Fuel Pump Function: one man's opinion
Symptoms, debugging info, whys and hows from my experience. In pondering the mysteries of the Bosch fuel pump in our 164 cars, I have found some information and come to some conclusions. I hope these will be of some help to others in identifying what and where to fix.
The Bosch in-tank fuel pump has an integral 1-way valve at its outlet to prevent loss of fuel pressure when turned off, thus delivering fuel more swiftly to the injectors when the car is once again started. As instant a response as can be hoped for. But the fuel line between the pump outlet and the exit fuel port may (and in our Alfa 164s _does_) have a rubber fuel line connection which can split. This can cause decreased fuel flow and decreased fuel pressure to the regulator. If too much fuel is diverted from the main line, the pressure can be too low to cold-start the engine. However, this was not the problem I experienced.
I had just cleaned up a fuel pump assembly and replaced the rubber "skirt" bushing Alfa uses to mount a plastic cage with fine fabric filter around the input port of the pump. The pump skirt has one penetration at its top which is used for a "soda straw" tube providing a suction port to the head space at the top of the fuel tank. This was described to me as an anticavitation measure by Paul, a senior Alfa mechanic at DiFatta in Baltimore. I had installed this the best I knew how, but I was finding that the fuel lines in the car had a mix of fuel and air in them, making for very irregular metering of fuel into the cylinders and consequent very rough running.
When I topped off the fuel in the tank, things ran better, but never well. This reminded some of the split in-tank hose issue, but the pressure in the fuel line appeared uniformly high, and seemed not to decrease with time after shutdown. Even days later, I'd still see a huge volume of air mixed with fuel coming from the fuel supply line when I disconnected it at the fuel rail.
When I pulled the pump again, its inlet filter screen was plugged with rust dust and sealer film from the fuel tank. I thought about what had happened and decided that Bosch/Alfa wanted the fuel pump to suck air instead of unfiltered fuel from bypassing the screening, so they provided the fuel strainer "soda straw" path to the top headspace. When the screening gets too dirty to allow sufficient fuel to the pump, the pump starts delivering a mix of fuel and air. Some fuel gets through to provide lube for the pump, so it doesn't self-destruct, and the pump doesn't suck on the blanked-off fuel strainer to tear the strainer and suck in a sudden and enormous amount of crud and destroy the pump that way. Sort of a best outcome solution, if you ask me. It tells the operator that there is a problem without destroying anything.
So I got some clear 3/4" od x 5/8" id vinyl tubing and a 2 gallon fuel can. I siphoned out the bottom of the fuel tank through the fuel pump mounting port until I could get no more rust and crud. I cleaned off the pump, rinsed the rust dust off the filter, reassembled everything paying attention to which of the four identical looking holes in the top of the fuel pump skirt (alfa part number 60801387, "seal gasket") actually penetrated to the pump inlet area. Only one does, and if you put the straw in the wrong one it will do nothing and the pump will pull unfiltered fuel through the correct hole. The car will continue to run for a while, but it may destroy the pump and you won't get much advance notice.
In hopes that this helps someone. I thought about the possibility of a split fuel line (high pressure side) and decided there was only one way it could cause air intake: if the unintended fuel jet caused a fine spray of fuel to inject bubbles into the fuel where it is taken through the filter, then one could get significant air into the pump. This seems not to have been my problem, but which seems to be plausible. I did look at the rubber fuel line connection when I took the pump out the second time. The first time it looked intact, and the pump had been running before its host car was totaled, so I put off replacing it. It turns out to have been aged and a little bulged, but it was intact. But it needed replacing, as a pre-emptive rather than corrective measure.
I looked up a picture to post below. I have another car which just now is giving hard starting behavior after sitting overnight. I _suspect_ this means it actually has a split in-tank hose and not just a crudded up filter. We'll see soon.
Last edited by MrT; 11-17-2008 at 09:29 AM.