I've posted on here before about the misfire that has beset my 1994 Passat 1.9 TDI estate ever since just after I boasted on here how well it was running!!
(see www.honestjohn.co.uk/forum/post/index.htm?t=24091 )
At first the suspect problem was the connector for the MAF unit, as the engine had cut to default mode once or twice and been restored to normal by fiddling with the connector.
BUT, seeing bubbles in the fuel line made me think the misfiring had another cause, air in the fuel I could find no fue leaks and so was baffled. Then I thought "airlock in fuel tank? - no, surely it couldn't be that!" I've heard of blocked fuel tank vents bringing vehicles to a stop, but it seemed too simple.
Today, before going to work, I removed the fuel filler cap (which is a Nissan one for some reason - been on the car since I bought it). Placed a strong piece of plastic sheeting tightly over the filler neck, secured in place with an elastic band, and made a small hole in the plastic. Drove to work and back, a distance of 30 miles. MISFIRE CURED! So simple!
The vent valve on the cap must have become blocked, but I can't get it to pieces to unblock it so I must buy a new one.
SAFETY NOTE: I wouldn't have tried this test on a petrol-engined car because of the more volatile nature of the fuel.
Cheers, Sofa Spud
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errrrrr.
Dont celebrate too soon.
When this happened on my 405d, there was NO vent in the cap. The tank was vented through a very complex and hard to get at breather system that was blocked.
Perhaps a nissan cap was fitted because the VW has no breather in it?
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The venting system may be your problem but to get air in the fuel there must still be a fuel line connection that can let hre air in. Most cars do not not rely on the cap to vent the the tank as in a roll over feul would be spilt. They use a dedicated vent pipe with an inversion valve in it to shut it off. I suggest you replace the cap run it until it misses then remove the cap to see if the tank now has a vacuum. If so then your main vent is block or the pipe kinked. Good Luck. Regards Peter
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I myself was puzzling how a blocked vent would lead to air bubbles. I suspected the vent problem because it took 3-4 miles before the misfire began. I can't find any fuel leaks.
If the fuel becomes more difficult to draw through from the tank because of the vacuum, I wonder if air could be sucked in through gaps in the plumbing through which fuel would not normally seep out.
cheers, SS
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i regularly notice a partial vacuum when i open the cap to fill especially if the tank is getting low
this has happened from new (1998)
wonder if it has anything to do with my current problem (see peugeot 306 d turbo engine problem)
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Re. My misfiring Passat TDI:
Today I found that the fuel return hose connection to the tank was not a tight fit. When I removed it to put a new jubilee clip on, no fuel dripped out. If air is drawn into the return hose and the bubbles are dischcarged into the tank, I wonder if some of them are drawn back into the 'live' fuel line back to the engine, given that the return and outlet are close together. If there is a partial vacuum in the tank because of a blocked breather, that might mean more air is drawn in through the loose return hose connection.
Cheers, SS
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