[xj] Fuel vapourisation

XJ
1

after a longish run in 30 degree heat when I stopped at
the first stop sign in town so did the car. Turns over but
does not attempt to start. Open the bonnet and the is
quite high heat from the engine bay although the
temperature gauge was only reading less than 90. Pour a
small bottle of water over the fuel injection rail and it
starts immediately, but runs rough for a while. This
happened some time back and on advice I replaced the fuel
pump as low fuel pressure was suspected. If the fuel is
circulating back to the tank via the return line why is
the fuel getting that hot that it vaporises? 1983 XJ6 vp–
Col
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2

In reply to a message from Testie2 sent Wed 7 Jan 2015:

I have to wonder if the fuel cooler is still in place. Is
it?

Does your fuel rail have the fuel temperature switch? It’s
brown plastic, has two vacuum hoses, and screws right into
the rail. I can’t remember when this item became standard
issue but am fairly sure that your car should have it.
Anyhow, when the fuel reaches xxx-temp the switch opens and
vacuum to the pressure regulator is vented to
atmosphere…thus allowing max fuel pressure in the rail.
So, you might want to check the operation of the
switch…assuming that you have it

Cheers
DD–
Doug Dwyer
Longview Washington, United States
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3

Doug is correct (as always) but a preliminary question: is your air
conditioning working? because the fuel cooler only functions if the AC is
working.

Gregory
Victoria, Canada-----Original Message-----
From: Doug Dwyer
Sent: Thursday, January 08, 2015 5:08 AM
To: xj@jag-lovers.org
Subject: Re: [xj] Fuel vapourisation

In reply to a message from Testie2 sent Wed 7 Jan 2015:

I have to wonder if the fuel cooler is still in place. Is
it?

Does your fuel rail have the fuel temperature switch? It’s
brown plastic, has two vacuum hoses, and screws right into
the rail. I can’t remember when this item became standard
issue but am fairly sure that your car should have it.
Anyhow, when the fuel reaches xxx-temp the switch opens and
vacuum to the pressure regulator is vented to
atmosphere…thus allowing max fuel pressure in the rail.
So, you might want to check the operation of the
switch…assuming that you have it

Cheers
DD

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4

after a longish run in 30 degree heat when I stopped at
the first stop sign in town so did the car. Turns over but
does not attempt to start. Open the bonnet and the is
quite high heat from the engine bay although the
temperature gauge was only reading less than 90. Pour a
small bottle of water over the fuel injection rail and it
starts immediately, but runs rough for a while. This
happened some time back and on advice I replaced the fuel
pump as low fuel pressure was suspected. If the fuel is
circulating back to the tank via the return line why is
the fuel getting that hot that it vaporises?

Remember that the same fuel is continually circulating between the hot
engine compartment and the not too cool tank, Col…

…particularly with low tank levels the fuel temp may actually reach
boiling point in hot weather. It usually is not a problem; while a fuel
cooler was fitted in some markets - ‘most’ POs bypassed it at some time, as
a possible leak source, and with few issues. The EFI is not prone to vapour
locks due to the high fuel pressures - but indeed, low pump pressure may
provoke it…

As Gregory says; for the fuel cooler to work, the AC must of course be
functioning. Nominally, a well functioning pump flushes out the fuel rail -
and disconnecting the vacuum hose at the pressure regulator helps raise the
pressure. But the first step is to change over to the other tank. Thefuel
there may be cooler…:slight_smile:

Frank
xj6 85 Sov Europe (UK/NZ)-----Original Message-----
From: Testie2
Sent: Thursday, January 08, 2015 8:41 PM
To: xj@jag-lovers.org
Subject: [xj] Fuel vapourisation

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5

In reply to a message from Frank sent Thu 8 Jan 2015:

Yes the air conditioning is working and was on! It gets
reasonable warm here at this time of year! Not sure about
the brown pressure switch but I will look tomorrow and
post the answers.
Col–
Col
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6

In reply to a message from Testie2 sent Fri 9 Jan 2015:

Yep, something is awry.

Known in past years as vapor lock. In carb’d domestic cars,
some more prone than others. Various ‘‘devices’’ to avoid it.
Some quite weird. A wood clothes pin or two on the fuel
line near the carb. A ‘‘cow magnet’’ secured to the fuel
line. A bit less weird was wrapping the line adjacent to
the carb with several layers of aluminum foil.

I’ve had a few FI cars. The Jaguar was the only one with
the slick cooler using the AC. Mine went to Texas to help a
fellow with one that had a stripped Schrader valve. The
Jaguar does just fine, sans the cooler.

The Jeep never had one and does just fine.

Now, the Jeep as do many domestics have fuel pumps
submerged in the fuel tank. That works a s a cooler.

