Fuel Pump

[Rapido5]

so I need a new fuel pump. pardon my naivety but when i looked up the part on bavauto there are two fuel pumps listed. How do I know which one I need? both are electric. one says inside gas tank one does not.

[Freshbread]

Brian the external pump does most of the work #12:


The internal pick up pump or suction device is in tank #7

[carsnplanes]

so I need a new fuel pump. pardon my naivety but when i looked up the part on bavauto there are two fuel pumps listed. How do I know which one I need? both are electric. one says inside gas tank one does not.

It depends how you you determined you needed a new pump. The in-tank pump is the aux pump and "lifts" fuel the main pump which is outside the tank. If your car runs, chances are the main pump is working and you might never know the in-tank pump is dead. If the in-tank pump is working and the main pump is dead chances are you will not run very well or at all. If the car starts and runs, put your hand on the main pump which is under the right rear wheel well area next to the filter. If you can feel it running or buzzing, then it's working. You can also remove the access plate to the in-tank pump in the trunk and do the same. Check also the fuse and relays to the pump before deciding the pump(s) is bad.

[bimas]

Also, if your main pumpkins working and your in-tank pump is dead, your car will usually run normal until you get to about 1/4 tank of gas. After that, it may show signs of fuel starvation.

[Rapido5]

Well here is the scenario. For the most part the car runs fine. Every so often I'll hit the accelerator and have no pick up. I'll floor it and it'll slowly get to where it should be. Since it's an intermittent thing I've discussed it with my mechanic and it is he who suggested it's a fuel pump issue. I prefer to ask here first before I agree to anything because chances are if you guys help I'm able to reduce my costs. Especially if the issue is something I can handle myself.

[RSheiman]

There is a list of things that can cause this, I would put the fuel pump towards the bottom. First thing is to check the fuel pressure at the rail (many a post on this) if you think the pump is bad. My experience, the pump is all or none, when it goes, car won't start. I know the coolant temp sensor and AFM and TPS are other sources once you establish the fuel pressure is ok.

[ldk]

... I would put the fuel pump towards the bottom ...

Agreed. However, after a fuel pressure check, also check fuel volume. Another check on the in-tank pump is whether your acceleration problem is worse when the fuel is low in the tank.

[CW6er]

Falling fuel pressure isn't necessarily the results of a failing fuel pump(s), a stuck injector or a bad FPR can cause the same symptoms.

Fuel pressure gauge:
A $10 gauge (80 or 100psi) from a place like Grainger Industrial Supply (they have local stores near major industries) or Harbor Freight, a piece of 5/16" (7.9mm) high pressure fuel injection hose from the local auto store and a "tee" with threads matching the gauge and some 5/16" hose barbs from the local home center.

You pull the fuel hose off of the fuel rail at the back by the firewall and insert the gauge in-line.


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I should mention that the fuel pump does not normally run when you turn the key on. It only runs when cranking the engine or if the speed sensor tells the ECU that the engine is running.

Jumping the fuel pump relay as in the photo below is used to test the fuel pressure and eliminate the possibility of a bad relay. (No need to jump the Main relay to test just the fuel Pressure)

With the engine off and the fuel pump relay jumped:
- Early, 633 cars with 2.5 bar FPR = 36.3 ±0.7psi (35.6 to 37.0 psi)
- Later, 635 cars with 3.0 bar FPR = 43.5 ±0.9psi (42.6 to 44.4 psi)

With the engine idling
- Early, 633 cars = 33.4 to 39psi (or 2.3 to 2.7 bar)
- Later, 635 cars = 40.6 to 46.4psi (or 2.8 to 3.2 bar).
- If you remove the vacuum line from the Fuel Pressure Regulator (FPR), the pressure should rise, and then go back when the line is replaced.

If the fuel pressure is low, slowly clamp off the return line from the FPR, if the low pressure rises back up to the correct value, you know that the fuel pump will deliver the pressure, so it is the FPR that is probably bad, but if it does not rise, then do a fuel pump output delivery test for a faulty fuel pump.

Bruceys Excellent Photo, relays jumped:


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Fuel delivery test:
The pump may be able to slowly pump up to the required pressure, but not be able to deliver the required volume at that higher pressure.

Pull the return hose off of the bottom of the Fuel Pressure Regulator (FPR) and replace it with a short piece of hose into a container to measure the output of the FPR. With the Fuel Pump relay jumped as above, you should collect at least the following in exactly one minute:

'82 thru '85 - 2.2 Qts (2.0 L)
'86 thru '88 - 1.9 Qts (1.8 L)

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If you do decide it is a bad fuel pump, Here is a copy of the OEM replacement for the external inline high pressure pump, they claim that their pump is quieter then the Walbro Pump, $70.:
http://treperformance.com/i-593443-b...1976-1989.html

[Rapido5]

and this is why I love this place. Instead of saying "uh huh" at the drop of a hat, I have an educated stance to take. Looks like I have some things to check out before I tell my mechanic to replace the fuel pump.

Thank you.

[Der Krieger]

The very FIRST thing to check is the vacuum line connected to your fuel pressure regulator, at the front of the fuel rail. If it's bad, then nothing else you do (spend a bunch of money on) will make a bit of difference. If you need to replace this line, it could cost you upwards of $3 and five minutes of your time.

[abouzmm]

Chris,
So when I check my fuel pressure by jumping the fuel pump relay, I get about 42 psi which is correct. I run the car and it goes down to 38 psi at idle. I clamp the return line slowly and it rises to about 42-43 psi. Ok. I changed the FPR. I have 3 spares. All 3 are 3.0 bar as my car is an 88 635csi and 2 of the 3 are fairly new units with a 3.0 bar labeled (Bosch). All 3 give the same results above. I am puzzled. My Injectors are new. Why there is a drop in pressure at idle? This is driving me crazy. I am going to test the fuel delivery rate tomorrow. I have not done that. Any other ideas?

Abouzmm

[tschultz]

It is meant to change based on vacuum of the engine, that's how it operates.

Maybe your pump is not working on flow but is up to par on pressure. I had that problem once...