NEED HELP: 1974 1.8L Will Only Start with Brake Clean, Runs with AFM disconnected only Options

[Nor.Cal.914]

Hello everyone,

I have a 1974 1.8L that needs a new fuel pump and the customer purchased this pump from Pelican:

https://www.pelicanparts.com/More_Info/0439...59&DID=4630

I have already figured out the line routing in terms of inlet and outlet, but I do have a few questions about some other aspects. Being that the stock pump is a 3-port pump, first question is, what was the reasoning behind the 3 ports instead of just an inlet and outlet? My first thought was maybe an internal FPR, but I see there is a one in the engine compartment so that doesn't seem to be the case, but please correct me if I'm wrong! Second question involves mounting the pump. I have a 1975 so I'm not as familiar with how the 1974's were setup. This car has the coil style ring bracket around the pump which I'm now trying to figure out if I can utilize to mount the new pump. Main issue of course is the fact that all 3 lines came out on one side of the factory pump, whereas this pump has inlet/outlet on either side.

So my question is, what have you guys done to mount your pumps either in the same location or where did you place it instead of the factory location? Thank you in advance for the help! (IMG:style_emoticons/default/beerchug.gif)

-Christopher

[wonkipop]

have rebuilt two of the old bosch 3 port pumps.
link to the topic with plenty of pics.
go to page 2. some cross sections through the 3 port assembly.

http://www.914world.com/bbs2/index.php?showtopic=350416

its really a kind of staged port assembly with a spring loaded valve.
i think part of what it does is not only relieve pressure if it builds too high for some reason - but also i think it helps dampen pulses from the rotary cell pump.
also remember in these old cars the entire injector circuit was pressurized but using flexible cloth reinforced hoses. so i think the idea was to pressure relieve as a fall back under the threshold where the hoses would fail. which would be catastrophic - you know another thing 914s were famous for. burning.

ultimately the fuel pressure regulator does most of the work relieving pressure and sends fuel back down to the tank via the return line. after the fpr its all just flow not pressure.
but if for any reason the fpr fails or the injector circuit blocks and can''t relieve the whole thing blows out the third port (which is called a return port but really its a relief port) and returns to the tank via the t with the return line coming from the fpr.

the third port blows at about 60-70 psi.
thats how you test them. in a bench test. you should be able to get the pump to build about 60 psi min before it relieves.

but in reality i think not only is fuel being delivered to the injector supply line and the pump building pressure in it, it also always dribbles a bit back through the third port.
esp as the pump ages.

i'm not entire sure why bosch went for these three ports.
i do know that there are some similar bosch pumps that are even slightly earlier that are fitted to mercedes that in fact have the pressure relief inside the pump at the supply (from tank) port and these blow back through the supply line. and are two port while still being roller cell pumps.

but as far as i know all the cars (different makes and models) that ran D jet and early L jet ran these 3 port pumps. not exactly sure what the mercedes two port pumps ran in.
carb? i don't think it was D jet in the case of the pump i was looking at.

however when the 914 went to a front fuel pump in 75 it ran the two port pump or so i think, never having actually seen an original one of those pumps.

i've rebuilt my original 74 1.8 pump and also a virtually identical one from a 74 SAAB that i have as a spare.

----
they evolved quite a bit during the years they were used for D jet applcations.
most of the evolution is on the interior. for instance the early ones have turbulence shrouds inside. later they don't. found to be unnecessary. typical initial german "over engineering". also they gradually shrunk them down in size.

[Nor.Cal.914]

have rebuilt two of the old bosch 3 port pumps.
link to the topic with plenty of pics.
go to page 2. some cross sections through the 3 port assembly.

http://www.914world.com/bbs2/index.php?showtopic=350416

its really a kind of staged port assembly with a spring loaded valve.
i think part of what it does is not only relieve pressure if it builds too high for some reason - but also i think it helps dampen pulses from the rotary cell pump.
also remember in these old cars the entire injector circuit was pressurized but using flexible cloth reinforced hoses. so i think the idea was to pressure relieve as a fall back under the threshold where the hoses would fail. which would be catastrophic - you know another thing 914s were famous for. burning.

ultimately the fuel pressure regulator does most of the work relieving pressure and sends fuel back down to the tank via the return line. after the fpr its all just flow not pressure.
but if for any reason the fpr fails or the injector circuit blocks and can''t relieve the whole thing blows out the third port (which is called a return port but really its a relief port) and returns to the tank via the t with the return line coming from the fpr.

the third port blows at about 60-70 psi.
thats how you test them. in a bench test. you should be able to get the pump to build about 60 psi min before it relieves.

but in reality i think not only is fuel being delivered to the injector supply line and the pump building pressure in it, it also always dribbles a bit back through the third port.
esp as the pump ages.

i'm not entire sure why bosch went for these three ports.
i do know that there are some similar bosch pumps that are even slightly earlier that are fitted to mercedes that in fact have the pressure relief inside the pump at the supply (from tank) port and these blow back through the supply line. and are two port while still being roller cell pumps.

but as far as i know all the cars (different makes and models) that ran D jet and early L jet ran these 3 port pumps. not exactly sure what the mercedes two port pumps ran in.
carb? i don't think it was D jet in the case of the pump i was looking at.

however when the 914 went to a front fuel pump in 75 it ran the two port pump or so i think, never having actually seen an original one of those pumps.

i've rebuilt my original 74 1.8 pump and also a virtually identical one from a 74 SAAB that i have as a spare.

----
they evolved quite a bit during the years they were used for D jet applcations.
most of the evolution is on the interior. for instance the early ones have turbulence shrouds inside. later they don't. found to be unnecessary. typical initial german "over engineering". also they gradually shrunk them down in size.

Thank you so much for all of the information! It was really interesting to read through it all and through the thread you referenced! I might hang onto this pump to take it apart later and learn from it!


UPDATE:

I was able to install the new fuel pump and get rid of all of my leaks that arose after the pump replacement, but now I'm having starting issues. The engine will only run with the AFM disconnected and will only start after spraying brake clean into the intake. Engine will stay running (poorly - idle speed about 1,200 RPM and feels rough) until you shut it off, but definitely doesn't like to rev up and feels like it's bogging down pretty heavily. I'm not very well versed on this system (L-Jet?), but I've been trying to do some reading in order to figure out where to go next. If the AFM is bad, will it prevent the engine from starting up on its own? What can I do to test it to see if it is actually bad or not? Any advice will be greatly appreciated! Thank you so much!

-Christopher