Fuel surging/poor pick-up in tank pre-pump

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DRPM635CSi
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Fuel surging/poor pick-up in tank pre-pump

Post by DRPM635CSi »

I've got a fuel pick-up problem which is now getting more and more noticeable as the car gets faster and faster with various new chips and engine mods. On balls-out WOT launches with 1/4 fuel tank indicated on the needle, the car takes off until the fuel surges backwards and then dies halfway across the intersection as the pump gets starved and sucks air.

The car is a May 1985 and I don't know if there was a later fuel tank design perhaps with more internal baffles to keep fuel in the pick-up bowl for longer or not? Does anyone know? My tank is original. It has been repaired for the filler neck corrosion problem, but it is still the original tank.

The in tank pre-pump has been rebuilt. Not with a complete new unit, but by buying the American-market in-tank pump from the 63 litre tank they got, being disassembled to gut the working motor components from it and then reassembled to the old tubing and anchor plate of my original pump assembly that came from my car. At the same time all the delivery and return hoses to the in-tank pre-pump were renewed. None of this work has solved the fuel surge problem.

I would really appreciate any advice for solving this please if anyone has anymore ideas?
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hornhospital
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Re: Fuel surging/poor pick-up in tank pre-pump

Post by hornhospital »

Putting an un-gutted actual working pre-pump back in the tank will very likely solve that problem. The purpose of the pre-pump was to prevent just what you're experiencing. I'm really curious why you gutted the pump. :-k
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DRPM635CSi
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Re: Fuel surging/poor pick-up in tank pre-pump

Post by DRPM635CSi »

hornhospital wrote: Fri Sep 21, 2018 5:16 pm Putting an un-gutted actual working pre-pump back in the tank will very likely solve that problem. The purpose of the pre-pump was to prevent just what you're experiencing. I'm really curious why you gutted the pump. :-k
At the time I was told this was the 'cheap' way of replacing the in tank pre-pump. At the time the European market car pump was three times the cost of the US market one, but the major working components were the same. The pump body was shallower I think to accommodate the 63 litre fuel tank of the US cars and the hose fittings at the top were a different size and the anchor plate locking ring was also different from memory.

I was told the cheapest solution was to buy the US market pump, disassemble it to get all the new pump bits from it that were the same from the Euro pump, and rebuild them into my body/chassis with the correct locking plate and hose connection pipes to suit my boot floor instead of the US car one. Hence, everything would be new except for the actual metal pipes and the locking plate securing it into the boot floor metalwork which are not critical items controlling fuel flow anyway, so they didn't matter. In doing all this, the saving was something like ~$200 on just buying a brand new Euro market in tank pump assembly complete.
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Re: Fuel surging/poor pick-up in tank pre-pump

Post by Brucey »

I think that what you are experiencing is quite normal with a standard (two pump) fuel delivery setup. My '87 has always done this on a hard launch if the tank isn't full enough. Early 635CSi had an extra part in the fuel system that may have reduced the chances of fuel starvation under certain circumstances but I am not sure it was 100% effective and it was deleted on later cars.

I think what is required is a swirl pot of some kind. Normally this would sit above the fuel tank and return air from the top and liquid fuel from the bottom, feeding the pressure pump. The volume of liquid fuel in the swirl pot needs to be enough to last as long as fuel starvation might, which in the case of a 6er (in the absence of steep hills) is only about six seconds. After six seconds you will be in one of the higher gears and the acceleration won't be enough to cause fuel starvation any more. [My autobox car shifts from first to second at an indicated 58mph after about five and a half or six seconds] As little as a litre of fuel might be enough. However if any air might still get through to the fuel rail at some point (as in the case with a simple inline reservoir) then this is exactly the sort of thing that can take the tops out of the pistons. I think that there is enough room under the car (right next to the pressure pump) for a unit (lets call it an Underslung Swirl Pot USP) of unconventional design that ought to do the trick and to retain the stock look.

