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timspu's Avatar
 
Join Date: Jul 2012
Location: Devon. UK
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rear fuel pump problem

Gents, i believe i may have an issue with the rear fuel pump on my 1988 930 as I cannot get any leaner the low 10 AFR's.
If i pull the relay for the front pump so it does not work, the rear pump cavitates and only gives fluctuating pressure in the range of 5 to 5.7 bar. Is this normal? Does the rear pump require a head of pressure from the front pump or is this an indication of my rear pump failing?
With both pumps running i get 6.2 bar, basically the same as if only the front pump was running. I think this is low and is being held low by the rear pump.

Any info would be appreciated before i take the plunge on a new pump.

Thanks

TIM


Last edited by timspu; 05-05-2013 at 08:10 AM..
Old 05-05-2013, 12:08 AM
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Hi Tim,
Search for my name and rear pump.
I can't post the link due to connectivity here but somebody posted the proper testing procedure for the fuel pumps and it makes a lot of sense. It saved me having to buy another one and yes, cavitating the rear pump when the front one is off is normal.
I replaced my two relays with the ones from the dealer (better quality than the URO ones I had there) and it resolved my problem (I also splitted the power supply to the relays so it goes through 2 fuses and not only 1, there are write ups on this here)
Good luck
David
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David
1985 Black 930, K27 7006, Garretson Intercooler, Adj boost, TB: 23 - 31mm, bielstein sport shocks, ER polybronze bushings front and rear, ER monoball joints front and rear with offset camber plates, 935 X triangulated strut brace, raised spindles, racing bump steer kit
1981 Black 911sc (sold)
Old 05-06-2013, 08:13 AM
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David,
Thanks for the info i have found the link.
I checked the voltage and i have 12 volts at the rear pump with the wires disconnected, i swapped both relays and the voltage remains the same.
What was your measured voltage for you to replace the relays?
Sounds like the pump is running correctly when both pumps are on. i was trying to link my sudden drop in AFR with a drop in fuel pressure. Maybe i am barking up the wrong tree.
Will complete the test procedure once i have the adaptors to fit a gauge.

Cheers

TIM
Old 05-06-2013, 10:24 AM
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The rear pump is in series with the front pump so you can never experience a higher fuel pump pressure from the discharge of the rear pump compared to the discharge of the front pump. The rear pump insures the high pressure/fuel flow required is delivered to the CIS system.
A healthy CIS system is dependent upon other components performing as Bosch intended. How's the fuel filter? An old fuel filter will restrict fuel flow dropping fuel pressure/flow to the fuel distributor.

BTW, measuring the voltage at the rear pump with the wires disconnected will tell you nothing in terms of the health of the pump. You have to measure the voltage at the pump to see how much voltage is dropping when its running. These pumps tend to get pretty noisy when on they're last legs. You can hear a bad pump whining from standing a few feet away.

I suspect the Warm Up Regulator is not adjusting the Control Pressure adequately with higher boost levels. A CIS Pressure testing set of gauges will bring that out if true.
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'85 930 Factory Special Wishes Flachbau
Werk I Zuffenhausen 3.3l/330BHP Engine with Sonderwunsch Cams, FabSpeed Headers, Kokeln IC, Twin Plugged Electromotive Crankfire, Tial Wastegate(0.8 Bar), K27 Hybrid Turbo, Ruf Twin-tip Muffler, Fikse FM-5's 8&10x17, 8:41 R&P
Old 05-06-2013, 12:38 PM
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Dave,
With wires connected I am seeing just under 10.6 volts at the rear. Pump is quiet when front pump is running. The filter is about 1500 miles old. With pressure gauges fitted the warm setting is 3.9 bar . At 0.8 bar boost I have 3 bar fuel pressure. All on the leaner side. Idle Afr is 13.5. With both pumps running I have 6.2 bar pressure measured at the WUR.
Do not understand why I cannot lean AFR above 10.4 under boost and why on odd occasions the AFR's stick around 11.6 until I switch it off and let it cool for a while. Its as if the WUR has gone onto to cold start.

Car runs fine otherwise and starts ok.

Thanks
Tim
Old 05-06-2013, 11:02 PM
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Check my post here how to test pumps:

Rear fuel pump?
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Old 05-06-2013, 11:24 PM
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Check the voltage on the wur red/white. It gets power from rear pump and needs voltage to heat the warm up circuit. You might be on cold start enrichment? I does heat soak so that it will go out of cold start enrichment when engine gets hot enough.
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Old 05-06-2013, 11:53 PM
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The reason why I rewired the fuel pump relays through two fuses is that my fuel pumps fuse was getting very hot as it is for many people here. I had replaced the fuse by a modern type fuse holder but the wires were quite hot. Having each relay on a single fuse rectifies that problem.
I replaced the fuel pump relays because sometimes, when I would start, I could hear the rear fuel pump whine and cavitate. Originally, I thought it was the rear pump dying but it seems that the front one would not work due to unreliable relay making the rear pump cavitate.
I did not check the voltage as it was running good most of the time and the new relays fixed it.
Good luck
David
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1985 Black 930, K27 7006, Garretson Intercooler, Adj boost, TB: 23 - 31mm, bielstein sport shocks, ER polybronze bushings front and rear, ER monoball joints front and rear with offset camber plates, 935 X triangulated strut brace, raised spindles, racing bump steer kit
1981 Black 911sc (sold)
Old 05-07-2013, 05:39 AM
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Hi guys,
Thanks for the info. looks like its not the rear pump.
I appear to have a problem with the Mass airflow plate vibrating causing my control pressures to fluctuate. This is giving me a rich AFR. It returns to normal when i apply pressure to the airflow plate. looks like i have to have my fuel distributor head overhauled.
JOY !!

Regards

TIM

Old 05-10-2013, 07:09 AM
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