filter and pump

regis101 · Post by **regis101** » Wed Oct 17, 2001 8:40 pm

Greetings again. Ques. #2. I have a couple of Bosch F.I. books that proved to be very valuable. The schematics all seem to show the filter after the pump. The numerous V.W. books that I have show the filter before the pump. I can understand the filter before the pump, but what was the Bosch intention ?

Steve Arndt · Post by **Steve Arndt** » Thu Oct 18, 2001 4:10 pm

EFI pumps push, they do not pull. Factory installations of EFI all have the filter after the pump. Sometimes we do it before because our nasty 30 year old bug tanks have so much crud floating around in them! As long as the tank can still gravity feed with plenty of oomph through the line and filter to the pump, I think you will be fine. Either way should work.
Steve

ray greenwood · Post by **ray greenwood** » Thu Oct 18, 2001 5:56 pm

Steve, thats not really correct. I can assure you that a D-type injected pump will indeed suck a hole through a filter when plugged. All D-type and some of the L-type injection systems use a floating roller pump, with very tight tolerances...especially in the relief/bypass valve area. In all type 3 and 4 installations, the filter is plumbed in before the pump, from the plastic "Y" coming from the tank. The other end of the "Y" is where the return from the fuel pump/fuel loop goes into. The VW diagram is correct. On some later pumps...this may or may not have been a problem...they may have changed the location. If you have a roller pump, it needs to be filtered. The d-type roller pump is a suction pump. Self priming with a pressure capability of about 100 psi. I run test engines from ground level gas tanks 20 feet away with standard fuel line. It takes about 3-5 sconds to prime from its own suction. Without filtering, the roller pumps fail in a relatively short time. The late bus pump is a linear pump. It should have rollers radially arranged to the shaft. It is more of a flow through design. All of the D-pumps also had the motor submerged and cooled internally in the jacket by gasoline. It flowed through the motor jacket first before entering the pump head. Can't have sand in scale in there...hence the pre-filter. The bus may have had less problems with sand and rust. I have found bus pumps with the filters after the pump and always thought that people have installed them backward, so I changed it...and they suck fuel just fine also. It may just be that they are no longer worried about sand damage on the late bus pumps and changed the configuration. By the way, if it pushes at high volume...it also sucks at high volume to replace what it pushes. It is correct to say that there is little suction on the feed side of the rollers, as they are squeezing the fuel ahead of the rollers, but there is a large amount of suction in the feed line due to the high rpm movement of fuel in the pump. Ray