Strange fuel pressure issue - help please
Hallo everyone.
First post - though I see I have been a lurking member for ten years. I wonder if anyone has experience of the following issue or can shed any light... V70 T5 2005 Monitoring simultaneously the duty cycle percentage from the pump head, the voltage across the pump head electrodes and the fuel rail pressure sensor the following anomalies occur: Under normal driving conditions fuel rail pressure is at or around 43 psi, voltage across pump 5-7v (dc setting on DVM) duty cycle 50- 55 %. All as it should be I believe. The above readings alter slightly when in idle or higher revs. No problems so far. However - with tank half full on level ground and accelerating as hard as possible things proceed normally for a few seconds then the fuel rail pressure drops suddenly to between 9 and 12 psi, the duty cycle quickly cranks up to 100% and the voltage across the pump goes up to 12v or more. While this is happening there is a noise that sounds like sucking from the pump (though it might not be sucking) and the pressure (real-time graphic display) spikes up and down as the pump tries to deliver fuel to the rail. In time with these spikes the revs leap and die kangarooing the car along until one backs off with the boot. The data suggests that the electronics are screaming at the pump to deliver and it can't. The effect is much less likely to occur thrashing up hill, and more prominent going down hill. Next - with a brim full tank - the anomaly does NOT occur on the level or going up hill, and only very slightly under extreme duress going down hill. Prior to all this: We replaced the full pump (3rd one - see below), which has slightly improved the problem. I also replaced the fuel pressure sensor and the in-line fuel filter. I have swapped the PEM on the side of the tank with a good second-hand one and checked that no damp has got into the CEM from above (A know problem with 2005 V70). My conclusion from the data so far is that there is a mechanical problem in the tank (Plastic). The evidence seems to suggest that the electronics are doing exactly what they should but that when the pump is really sucking it gets blocked or falls down on the job in some way. The first pump (original) stopped dead one day and was burned out. The replacement - Bosch - seems to work well except for this odd behaviour - only worse - so we got a new one under warranty and put it in - same problems occurred only not quite so bad. I didn't replace the pump s myself and the lads at the garage didn't deliberately check to see if there was any debris in the tank - which seem to be the obvious conclusion. Before I get them to pull it out and look - has anyone got any wisdom to share? Seems extraordinary that a plastic tank should have enough crap in it to block the pump strainer - unless there is a plastic bag floating around in there. Or has the pump got some sort pressure release valve that is not doing what it should. And why does filling the tank up make a difference? One thing that might be relevant - I put jet cleaning fluid in the tank about 300 miles before original pump burnt out. Thanks in advance for any useful thoughts or advice. Frank |
I think there is a succion tube that goes into the other half tank, could it have a problem? (I think there is a way to attach it before removal so as not to lose it inside the tank)
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Best start off with a new sender/pump unit I guess... |
Thanks for suggestions so far. We changed the whole fuel pump unit on both occasions. As far as I am aware the swirl collector pot and so on are an integral part of the whole thing - it has a float arm with a ball on the side and a braided umbilicus that passed through to the other side of the tank for attachment to the other sender and to dip into the other side of the tank.
The uptake sieve on that side seems to need to be clipped to the bottom on the sender unit so - presumably - it pushes down onto he bottom of the tank on the other side from the pump. Perhaps - as suggest - if that is floating around - and surfacing - more likely when tank half full - then air could get sucked in from that side of the tank and cause the problems described. I wonder if anyone has actually experienced the same symptoms as me - and even better found the remedy... Thanks again for replies so far. Frank |
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Problem is much worse going down hill and on level and with tank half empty rather than full. Doesn't do it on the level when tank full and much less so going down hill. My money is on the left side inlet having come un-clipped from the spring-loaded bit on the bottom of the L/H sender module. It this is detached is it conceivable that it could surface and suck air? If so do you think that would that cause air to enter the pump during high demand even though the main pump inlet is still well submerged in petrol? Thanks, Frank. |
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Struggling to think of a logical explanation. I talked to the - very able - mechanic that fitted them both and he clearly knew what he was doing. Says that everything was clipped together as it should be - so that the LH collector was pressed against the bottom of the tank. However - one thing I didn't have space to describe in my original post: With the first replacement pump fitted - the car ran out of fuel the first time I let the gauge go below '100 miles left in tank'. I know that the pump was dry because I heard it priming after I emptied the spare can in. And I know the tank wasn't empty because refilling it a few miles down the road took much less petrol than normal. I never let it go as low again until we replaced it and I still haven't had the nerve with the new pump. Will see what I can do... The above does strongly imply that the LH pick up was detached on the first replacement pump. I am passing the mechanic's place tomorrow and will find out if they remember what they discovered when they took the first replacement pump out. In the first instance we have agreed it would be sensible to take the left hand sender unit out and see if there any obvious anomalies - much easier than the RH side... Long shot I'd say. Thanks for the ideas and wisdom - keep it coming, Frank |
Further information and Solution
2 Attachment(s)
Further symptoms persisted. On long runs keeping the tank filled to the brim almost completely solved the symptoms. Nevertheless with the analyser plugged in the fuel rail pressure could be seen to drop on right hand turns but not left. There was a corresponding increase in the main pump duty cycle (tapped into white and white & black wires on pump head).
Conclusion being that the swirl pot was not filling properly or staying filled - therefore either the no return valve was stuck open or the there was a blockage in supply from the ejector pump/venturi system. With the tank brim full fuel was able to slop over into the pump pot. Opened the tank yesterday and removed the pump. Where the Venturi return pipe tuns at the bottom of the pump body it had been bent and kinked during installation and stayed kinked. The result was to very much restrict the flow of fuel from the ejector/venturi loop. The kind of plastic used and the thinness of the walls of the pipe lend themselves perfectly to this anomaly. Once the pipe is kinked it tends to collapse and stay kinked. We fixed an external stent in the form of a piece of (proper) petrol hose and clamped it distally with a jubilee clip. All symptoms have no disappeared. Presumably in the process of fitting the first replacement pump something similar happened and the pipe was kinked in the same or a different place. Examining the piping closely and the process of fitting I am surprised this doesn't happen more often. Decent Bosch pumps too. And careful mechanic - no problems in twenty years. I will attempt to attach a couple of pictures of the kinked pipe. Thanks to everyone who helped with advice. Frank. |
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