Question about 505 GTi Wagon Scavenge Fuel Pump.
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  1. #1
    Fellow Frogger!
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    Default Question about 505 GTi Wagon Scavenge Fuel Pump.

    G'day all,

    My understand is this is a low pressure, in tank pump which is intended to keep the main fuel pump (high pressure) primed, especially when fuel is getting low. I am pretty sure this pump isn't working in my car, and probably hasn't been since before I bought the car ages ago. I have found all sorts of issues with the ignition and fuel injection system which were making it run poorly, and have put them right, and the car runs a lot better, but have always found this car to be noticeably down on grunt, and less fuel efficient by comparison to my previous 505 GTi wagon which was virtually identical, same model, same age same system (standard factory, no mods). It is a bit frustrating, but as I said, the car runs quite well now, just not as well as I 'd like.

    So (finally) to my question: If the scavenge (in tank) pump is u/s, will this affect the power? I don't have any probs at all starting, or running, even with fuel very low level. This would tend to suggest the pump is actually working, especially when you consider that I live on quite a steep road, and again, even with very low fuel (only about 4 litres in tank), fuel still manages to get to the main pump when driving up it...but I have had the access covers off today, and put a short length of hose on the top of the pump and put it to my ear with the engine running and I couldn't hear any trace of a sound I'd expect to hear from an electric pump. I disconnected its earth, and put a multimeter between the earth wire and the earth point, with engine running, no voltage. Then I attached a length of wire to the detached earth wire for the pump, started engine, then alternately contacted/disconnected it to the earth point, and no change at all to engine idle or note, and still couldn't hear any sound at all produced by the pump. But again, my understanding is it is a small pump which may be very hard to hear anyway, and in any case, as long as the main pump is being primed, the pressure on the output side (delivery side for engine fuel rail) of it would not be any different with or without the scavenge pump working, so it wouldn't make any difference to performance. Do those of you who know a thing or two about this type of fuel injection system agree with my thinking?

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    Appreciate any thoughts about the pump.

    The engine itself is good, compression tests fine, all fuel injection components check out ok, and earth points have been cleaned up, or put right, so everything tests ok now, including O2 sensor (replaced). Ignition system good, including dizzy, advances all working working ok, timing set, and no air leaks (supp air device doing its job and adjusted to about right).

    Thanks and cheers.

  2. #2
    1000+ Posts Wildebeest's Avatar
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    Simon,
    Talking Volvo [what again?] When the pre-pump is inoperative or the gauze strainer is dirty it resulted in a noisy main pump.
    If there is no excessive main pump noise, remove the pre-pump fuse, if main pump noise increases the pre-pump is probably OK.
    The gauze strainer could still be dirty causing loss of performance.

  3. #3
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    Thanx Wildebeest,

    Good points.

    Main pump isn't noisy...and not familiar with architecture of the in tank (scavenge) pump, not even sure if it has a strainer, but I presume it would. Fuse is shared by both pumps....so would have to run the noise test using the earth point (remove/replace and compare).

  4. #4
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    Could it be that the pressure regulator on the end of the fuel rail is not maintaining a high enough pressure have you tried injector cleaner .pugs

  5. #5
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    I have pugs, reg seems ok, but would need to be measured to be sure. I swapped it from an engine which was going well, and it didn't change anything when I did it.

    The question I have for now is whether or not the tank pump would be likely to make any difference to main pump output pressure if it isn't working, assuming that fuel is actually getting to the main pump, which it clearly is. If say the main pump is pulling the fuel too hard to it without it being pushed by the tank pump, could it give rise to some cavitation and thus varying pressure output? I 'd imagine, if that is the case, then the higher the revs, the more the problem would be manifesting itself, which to some extent seems to be the case. The engine revs out, but is a bit gutless at higher revs (unlike other one I had), but it has plenty of power at low revs, in fact I'd say it has excellent low rev torque and power. I have another tank with pump, etc, so no prob with having parts, just thought I''d ask other peeps thoughts on this before I go through the exercise of swapping it over ( I figure that would be about as easy as trying to change the pump in situ).

    Cheers
    Last edited by Simon W; 26th September 2016 at 08:31 AM. Reason: typo

  6. #6
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    I did have this problem with my 505, -now sold on-I simply removed the pump and drove without it for about 5 yrs , put in a new fuel filter and I had no problems at all. the fuel pressure was 35psi. Max

  7. #7
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    Thanks Maxy,

    Was this on a fuel injected model (was it the L Jetronic)? If so, did you find that it went any better after you removed it? When you say new filter, I assume you mean the filter just downstream of the external, high pressure fuel pump.

    Cheers

  8. #8
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    The main pump is the one that does the serious work. The way the in-tank pump might cause a problem is if it were to restrict the flow to the main pump's inlet. In that case, the main pump would be noisy. Yours isn't, so I don't think that's your problem.

    How's the plug colour. Lean? Seems unlikely since you also say you're using more fuel than expected.

    I'd be checking the advance curve on the distributor. Vacuum advance can make a lot of difference to cruising economy, and centrifugal advance can have a big effect on power.

    Have fun,

    Rob.

  9. #9
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    Been ages since iv owned one of these ,i would be replacing the fuel filter and the pick up in the tank ,i somehow feel this may be a mission ,.can you disconnect the fuel line at the filter before the pressure pump ,stick it in a coke bottle ,turn the ignition on and see if it is in fact delivering fuel ,im inclined to agree with others that it can survive without the in tank pump ,from left field ,get underneath and give the muffler a good thump see if it rattles ,could be partialy blocked ,although i would expect the engine to be overheating if that were the case ,pugs

  10. #10
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    The fuel pressure regulator has a surplus fuel return hose. Uncouple this, block it off and fit a length of hose to the regulator with the other end of it in a container. Start the engine. At revs above fast idle excess fuel should drain into the container. If it does this implies that there is enough fuel to service much higher revs. Be careful to avoid the rotating fan blades and spillage on the ignition system.

    Pavel

  11. #11
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    Quote Originally Posted by Simon W View Post
    Thanks Maxy,

    Was this on a fuel injected model (was it the L Jetronic)? If so, did you find that it went any better after you removed it? When you say new filter, I assume you mean the filter just downstream of the external, high pressure fuel pump.

    Cheers
    yes, L;-jetronic; and yes, the can filter next to the pump.Didn't really notice any change in performance but it had a few issues with the injection system that were fixed later down the track that did make a difference,

  12. #12
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    I'd agree with Rob:

    1. Check that the distributor centrifugal advance is working correctly and not seized as a first stop.
    2. Check the accelerator cable. Is it adjusted correctly so that you are actually getting full throttle?
    3. Check the all the pollution control and intake air ducts for air leaks. There's a multitude of them on a ZDJ.
    4. Check the air flow sensor (generally seem to be pretty reliable though)

    Cheers.

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