test the pressure and unfortunately Z´s are rare around here and I don´t know anyone else with a Z other than Jonce who´s in the UK now for a few months.

 

I will remove the fuel pump and take a look at it, possibly buy a new OEM one too plus I´ll see if I can´t get a new cap from Nissan here.

 

Dave

 

That theory sounds good to me. Can I bypass the carbon canister without removing it (and the front facia to get to it)

 

I don´t think it has to do with the fuel vapour or gases though as the pressure is quite high after only 3 mins of running the car, it doesn´t go that quickly, surely ?

Just disconnect the carbon canister (under passenger front wing) on the tank side as Topless says. If you remove the assembly you should be able to test purge valve anyway.

 

Regards

 

Dave

There's a roll-over valve in the vent line from the tank to the carbon cannister that allows the pressure to vent to the tank, as far as I know the cap valve is vacuum relief only.

Worth checking that the valve is working OK and not stuck.

Dave,

 

,to disconnect the carbon canister on the tank side what do i have to do...removing the plastic cover beneath the drivers side wing reveals the carbon canister, but the only hose i can see from there going into it is at the bottom of the cannister ..do I have to take more apart ?

 

or can I bypass it by removing and capping the vac hose on the throttle body and the one on the balance bar ?

 

John

 

are you refering to the fuel vapor vent line check valve on the drivers side behind the tank ?

OK

 

New petrol cap ordered, new OEM pump ordered and I´ll check the whole carbon canister and release valve stuff and if need be remove the whole thing :)

 

Thanks for all your help guys, really appreciate it

mine sucks,is there not a return to the tank from high pressure rails,excess fuel that is not used.

your "what" sucks mate ..fuel pump , pressure in the tank ?

I take back the advice on new fuel cap ( sorry too late I know, youve ordered one)

 

Just had a weired occurence with my tank thats been removed from my car for about 5 months, with the cap on and the pipes blocked off with some hose.

 

Was painting it yesterday and noticed something rattling about inside it, took out the fuel pump assembly, removed a lump of tank repair filler that was loose inside (some nice chap has bodged up the filler at some time). I refitted the pump and went to do some other work on the car.

 

30 mins later and a strong smell of petrol I looked across to see petrol spraying out the return pipe . removed the fuel cap to find a fair bit of pressure and the leak stopped. refitted the cap 30mins later the same again fuel spraying out over the top of the tank.

 

Took the blanking hose of the pipe to the carbon canister and that cured the fault.

 

From this I take it that as some thought the cap does not vent excess pressure this must be done through the carbon canister...... carbon canister removal time then? or you may even find youve kinked a pipe when fitting the new fuel pipe.

Each time I release the petrol cap there is rush of air pulling into the tank and have wondrerd if that was normal.Performance is good and have never had petrol comming out. No help to your problem just thought I would add some information.

Each time I release the petrol cap there is rush of air pulling into the tank and have wondrerd if that was normal.Performance is good and have never had petrol comming out. No help to your problem just thought I would add some information.