Fuel tank breather?
Discussion
After filling up from a local garage (see not used very often and my usual garage was having a delivery) my car developed a low speed, light throttle misfire, kind of a gentle kangaroo effect that dissapered as the throttle possition increased, so I assume cacky fuel. I filled up with Optimax and added half a bottle of injector cleaner after which all was well. Now I rarely let it get bellow a quarter of a tank mark but did on one occasion due to an absence of garages let it get well bellow the empty mark before I found fuel, as much as three needles widths so mine is a pretty pessimistic gauge.
Anyway all was well until I got to the quarter of a tank mark and the misfire came back. Filled it up, and it went away again. Ok so there must still be cack in the bottom of the tank thought I, however gradually the misfire has been returning sooner in relation to the fuel level. I have noticed when I fill her up, I am getting a "whoosh" as the petrol cap is removed and it has been harder to undo lately.
Driving home yesterday the misfire had started again so I stopped undid the fuel cap, had the usual "whoosh" and hay presto the misfire vanished. Now it seems to me that the tank is pressurising as the fuel level drops so is there a breather somewhere that’s blocked, or am I barking up the wrong tree?
Anyway all was well until I got to the quarter of a tank mark and the misfire came back. Filled it up, and it went away again. Ok so there must still be cack in the bottom of the tank thought I, however gradually the misfire has been returning sooner in relation to the fuel level. I have noticed when I fill her up, I am getting a "whoosh" as the petrol cap is removed and it has been harder to undo lately.
Driving home yesterday the misfire had started again so I stopped undid the fuel cap, had the usual "whoosh" and hay presto the misfire vanished. Now it seems to me that the tank is pressurising as the fuel level drops so is there a breather somewhere that’s blocked, or am I barking up the wrong tree?
Just had a word with our mechanic and he's come up with a list as long as your arm as to what could be the problem (sorry). Basically it could dirt in the fuel or fuel filter, dirt in the air filter?? or some type of electrical problem. As for the pressure being released after you take your cap off that can be normal suction. Why it should sort out the running of the car he's not sure. Got a feeling it’s a matter working through all of the possibilities.
nsparey said:
Now it seems to me that the tank is pressurising as the fuel level drops so is there a breather somewhere that’s blocked, or am I barking up the wrong tree?
The original "breather" mechanism for my fuel tank was a 3mm hole drilled through the fuel cap and when I eventully changed it for a new shiny leven fuel cap I get round the misfire prob by not over-tightening the cap.
>> Edited by SlvrCrb on Saturday 20th May 21:02
When i bought my car the previous owner had a nice shiny Leven cap for the fuel tank. He said he removed it because he found that fuel leaked past the cap, ran down the gulley around the boot, down the rear of the car and dripped off the bumper, near the exhaust pipe 
I recently fitted the Leven cap back on (got a new rubber seal for it) and :touchingwoodsmiley: have no leaks. But like nsparey when i remove the cap there is much more of a "whoosh" than the original cap.
The original cap has a cork seal could this allow some pressure out of the tank? where as the rubber seal on the Leven one does not?
No idea if this is contributing to your problem though Nick, nice to met you and your other half at the Hants met last week. Hope you get it sorted.
I recently fitted the Leven cap back on (got a new rubber seal for it) and :touchingwoodsmiley: have no leaks. But like nsparey when i remove the cap there is much more of a "whoosh" than the original cap. The original cap has a cork seal could this allow some pressure out of the tank? where as the rubber seal on the Leven one does not?
No idea if this is contributing to your problem though Nick, nice to met you and your other half at the Hants met last week. Hope you get it sorted.
its the vacum thats stopping you getting the fuel as you use the liquid part the tank space is filled with a vacum that the woosh when you open the cap it will get worse as summer approachs you have to open the cap before each journey to break the vacum or get a motorcross breather tapped in the cap ??
hbeck said:
When i bought my car the previous owner had a nice shiny Leven cap for the fuel tank. He said he removed it because he found that fuel leaked past the cap, ran down the gulley around the boot, down the rear of the car and dripped off the bumper, near the exhaust pipe
OMFG!!!!!!!!
