BTW just to clarify, the oem fuel pumps in the in-tank versions were walbro’s.
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Correct, and I believe the unit also included the fuel gauge sending unit. It took Jaguar 3 model years to realize that the in-tank pumps worked much better in extreme heat conditions.
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I still prefer the earlier cars that have it external even with the problems. Nobody likes to pay hundreds and hundreds of dollars to replace an in the tank pump. Will all the labor costs i’m saving, i could replace 3 external pumps for the price of 1 internal.
Perhaps you live in a climate that does not reach 90F plus. In these climates, in my experience, if the car is driven for multiple hours the external fuel pump will cavitate causing a vapor lock and immediate stalling of the engine. This is because, unlike the V12 XJS and Series III sedans, Jaguar left off the fuel cooler as part of the air conditioning system. (Jaguar had a bulletin fix for this on the XJ40 compromising a rather complex installation of a second fuel pump under the car) After multiple instances of this happening over 29 years of ownership on my car, I think I have solved the issue with an added underfloor fuel cooler and wrapping all the accessible exhaust system in the vicinity of the fuel pump and fuel lines with insulation. I await a strong summer heat wave to do a definitive test of this modification. Photos show air scoop and fuel cooler and insulation on both rear silencers and pipes.
And still on the original exhaust system?
Yes, definitely, why it’s only 29 years old?
that was sort of my point, obviously made from good material.
I believe Jaguar was using stainless on the exhaust systems in this era but even on my 1965 3.8S (owned since 1966) the non stainless pipes and main mufflers are original from 1965. Rear resonators, have been replaced however.
Sadly i do live in a very warm climate here in the United States. Your pictures were excellent but what is the underfloor fuel cooler? I see in your pictures that you have bolted a piece of material to the underside of the body but i’m puzzled on what that does to protect the fuel pump from heat? Wrapping the mufflers makes a ton of sense but some elaboration on that added piece would be great. Thanks for all your dedicated hard work. What year car XJ40 do you have?
Thank you for your question. It’s a 1990 XJ6, owned since new, now with 72K miles total. The underfloor fuel cooler is in the return line to the tank and the metal piece is a scoop to direct air to it. It is my understanding that the fuel returned from the engine bay is dumped into a swirl chamber which is also the location of the pickup for the pump. Thus the heated fuel from the engine bay is also the bulk of the fuel fed to the pump which exacerbates the cavitation issue. My measurements indicate engine bay temperatures can be in the range of 130-140F on a 90F plus ambient day and thus the fuel in the fuel rail is heated by this before it reaches the swirl chamber in the tank. The return line cooler is an attempt to shed some of the heat absorbed by the fuel in the engine bay. Keep in mind that the bulk of the fuel pumped to the fuel rail is simply returned to the swirl chamber, thus over extended driving periods it gains more and more heat making vaporization more likely with the resultant pump cavitation. I would love to see a cut open fuel tank to confirm the location of the outlet for the returned fuel and the pickup for the pump to confirm their proximity. If Jaguar had returned the fuel to the other side of the tank the hot fuel would be diluted by the bulk of the fuel in the tank not just that in the swirl chamber. I have noted that in instances of stalling from vapor lock, dumping 1 to 2 gallons of ambient temperature fuel into the tank filler (which I assume deposits it near or in the swirl chamber) will immediately get the car running again. I also suspect that the reid vapor pressure of the gasoline has an effect on the vapor lock condition. Winter blend fuel, if left in the car until a summer heat wave, can bring on the condition as well as letting the fuel level drop below 1/3 of a tank. Has anyone cut open a 1988-1991 XJ40 fuel tank to inspect?
Your work and knowledge that you have put into this issue is amazing. You seem to be the certified expert on this issue. I don’t even own an XJ40 yet but i plan to purchase one in the near future and i want to to be an early 1988-1989 car. I would consider a 1990 if it was the base XJ6 only because i like the quad round front end lights on the earlier cars. I think the earlier cars have more personality and class since Jaguar was still an independent company then. Once Ford took over, yes the quality increased, which is a great thing. But in my opinion, the XJ40s starting losing some of their quirkiness that came with the earlier cars. What color is your 1990 XJ6?
White, with biscuit interior. And mine is an XJ6 not a Sovereign so no ride level and no sunroof. I had a 1988 earlier but liked the 1990 type instruments and the 3.54 final drive and 4.0 liter engine better. No sunroof gave me a bit more head room for my 6’5" frame. 1989 cars did get the 3.54 final drive but still the 3.6 liter engine and vacuum florescent gauges. And full disclosure, I was a 32 year employee of Jaguar Land Rover North America acting as the Western Regional Warranty Manager.
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Lovely car John, love a white XJ40. Don’t see too many.
Interesting discussion re failed fuel pump. Mine (1990) failed at 35,000 miles during my driving it from Minneapolis to Maine (home) after its purchase. Moderate weather temps at the time. I’ve been concerned about it since and don’t want another road trip interrupted by another fuel pump failure. With the car having such low mileage, I’m wondering why it failed? I can’t help but wonder whether modern fuel containing alcohol had anything to do with it.
But if the above posts are true regarding the fuel return dumping in or near the swirl pot, would it alternatively make more sense to relocate the fuel return line? Obviously, a major project but it might more directly address the issue of the heated fuel being immediately returned to the pump and instead allow it to mix with the entire tank of fuel.
I haven’t looked beneath my car to assess the feasibility of tank removal and modification (and would probably walk away from this project if I did) but it might serve to increase road trip reliability which is what I’m ultimately after. But again, only if the pump failure is a heat issue and not old materials which are not compatible with modern fuels.
Does anyone know if the current batch of Bosch pumps are compatible with modern fuels containing alcohol? If the pumps are being remanufactured, that would only make sense. And on another note, that makes me wonder whether the replacement pump installed during my recent Minneapolis / Maine trip was a NOS or a modern pump. Hmmm.
Your 1990 has an external pump?
Yes, my '90 has a stock external pump.
I think you will find the external fuel pump cars don’t have a swirl pot, the swirl pot is for the internal FP and the fuel is returned there to ensure the pump is always submerged as the fuel level drops. If the pump was left exposed it wouldn’t be cooled as the level dropped leading to a very quick demise. This is the in tank system:
This is the external pump system
Yes …quite familiar with it, much to my dismay in 2015 
Ditto, those clips on the in and out pipes are a steep learning curve
I solved the clip deal with a pair of 18" long needle nose pliers. Indispensable!
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