My idea is to have a return from the top of the USP that delivers air back to the tank via a bypass hose that T's into the same (low pressure) return line as comes from engine bay. The clever (well what passes for clever in my book.... :wink: ) bit is that the bypass exit from the USP would be a restriction (nozzle) that would result in 'choked flow' for liquid fuel but would let air through far more easily. This should mean that the overcapacity/pressure required in the lift pump needn't be as large as with a more conventional design of swirl pot. Provided the nozzle isn't so small that it might be blocked by any debris that passes through the pickup filter, this should work OK.

BTW I have not built such a thing yet because I don't really need it any more; running on LPG there isn't the same problem.... :shock:

cheers
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DRPM635CSi
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Re: Fuel surging/poor pick-up in tank pre-pump

Post by DRPM635CSi »

Brucey wrote: Sat Sep 22, 2018 3:29 amEarly 635CSi had an extra part in the fuel system that may have reduced the chances of fuel starvation under certain circumstances but I am not sure it was 100% effective and it was deleted on later cars.
Is this the fuel damper you're referring to mounted near the right rear wheel arch? My car still has this. It was very badly corroded a little while ago and my mechanic wanted to just delete it entirely and get rid of it since he said it was unnecessary and in fact deleted on later model cars entirely by the factory, but I went to the expense of buying a new one and having it refitted anyway when the fuel line hoses were replaced. I was told it's only purpose was to reduce the mechanical noise of the fuel being pumped from being heard inside the cabin. When quieter pumps became available, it's necessity disappeared, so it was dispensed with. I don't believe it ever had any purpose at all in nullifying fuel surge issues.
After six seconds you will be in one of the higher gears and the acceleration won't be enough to cause fuel starvation any more.
Unfortunately the acceleration & thirst for flow of fuel in an M88 engined car is vastly more aggressive than to take as much as 6 seconds. My car won't even make it completely acrosss the intersection before it dies on a full encilada launch with extra beans.
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Re: Fuel surging/poor pick-up in tank pre-pump

Post by Brucey »

apologies if this is b. obvious but the amount of fuel that the engine actually burns is almost irrelevant; the fuel pump is pumping it at an almost constant rate through the FPR so the same volume gets delivered to the fuel rail whether it is burnt or not. This means that only important things are

a) how long a high G condition exists
b) the fuel level in the tank vs the pickup
c) the volume flowrate of the pressure pump
d) the volume of available fuel in the pipework between the pickup and the pressure pump.

Needless to say as soon as the pressure pump is sucking on air you have no real fuel pressure any more and it will feel like a hard cut. Making d) larger is how a swirl pot works.

The 'damper' unit (which I didn't think was ever a standard part on M635CSi BTW ) is normally installed between the pump and the filter, i.e. downstream of the pump. But if it were installed upstream of the pump instead it could increase d) and therefore the time between onset of high G and onset of fuel starvation at the pump. What it won't do of course (not without a bypass hose such as the one proposed above) is prevent any air that goes through the pickup pump from eventually getting to the fuel rail (or filling the 'damper') so if enough air gets into the system and thence into the fuel rail it may allow a fuel emulsion (mixture of fuel and air) to be injected and therefore a lean running condition.

Anyway if all you want to do is smoke away from the lights you may be able to do a fifteen minute fix and just plumb the 'damper' in a different place. Not a good idea if you are going drag racing but might be good enough for the urban dash to a more modest speed.

cheers
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Re: Fuel surging/poor pick-up in tank pre-pump

Post by GazM3 »

I'm starting to get this issue on the m635csi as the chassis gets upgraded, Especially as im turning right. Im going to first check and see whether the in tank fuel pump is actually working. Ideally I don't want to install an aftermarket swirl pot in the trunk unless I can hide it well. Unfortunately the car now is in the paint shop (been on the cards since 2012) so only 10 years to wait so it will be one of the things to resolve when i get it back in hopefully a few months.
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