Question-------------------------
Followup To
Question -
-------------------------
Followup To
Question -
I have a 1980 924 that I am in the process of replacing a number of components in the rear while the transmission is out for repair. One of the items I am replacing is the fuel pump. On removing the wires from the external fuel pump I discovered that there appears to be a fuel pump in the gas tank as well. My question is two part. Why the second fuel pump and is there a way to test it without putting the whole system back together. I can get a fuel pressure guage on the end of the line coming out of the tank pump but I don,r know what kind of pressure to expect. Also should the fuel pump come on when I move the ignition switch to the run position.
Thanks.
Answer -
Hi Joe. I'm trying to visualize what you are seeing that would suggest to you there is a fuel pump inside the tank. If its in the tank, you couldn't see it, could you?
If there is one, my guess is the prior owner installed the external pump when the internal pump failed...not wanting to go through the effort of removing the bad one. There's never any good reason for two pumps.
The way to test it would be to access the 12v wire leading the the internal pump and energize that wire with 12v from a battery. Since the pump is grounded, either through contact with the body or via a ground wire...if the pump wants to work it'll work when it gets 12v. This test will bypass the fuse and the realy. The fuel will come gushing through the feed line from the tank to the engine. Adding your pressure gauge in that line will give you the reading you want. Sorry, I don't seem to have a spec on the exact pressure in front of me, but more than 15+ psi seems a very minimum.
As for the fuel pump coming on when the ignition is in run?...Every electric pump should work this way...and looking at a 944 wiring diagram from a similar vintage, every indication is that the pump should be pumping as soon as you move the ignition switch to run. That assumes the pump relay is working and the pump fuse is intact.
Let me know if I've answered the right question...and how you know that internal pump is there, OK? Thanks.
Dave
Dave;
I have checked a number of parts houses and they list both a internal(in the tank) and an external fuel pump. But nobody has been able to tell me why two. One parts person said that the in tank pump may be to pressurize the tank and keep fuel going to the external pump which is a little higher than the middle of the tank. The same two wires that go to the external pump go over to the tank. In my manuel it shows the external pump and the wires then going to the one in the tank but never talks about the second pump. Very confusing.
Thanks
Joe K.
Answer -
Hi Joe. I'm stumped on this one, but here's an exceprt from a 924 forum I found that seems to indicate there were two pumps:
"Finally, how to determine if your car has one or two pumps. All cars have a high-pressure pump external to the tank, clamped to the body behind the right rear wheel. As of sometime in '79, an in-tank supply pump was added. However, not all '79 cars have one. The surest way to figure out if you have 2 pumps is to slide under the car and look. If the car has an in-tank pump, the outlet from the fuel tank which goes to the external pump will have 2 electrical connections and a huge hex on it, and is actually the fuel pump. Single pump cars only have a nipple brazed into the tank wall, no electrical connections."
Since you already know you have two pumps...perhaps I should ask this question first:
What do you want to do about it?
Are you intent on keeping the car "original"? Or are you just trying to to make sure you've got updated parts back there...'since you're there anyway'!
If it were mine, I'd try to justify by-passing the in-tank pump...and using a high quality pump external to the tank; but, that means actually understanding what the heck the Porsche engineers were trying to fix by doubling up on the pumps.
While you ponder what you want to do I'm going to see if I can do some more research on the two-pumps-are-better-than-one idea.
If you don't here from me in 2-3 days...or if you are positive you have a plan of attack sooner than 2-3 days, i.e. you're going to keep it original...or not... write back to me and we'll take it whichever way you want to go.
Dave
Hi Dave,
My goal is to keep the vehicle original and try and determine if the intank fuel pump is operating to its specification. I have put a pressure guage on the in tank pump and only get about 2-4 lbs pressure when it is turned on. This does not seem like a lot but I am still not sure of the purpose of the in tank pump. It might just be to get a flow to the external pump which them provides all the pressure. I'm just not sure.
Joe K.
AnswerHi Joe, It's not clear why the Porsche engineers made the decision to use two pumps...and I'm not finding any real or anecdotal evidence even discussing rationale...just what we've shared confirming there could be two pumps in some cars.
The manufacturer of the pump would be the best source for establishing the pump's rating. So...you could pull it to get the serial numbers etc. Two to four psi seems low as well, however, the reading I have done seems to indicate the external pump really does the lion's share of the work pushing the fuel forward as it arrives. It appears the sticking point arises when the tank level is low and the external pump begins to falter because fuel isn't arriving at its location as reliably as before.
I'm sorry, but I don't have any more ideas as to confirming the why question.
My only other suggestions would be to find a grey-haired German mechanic at the dealer...or, find a friendly forum on the net of 924 owners. Porsche owners just love to talk about their cars!
Dave
