Fuel pressure question - 1988 635csi

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abouzmm
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Location: Detroit, Michigan

Fuel pressure question - 1988 635csi

Post by abouzmm »

Hello,
The car is a 1988 BMW 635csi manufacturing date 09/87. My question is about fuel pressure. Long story short is: new fuel pump, new 3.0 bar Bosch FPR, new in- tank pump. Measuring with gage T-Fitted to the fuel rail and jumping the relay, I get about 43 psi which is correct for a spec per the Bentley manual. I did a flow test and it exceeded specs of delivered quantity. Started the car with the relay, pressure drops to 36 psi which is not to spec. Removing vacuum line, pressure goes back up to ~ 43 psi. What could possibly cause the pressure to go down to 36 psi when idling? I installed a new set of Injectors purchased on eBay. They are not Bosch but look similar to the once’s I took out. The listing states they are for 635csi. Not sure what to do. Shall I just buy a new set of original Bosch. I can get them for roughly $250 and they are the new style as can be seen on FPC site. The car has a completely rebuilt cylinder head and all new ignition plugs, wires, cap, rotor, coil. Engine sound very smooth and valves adjusted. No vacuum leaks. A new idle valve as well straight from Germany. The idle has a surge and it feels it’s missing. It is not smooth and I don’t expect it smooth as M30 are not. But there is a miss. Any idea what I should do next? I am restoring the car and don’t want to move to the next project until I get the engine running like it should.
88 BMW 635csi Auto
VIN 3266761
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hornhospital
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Re: Fuel pressure question - 1988 635csi

Post by hornhospital »

How does it idle, run and drive with the vacuum line disconnected, presumably with 43 psi the whole time?
Ken Kanne
'84 633CSi "Sylvia"; '85 635CSi "Katja";'85 325e "Hazel Ann"; '95 M3 "Ashlyn"
abouzmm
Posts: 10
Joined: Tue Nov 04, 2014 1:59 pm
Location: Detroit, Michigan

Re: Fuel pressure question - 1988 635csi

Post by abouzmm »

The engine shakes a little specially during cold start and smooths out a little when hot. I have not driven it with vacuum line off but will try that and report the results. I just want to understand why pressure drops down to 36 when it idles. I am considering removing the fuel rail with the Injectors attached and pressurize the system to see if they are leaking. I will also activate the Injectors to see how they spray. Thanks
88 BMW 635csi Auto
VIN 3266761
abouzmm
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Joined: Tue Nov 04, 2014 1:59 pm
Location: Detroit, Michigan

Re: Fuel pressure question - 1988 635csi

Post by abouzmm »

I should probably mention that the fuel pressure has dropped down to ZERO after about 1.5 hours. I don’t believe that is normal.
88 BMW 635csi Auto
VIN 3266761
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hornhospital
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Re: Fuel pressure question - 1988 635csi

Post by hornhospital »

abouzmm wrote: Sat Aug 31, 2019 12:13 am I should probably mention that the fuel pressure has dropped down to ZERO after about 1.5 hours. I don’t believe that is normal.
Nope. you are correct. It should hold pressure for several hours if not overnight. The check valve is leaking. Shouldn't do that with new pumps. It also makes me wonder about the health of the FPR, new or not.
Ken Kanne
'84 633CSi "Sylvia"; '85 635CSi "Katja";'85 325e "Hazel Ann"; '95 M3 "Ashlyn"
abouzmm
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Location: Detroit, Michigan

Re: Fuel pressure question - 1988 635csi

Post by abouzmm »

I highly doubt the pump is leaking. I am going to install a shut off valve at the inlet of the fuel rail and another on the return line. I’ll run the pump and close both valves to see if pressure drops if it did, this means the Injectors are leaking. If pressure doesn’t drop, then I will close the valve on the return line and open the rail inlet valve. If pressure drops, then the pump check valve is leaking. The pump is brand new and I haven’t driven the car since installing it. I will finally close the rail inlet valve and open the return fuel valve. If it drops , then the FPR is leaking.
88 BMW 635csi Auto
VIN 3266761
KneeGrowNetwork
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Re: Fuel pressure question - 1988 635csi

Post by KneeGrowNetwork »

I know this is an old post, but I have the same issues with my 85 635csi. Did you ever fix the problem?
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86_6series
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Re: Fuel pressure question - 1988 635csi

Post by 86_6series »

I would PM abouzmm in case he's not monitoring the forum.
86-635CSi
96-MB C220 SOLD
98-740i
71-Continental MK3
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