I have a fuel pump problem that I don’t understand and need help. I’ve a 1973 OTS with 125,000 miles I’ve owned since 1975. It’s recently been losing power, missing and stalling after say 30 minutes driving. Allowing 15 minutes cool down starts right up and runs for another interval. At first I suspected the amplifier or coil, both part of the SNGbarrett upgraded ignition package but after replacing both coil and amp the problem continued.
Installed a fuel pressure gauge to monitor the 7 yr old replacement SU fuel pump. At cold start up pressure is just a tick under 3 psi, just where it should be. After 25/30 minutes the pressure has fallen to 0-1psi and it stalls. Has new fuel filter, opened tank cap to rule out blocked vent line, no change. I ran the pump for 45 minutes with engine off and pressure remained at 3psi with fuel flowing back to the tank so it seems the pump was ok with engine off seems to rule out any overheating of the pump. The non return valve is allowing fuel to pass thru even with engine off. I boosted the battery for engine off test so was getting full 12 v+ power to pump.
I’m at a loss to understand this. Sorry for the very long post. I’m happy to install a new pump but I’m not sure that’s the problem considering the static test.
I welcome your suggestions. Thank you
Mike Schiebert
First of all, I think a fuel pressure gauge is a critical tool to diagnose these issues. One glance and you know you don’t have an ignition problem. I ve had one for years
I suspect you have a problem with the return valve
years ago I had a similar issue where after an hour or two the valve would stick open. And I put all fuel back to the tank. I don’t have a pressure gauge back then but at first the symptoms where like the engine was running of 9 cylinders. A few minutes later it would get worst like the left bank was out. And eventually even the right bank would be starved. It took me a while to figure it out …stop for 10/15’ and everything would fine for another couple of hours.
The valve is the only thing affected by heat. The pump isn’t besides its own heat build up which would be the same whether you’re driving or not. My WAG is either that the body of the valve expand more than fuel cooled ball. Or the temperature affects the spring
Open the valve and clean it up then try stretching the spring a little. Or put a nut/washer behind the spring to compress it more. On a bind you can also pinch the return fuel line to restrict fuel return. I did that on the trip to New England when I was having to stop every couple of hours. Details are a bit foggy but if I remember correctly I didn’t know the valve could cause this until I got there and talked to Stew Jones, Mr V12 E-type 
Great suggestion. I try that tomorrow. Thanks
The SU pumps should stop once they reach pressure (2.8 psi), then restart once it drops.
In the 6 cylinder cars there is no return line.
My knowledge of S3 cars is limited, but could you simply block off the return line and see what happens?
If the pump is working properly and behaves as I expect it should fix your problem. You can then focus on the return valve as the source of your trouble.
If the pump is failing somehow, even with the return line closed, then this will become evident as you’ll still get failure.
Mike,
I had an issue with my ‘73 OTS where it would run fine for a while and then lose power. I too rigged a dynamic fuel pressure gauge and could see it drop to near zero after a while at speed. I replaced my fuel pump and it solved my issue. Subsequent tear-down of the old pump I found the ground on one side was bad basically making the pump a single side and eventually starving the carbs at higher fuel usage.
Sounds like a defective rotor to me.
UPDATE. I’ve tried various suggestions from this and the E Jaguar Facebook forum. After extensive testing with no positive results, I have ordered a replacement SU electronic pump which shall arrive next week. I’m looking forward to driving without breakdowns - - at least till the next gremlin raises its ugly head. After 45 yrs ownership, nothing about this car surprises me anymore. Again, thank you all and happy Thanksgiving…
This is what i installed a couple of years ago after trying many incl the electronic SU
This thing is bullet proof and quiet
Oh and i relocated the pump from the fender to under and inside the spare. Much better protected from SUV collisions
Pascal: . . . and I ordered one of them the day you initially posted the specs (months ago) – based purely on your recommendation.
It’s been in my spares cabinet ever since waiting for that stage of my restoration.
Just not sure if I’ll put under the rear fender (a la original) or under the spare.
Craig
I really don’t know why the fuel pump was installed in the fender in the first place. It really doesn’t make any sense to me and it s a lot harder to replace.
Safety wise I like having it under the spare as it is protected. The fender location is just about suv bumper level