Garethgtt said:
When i bought my car the previous owner had a nice shiny Leven cap for the fuel tank. He said he removed it because he found that fuel leaked past the cap, ran down the gulley around the boot, down the rear of the car and dripped off the bumper, near the exhaust pipe
OMFG!!!!!!!!
Been there I'm afraid: www.pistonheads.co.uk/gassing/topic.asp?t=164050
Not sure if there is a carbon canister as I used to have on my Chimaera, but I think if you overfill the tank the fuel can expand due to the heat from the exhausts and expand down the breathers... I only fill to the first pump cut-off now, just in case.
I have a 4.2 97 Cerb, so a fairly early one. The charcoal cannister sits in the nearside rear arch, and a vent tube from the tank connects to it via a solenoid that operates when the fuel pump is energised. You could disconnect this hose and directly vent the tank and see if your problems persists. Obviously if the relay has gone down the tank won't vent creating suction which will hamper the fuel pumps efforts to deliver fuel at the correct pressure. Worth a shot.
longbow said:
I have a 4.2 97 Cerb, so a fairly early one. The charcoal cannister sits in the nearside rear arch, and a vent tube from the tank connects to it via a solenoid that operates when the fuel pump is energised. You could disconnect this hose and directly vent the tank and see if your problems persists. Obviously if the relay has gone down the tank won't vent creating suction which will hamper the fuel pumps efforts to deliver fuel at the correct pressure. Worth a shot.
Gorgeous looking car you've got there, is it lowered?
longbow said:
I have a 4.2 97 Cerb, so a fairly early one. The charcoal cannister sits in the nearside rear arch, and a vent tube from the tank connects to it via a solenoid that operates when the fuel pump is energised. You could disconnect this hose and directly vent the tank and see if your problems persists. Obviously if the relay has gone down the tank won't vent creating suction which will hamper the fuel pumps efforts to deliver fuel at the correct pressure. Worth a shot.
Will go and have a look at lunch time, cheers.
longbow said:
I have a 4.2 97 Cerb, so a fairly early one. The charcoal cannister sits in the nearside rear arch, and a vent tube from the tank connects to it via a solenoid that operates when the fuel pump is energised. You could disconnect this hose and directly vent the tank and see if your problems persists. Obviously if the relay has gone down the tank won't vent creating suction which will hamper the fuel pumps efforts to deliver fuel at the correct pressure. Worth a shot.
...hope this does solve the problem so I can put this one in the "what to do when it goes wrong" diary.
It shouldn't have an open to atmosphere vent on the tank. The tank vents via a hose to the carbon canister which allows the fuel vapour into the inlet manifold when a solenoid valve is opened, controlled by the ECU when running. If the tank is not allowed to vent then the pump cannot suck fuel and creates a partial vacuum in the tank, and reduced fuel pressure, hence the "whoosh" as air enters when the cap is removed. Used to see quite a few diesels years back where the tank had collapsed to the point where its capacity went from 60 litres down to about 10
Hmmm I was a bit hasty me thinks, filled up with fuel a couple of days back and it started missing again, so much for low fuel/pressure build up. Seems to have got worse too, today I could barely pull away and it stalled a couple of times. Ran it over to Richard at Henley TVR and he's going to have a look at it short notice for me, despite have the busiest TVR workshop I have ever seen, so thumbs up to them and fingers crossed.
Ok so the plot thickens, took the car over to Henley, popping and missing like a gooden, got stuck in traffic at Sonning. After sitting idling in said traffic the problem disappeared dropped the car off and the bloody thing didn’t misbehave once while at Henley, no fault codes, engine balanced perfectly, no fuel pressure problems, just spot on in fact. Picked her up, did a 100-mile round trip with no problems at all. Next day car started as normal until it warmed up then the problem returned again! What I have noticed is as I change up through the gears, when you dip the clutch the rev normally drop instantly but when its misbehaving, the revs hold for a second, even rise a couple of hundred rpm, as if you are changing gear with a little throttle, all very odd. Idling or stopping and restarting seems to be the key to clearing it and I duplicated it again, she is now running fine again. Wondering if its not fuel starvation but overfueling? Lambdas or ECU, looking at the history, she had new lambdas at a few hundred miles. Cars back in at Henley on Friday so fingers crossed.
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