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TOPIC: fuel pump questions for a 1979

fuel pump questions for a 1979 10 Jun 2018 13:15 #20409


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Hi there. It's been awhile since I've worked on my 1979 928 but the last 3-4 weeks I've been hard at it. I'm at the point now where I need to decide what fuel pump to go with. My car had the 2 pump setup but I've opted to delete the in-tank pump and go with just the external pump. I've read that going with a Bosch 044 pump is an option but requires an upgrade to the wiring? I read in one forum at another site that the external pump isn't strong enough on it's own and needs the internal pump as a helper pump. Anything to that claim? I've always thought the 2 pump setup was to avoid vapor lock possibilities during spirited driving in hot weather. Anyway, my thought was to go with a new standard OEM external pump. Any suggestions?
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fuel pump questions for a 1979 29 Jun 2018 09:49 #20420


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There aren't any replies about in-tank pump delete on CIS 928s (1977 - 1979). I replaced both pumps in my 1979 back before the in-tank pump became insanely expensive and hard to obtain.

If you do go single pump, the key thing that has to be monitored is the fuel pressure. We'd love to hear if you are successful.
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fuel pump questions for a 1979 30 Jun 2018 10:31 #20422


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Well I went for a direct replacement Bosch external fuel pump with just the in tank screen filter in place. As well as a new fuel filter in the wheel well. Left everything else as is and gave it a shot firing it up. I have no idea the last time it ran but it's been many years I believe. It didn't take long for the car to fire to life however it was short lived and after about a second it dies. Almost like it's being cut off. Any time I try starting it now it's the same thing. Fires up right away but cuts out within a second. Another forum I've used has said the cold start valve is working fine but there's a problem after that. I've been trying to read up on how the CIS system works in these cars so I can keep trouble shooting. One thing of note is that the previous owner said there was an issue with the cooling system so it had been drained. I hadn't bothered to button the cooling system back before attempting to just see if it would start, run and idle for a few seconds. After reading more about the CIS system and how it uses coolant temp as part of the warm up procedure I'm wondering if I need to get the coolant flowing again because the system is shutting the car down.
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fuel pump questions for a 1979 01 Sep 2018 23:21 #20585


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new928 wrote:
Well I went for a direct replacement Bosch external fuel pump with just the in tank screen filter in place. As well as a new fuel filter in the wheel well. Left everything else as is and gave it a shot firing it up. I have no idea the last time it ran but it's been many years I believe. It didn't take long for the car to fire to life however it was short lived and after about a second it dies. Almost like it's being cut off. Any time I try starting it now it's the same thing. Fires up right away but cuts out within a second. Another forum I've used has said the cold start valve is working fine but there's a problem after that. I've been trying to read up on how the CIS system works in these cars so I can keep trouble shooting. One thing of note is that the previous owner said there was an issue with the cooling system so it had been drained. I hadn't bothered to button the cooling system back before attempting to just see if it would start, run and idle for a few seconds. After reading more about the CIS system and how it uses coolant temp as part of the warm up procedure I'm wondering if I need to get the coolant flowing again because the system is shutting the car down.

I’m just getting back to working on my 79. I had the same decisions to make about the fuel pumps and did the same as you, just replacing the external one. The car starts and runs fine, up to 3000 rpm, but will not rev higher. I just got fuel pressure gauges to measure the system and control pressures. Hopefully, I’ll be able to report if one pump is providing enough pressure in a week or so.

New928, your symptoms sounds almost like the fuel pump is not staying on after the initial start. As I recall, the distributor has to send a signal that the engine is running to keep the fuel pump on. A possible way to check is to jumper the fuel pump relay so that it is just on until you disconnect the jumper. Just a thought.
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