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MG MGB Technical - Fuel pump discovery

My '73 GT has been running with the aftermarket "rectangular box" fuel pump since I bought the car in 1986. The pump finally died on me about two months ago. The good news (and, as it turns out, the bad news) was that I was less than half a mile from a NAPA and I was able to buy the current version of these pumps carrying the brand name of "FACETS".

This pump held out well for me during a summer of driving that included a 2,000 mile trip followed by a 1,500 mile trip, although - particularly on the second trip - there were moments of hesitation which progressed to the point where the pump would work like crazy with no result. I'd disconnect the fuel line to the carbs and nothing more than a few drops would emerge.

Now, along with this failure, I did discover that my fuel tank breather line (to the canister) was blocked. Often when refueling, opening the gas cap released a substantial vacuum. That may have overworked the FACETS unit and contributed to its demise.

OK, so this time I decided to go back to a real SU pump. I have a voltmeter on this car, which has been EXTREMELY useful as I have A/C, two cooling fans and halogen headlights served by a 105-amp Saturn alternator.
On start-up; first I noticed I didn't have to use the choke at all for a cold start. Then I noticed my voltmeter was showing at least one more full volt at all rpm ranges and with any combination of electrical devices working.

Conclusion? The little FACETS pump draws a LOT of current. Whatever one argues about the effect of the plugged breather line, the pump was drawing this much current when first connected. And, near its demise, removing the gas cap had no effect on its current draw or its performance.

I've used these aftermarket pumps on a number of cars over the years, but I think this experience has taught me something. I see the SU pump action spiking on the voltmeter, but most of the time it draws nothing, and with my A/C on, I'm charging at 14 volts rather than just over 12. From now on, I'll stick with the SU pumps.

FWIW,
Allen
Allen Bachelder

Allen,
Don't know the exact works inside the Facet pump, and I've never measured the current draw, but I don't think they normally pull any great amount of juice. Certainly not enough to drop your Niagara Hydro power system by a full volt. They don't get hot, and they work fine on all manner of raggedy-ass electrical systems. I suggest that it was bad from the start. Further, where is your voltmeter hooked into the system, especially relative to the pump feed?
The blocked tank breather may cause the same condition with the Facet as a blocked suction side will on an SU - stalling in a current-on condition. It'll kill an SU. Might be what killed the first square pump too. Dave DuBois should be along shortly to enlighten us.
The pump running madly sounds like the opposite trouble- an air leak on the suction side, but under some conditions the fuel could be boiling under vacuum. Congratulations, you may have finally made an electric pump go into true vapor lock!
FRM
FR Millmore

I am with Fletcher regarding the current draw of the Facet pumps, normally they are quite low. If you were dropping a full volt, the pump had some kind of problem inside of it. I don't really know how the electronics are configured in the Facet pumps (the electronics are all encased inside a block of epoxy, a good way to insure that they remain tamperproof). The Facet does not react to a stall situation the way a SU pump does. When a clog occures on the inlet side of a SU pump, the pump stalls in a current on condition as Fletcher states. The Facet pumps, in addition to the check vlaves, have a by-pass valve that determines the pressure that the pump operates at, so if there is a clog on the inlet or outlet side, the pump just keeps on hammering away even though it is not moving any fuel. I haven't tried doing so on my test stand, but I believe that the pump will speed up slightly on a clogged inlet.

Going back to current draw, the Facet may have more total current through them when operating than the SU pumps do, but this is the result of the duty cycle of the two pumps. The SU pump, when operating properly will draw up to almost 5 amps in a constant on condition, but when operating normally, they have a very short duty cycle (time on vs time off), so total amount of current is very low (on the order of 0.5 amps). The Facet, on the other hand, while it's total, constant on current may be no higher than the SU pumps, has a much higher ratio of time on vs time off than the SU pumps do, so the total draw will be higher than that of the SU pumps. Even so, it should not be sufficient to cause a 1 volt drop from an alternator that puts out 105 amps. To do that the pump had to be malfunctioning from the git-go. Cheers - Dave

PS - I still advocate using the Facet pump as a permenantly installed backup pump, with it and the primary pump being powered through a single pole double throw switch.
David DuBois

Thanks guys,

I have gotten along fine with these pumps (the rectangular ones) for many years, so I too was quite surprised by the current draw. It's entirely possible that it was defective from the beginning. The vacuum condition in the tank may have been a factor in the demise of the 19+ year old one. I blew out all the lines with my air compressor while it was down. I would still like to wire/plumb in ANOTHER Facet as a back-up. Did somebody say to plumb it in parallel with the SU?

Allen
Allen Bachelder

And what's the connection with the fact that your car no longer needs the choke for a cold start ??
Iain MacKintosh

Iain,
Good question now that I think of it! I was surprised however - it has not done this before. The fuel pump was the only change and now it doesn't need the choke. Go figure! The implication is that there is now more fuel available, but on the downstream side of the float bowls, there should be no difference.

Allen
Allen Bachelder

May be that the Facet was never producing correct pressure, therefore the effective fuel (float) levels were low, now it's fixed.
FRM
FR Millmore

Allen - You have mail. Dave
David DuBois

David,

Indeed I do have mail. Thank you very much. I hope to get to this soon.

Allen
Allen Bachelder

This thread was discussed between 30/08/2005 and 31/08/2005

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