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 Post subject: Silly question regarding the fuel pump...
PostPosted: Fri Sep 21, 2018 8:33 pm 
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Joined: Wed Jan 17, 2018 9:15 pm
Posts: 90
TIPEC membership: 8169
Since fitting the correct FPR, I have noticed that the noise from the fuel pump is a constant mmmmmmmm. Is it supposed to cut out when fuel pressure reaches the optimum point or does it always run?
Also, seems to be a bit rich. On start up, I can sometimes get a back fire.
Now, as it was running too lean with the lower pressure FPR previously, I have little benchmark on fuel consumption,

Any help appreciated.

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Fun - 944 2,7 Lux '89 acquired Nov '17, needing less TLC than when I bought it.
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 Post subject: Re: Silly question regarding the fuel pump...
PostPosted: Fri Sep 21, 2018 9:25 pm 
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Joined: Tue Nov 19, 2013 11:48 am
Posts: 1667
Location: West Essex
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It's a constant flow system that bleeds off the unwanted fuel back to the tank down the 8mm return line, the pump is capable of 5bar or more and the FPR manages the pressure reaching the rail and injectors

The only way to be sure is a pressure gauge on the take off on the rail

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 Post subject: Re: Silly question regarding the fuel pump...
PostPosted: Fri Sep 21, 2018 9:36 pm 
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Waylander wrote:
It's a constant flow system that bleeds off the unwanted fuel back to the tank down the 8mm return line, the pump is capable of 5bar or more and the FPR manages the pressure reaching the rail and injectors

The only way to be sure is a pressure gauge on the take off on the rail


Okay, good. Ta.

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Rob

Fun - 944 2,7 Lux '89 acquired Nov '17, needing less TLC than when I bought it.
Everyday - Merc C220 '95, Elegance, almost mint
Work - Fraud Transit van, skip, bus...
944 C Reg 2.5 n/a, breaking for spares. PM me if you need bits.


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 Post subject: Re: Silly question regarding the fuel pump...
PostPosted: Sat Sep 22, 2018 1:50 pm 
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Joined: Sun Mar 11, 2012 11:03 am
Posts: 448
Location: Bolton U.K.
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I bought a fuel pressure guage from Arnwerx but beware the plastic face of the guage dissolves in petrol (design error). I changed the FPR which ended up was ok, the problem was bad hot start traced to the non return valve in the fuel pump. The contant flow keeps the pump cool and they are designed for constant running unlike old SU ones which had a tendency to stick but that was on carbs. Always a good idea to change the fuel filter, I think mine had been on for 20yrs. Iirc the mixture screw only affects idle running, I can get approx 35 mpg on a motorway cruise up to Glasgow average around 25 mpg on track nearer 15 mpg. Pops are usually a sign of a weak mixture in my experience rather than rich which will cause misfires on WoT at below 3000rpm, but again that was on carbs (twin Webers on my old MGA Twin -Cam)

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 Post subject: Re: Silly question regarding the fuel pump...
PostPosted: Sat Sep 22, 2018 1:58 pm 
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Joined: Wed Jan 17, 2018 9:15 pm
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Oh ok.

It's just since fitting the correct FPR, the noise is much more discernible from when it had the wrong FPR.
I was trying to get a proper description of the fuel system but despite looking at Clark's and a few links on here, I didn't manage to.

What part of Scotland are you in?

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Rob

Fun - 944 2,7 Lux '89 acquired Nov '17, needing less TLC than when I bought it.
Everyday - Merc C220 '95, Elegance, almost mint
Work - Fraud Transit van, skip, bus...
944 C Reg 2.5 n/a, breaking for spares. PM me if you need bits.


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