Car is a 1992 XJS, I started another thread about a running problem I’m having where the car stops revving at 4k RPM. It felt ignition related because it was smooth, just wouldn’t rev.

But new coils, rotor and distributor cap didn’t resolve and so I dug into the fuel system. I purchased a new fuel pump and installed in the original housing (facelift cars are in tank). But Before I installed the housing and pump back into the tank I decided to test it by placing it in a plastic tray filled with fuel—it didn’t work. The fuel pump sputtered and the fuel was aerated and weak. Hmmmm….

so I took the new pump out of the housing and tested it by itself, worked just fine. Hmmmmm…

so then I reinstalled old pump in the housing and tested—didn’t work. So I took the old pump
out of housing and tested by itself, worked just fine. Hmmmm…

so, the pump works outside of the housing and doesn’t work when installed.

see my crude drawing below for my theory why.



The way the housing works is the fuel pump has its own pickup at the bottom, INSIDE the housing, and the outlet for the fuel pump fits inside of a foam gasket in the top of the housing. There is ANOTHER pickup, (right hand side in above drawing) that takes from OUTSIDE the housing and travels into the same tube/cavity as the fuel pump outlet. It appears that this second pickup works via siphoning/vacuuming from the fuel pump outlet. (Other theories welcome).

anyhoo, on the above drawing, the blue ink is areas that are pickups (on the bottom) or are sealed connections.

I think my problem was that there was an inadequate seal in the right hand side pickup and/or the seal at the top of the pump.

and the reason I think I didn’t notice the problem is this is the first time since I’ve had the car that I ran the tank down below 1/2 tank. With a tank half filled* (rough estimate) , the entire fuel pump housing is “under water” so to speak, so any failed seals would be submerged and sucking gas.

I think this problem would only show up on a car that sits unused /undriven like mine.

am I crazy? Any additional ideas /thoughts?

 

Theory sounds good to me.

 

Well, i wish I could report back with an update after I made some changes to the stock fuel pump and housing but as soon as I started the car up, fuel began pouring out from under the car. I guess I was a little too rough with the tank with the hoses still connected. This car is killing me, it sat unused for some amount of time in Miami and so everything I touch is fragile. Time to replace some tank hoses on a facelift car, which appears to be a nightmare...

 

And for future searchers, here is a video that describes my exact problem and they suspected the o ring on the right hand side of my above drawing.



i will have another post describing how I fixed it (changed the stock set up) because even after I changed the o-ring, it still wouldn't seal. Even in my 4 boxes of various size o rings, i couldn't find one that fit. Which reminded me of a post by some old machinist who said there are three sizes of o-rings in his experience: "Too big, too small and doesnt fit" and that has also been my experience using universally sized o-rings.

 

Andy,

As I think you may have realised from your video, the fuel pump system doesn't work the way that you've described in the diagram.

As you now know, there are 2 submersible pumps in the late 6 litre cars. There are also 2 Y-pipes in the tank. One of the Y-pipes has a single pick-up end which takes the fuel and then the pipe is spurred into two, providing an intake to each of the two pumps. This is the only feed of fuel to the pumps. The other Y-pipe works in the opposite way. Each of the pumps has an outlet port and the pipe is then joined together to a single pipe to enable a single fuel flow up to the fuel rail. Hope that is clear.

The other thing you probably know is that only one pump runs to start and run the engine at all revs. The other pump only kicks in at 2840rpm to provide additional flow.

Hope that helps,

Paul

 

Interesting. My car is a '92 and has a 5.3 and only has one fuel pump.

 

Andy,

I'm confused, the car in the video is a 1994 and has a 6 litre and the mechanic also references the 2 fuel pumps? So, that's not your car, then? If not, apologies, ignore my previous post!

Paul

 

Sorry, let me clarify. My car is a 1992, and the car in the video is a 94 with the set up you described. But the mechanic in the video suggested that they may not have had the housing reassembled correctly and therefore it was sucking air (at the pickup that has o-ring). But it also appears they mixed up their fuel lines.

I haven't been able to find much info about this problem (if in fact it is, I still haven't driven my car to confirm the fix) but after looking at the contraption that Jaguar uses, and testing it off the car, it is a precise set up, especially for something as simply as sucking up a liquid.

I didnt even mention the one-way valves in the lower pickup and in the brass outlet barb, two more points of movement that could fail (at least on my '92)

 

RESOLVED!

Follow up for future searchers. I will have a different post describing how I changed the fuel pump set up on my car but suffice to say, by situating the fuel pump to pump directly from the bottom of the tank, without using the stock housing, allowed for normal flow. This was confirmed on my test drive where I had full acceleration past 6k RPM whereas it was previously limited to 4k RPM.

So, for future searchers, there is a possibility of a bad fuel pump but there is also a possibility that the seals within the stock housing (at least on a '92 model) have failed and the pump loses its prime.