fuel pump questions for a 1979

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?

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.

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.

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.

joejabq wrote:

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.

I finally was able to measure the system pressure, control pressure, and flow rate for my one fuel pump '79. My car runs, but I have never been able to get it to rev past -3000 rpm. I hope that I have found my problem.

Apparently, going to one external pump is pretty common. I spoke briefly with Jim Doerr, 928 Classics, who said that it's a good idea for most situation, but be sure to use the official Bosch pump.

Flowrate: I measured flow rate at the return line at the top of the fuel tank. Amazingly, I got 1360 cc/30 sec, which is the minimum rate for a two pump system. This is with an official Bosch external pump. There's some evidence that the flow rate may be of secondary importance as reported in this Rennlist post,

rennlist.com/forums/928-forum/997461-in-need-of-cis-help.html

This person reports good performance with only 700 cc/30 sec.


System Pressure: I used a Harbor Freight fuel injection test kit. I removed all of the internal check valves, but in retrospect I may not have needed to do that? Oh well.
I measure system pressures between 35-40 psi. It should be between 65 - 75 psi. I hope that this is my problem!

928classics.com/resources/cis-quick-reference-guide-78-79

I have ordered new o-rings for the pressure regulator inside the fuel distributor. This was the suggested first step if the system pressure is low.

Control Pressure: I only measured a cold control pressure and got 25 psi. This is about right, but I'm not sure how this might change once the system pressure goes up.

I'll report what happens with the system and control pressures once the o-rings arrive! It looks like one external pump should work!

joejabq wrote:

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.

Hi New928!,

I have been thinking about your problem and wondered if you have been able to resolve it?