86 325es which fuel pump is bad?

My 86 325es has started giving what I think is a fuel problem, with the in tank transfer pump and the main fuel pump, I think the fuel filter is in the engine compartment (thought that was a 318 location) anyway what is happening is after 20 to 30 minutes the engine would lose power like it wasn't getting enough gas. this could be a filter problem but I think one of the pumps is giving up when it warms up. I don't mind replacing one of them but both makes the cost quite expensive. so how would I determine is a pump is failing and which one it is.

If your '86 is like my '90 there is only one pump, the part that sits in the tank, under the rear seat cushion,with the funny looking top (three or four leaf clover) is the sending unit, the other device next to it in the tank the pump. I had a bad sending unit and had to ask the question to figure out which one of the devices was the pump.

If your '86 is like my '90 there is only one pump, the part that sits in the tank, under the rear seat cushion,with the funny looking top (three or four leaf clover) is the sending unit, the other device next to it in the tank the pump. I had a bad sending unit and had to ask the question to figure out which one of the devices was the pump.

some info
the 86 has two fuel pumps the one in the tank is a low pressure with a larger diameter hose and it feeds the main fuel pump (located where your filter is) that is a high pressure fuel pump that supplies the puel at the higher pressure that the injectors need.
below the main fuel pump is where the filter sets on some models others have it in the engine compartment near the starter mounted on the firewall. the fuel pump on your 90 is larger than my in tank pump I have seen them priced from $160 to $330 each so replacing both when only one may be bad is expensive. sending unit may be the same but that works good. I think they changed to the single pump in 88.

the fuel system has two dampers, filter, regulator, as well as the two pumps.

i have a 85 318i and i think the pump in the tank went out and i installed a inline in between the 2 pumps and so far so good, i would not recommend doing this. this is like putting a band-aid on a broken leg

i have a 85 318i and i think the pump in the tank went out and i installed a inline in between the 2 pumps and so far so good, i would not recommend doing this. this is like putting a band-aid on a broken leg

the in tank pump is so easy to get at and if you have a junk yard (pick a part where you pull your own parts) you will learn how to R&R the pump when take one out. under the back seat 4 screws and then disconnect the hoses and undo the lock ring. I cut the hoses off the junk yard assembly but thats me.
they have half price days wher the pump assy. will cost less than $15 and som have a free warranty. My problem turned out to be a clogged radiator overheating and the computer not being able to compensate. all is good now.