So, in the case at hand, something else is awry as
mentioned, but also:

Fuel line too close to an exhaust or other heat source?

As cooling the line with water seems to cure it, is the
line hot to the touch?

Under bonnet temperatures higher than ‘‘normal’’.

Why, in another post at an other time.

Carl–
The original message included these comments:

Yes the air conditioning is working and was on! It gets
reasonable warm here at this time of year! Not sure about
the brown pressure switch but I will look tomorrow and
post the answers.

–
Carl Hutchins 1983 Jaguar XJ6 with LT1 and 1994 Jeep Grand
Walnut Creek, California, United States
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7

In reply to a message from Frank sent Thu 8 Jan 2015:

Yes the car have the switch to which you refer, and yes the air
was on. How do I go about checking the valve to see if it working.
There is nothing in the workshop manual about it that I can see.–
Col
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8

Ok in the middle of a heatwave down here and just after filling both tanks she died just before my drive. Yes, the air was on. After some time, cooling the radiator and fuel rail it still refused to start. Tried switching tanks…
Question… do the fuel filler caps have a breather hole? After opening the fuel flaps she started.
I know this post was a few years old but it has been running fine until now.
Col

9

**
Negative, Col…

Tank venting is through a closed vent system (US), or a simple vent hose from each the tank to ground (European).

You may have suffered a vapour lock due to fuel overheating - a very seldom occurrence on EFI. Opening the filler lids would not help, but cooling fuel rail, or waiting, would…

Faulty venting may cause excessive tank vacuum, and the powerful pump may cause partial tank collapse - emitting classic noise, but usually the engine keep running.

Opening the lids will relieve tank vacuum - and the engine will start, but the cause of your problem may have other causes as well…

However, with fresh fuel, likely cool(?) vapour lock seems unlikely.

As an aside; with full tanks - a faulty vent system (US) may quickly draw high tank vacuum. Watch for repeats - and if opening filler lids provide instant remedy; check venting…

Frank
xj6 85 Sov Europe (UK/NZ)
**

10

So if there is a simple venting system where is this located and at what point does it leave the tank. I will look under the car tomorrow to find where it exits to atmosphere. Is there something else I should be looking for in the fuel system? If when it doesn’t start can I remove the hose to the fuel rail(safely) and see if there is fuel pressure? Could the fuel filter be clogging up? Really hard to replicate the condition except by leaving the car in the sun all day then it is too hot to work on!
Can the charcoal canister have anything to do with the symptoms?
Lots of questions but I need to find the cause of this problem.
Col

11

I’d like to know myself…as I’ve never seen this ‘simple system’ illustrated (or mentioned) in any Jaguar literature.

In the filler necks, directly below the gas caps, you’ll see a tiny vent hole with a nipple and small diameter hose. Presumably the hose is just routed downward alongside the fuel tank?

Cheers
DD

12

“Safely” depends on your comfort level with a large spurt of raw gasoline…and whether or not there’s a source of ignition :slight_smile:

Bear in mind that, while zero gasoline would be telling, mere presence of gasoline at the rail isn’t much of a diagnosis. Only checking with a pressure gauge will finalize the diagnosis…showing whether or not you have the correct fuel pressure.

Cheers
DD

1 Like
13

**
Only the ‘European’ have the plain tank venting, Col…

‘US’ has the closed system - where a clogged charcoal filter may indeed cause loss of tank ventilation…

Suggestion; start the car with the filler lids closed. If the tanks are full (little air in the tanks) a clogged ventilation system will rather quickly build up tank vacuum - and the engine will falter and/or die. If so, open the filler lids and try restarting. If it does; the problem is tank ventilation - to be pursued…

If the problem is vapour lock causing the engine to stall, the engine won’t restart without cooling down - and overheating fuel require other remedial actions.

Vapour lock may only be provoked by driving - just parking in the sun will not be enough. What happens is that fuel is continually circulating through the fuel rail, picking up heat - until the fuel heats up enough to boil. Which takes some time with full tanks (lots of fuel) - with nearly empty tanks it happens quicker. If it happens - which is seldom…

However; there are some other heat sensitive items - like the ign amplifier, and sometimes the coil - which may simulate vapour lock, and requiring other actions…

Point is to clarify the problem before actions are taken…

Frank
xj6 85 Sov Europe 9UK/NZ)
**

14

Ok next symptom. Started the car and backed it out to wash dried it and drove back in to the garage with engine still running, opened the driver side fuel flap and it was like looking into a boiling pot. Stopped as soon as changed tanks. The second tank is not as full. Syphon 1 gal out of the tank and give up for today.
It seems to me as if the breather to the charcoal canister is blocked somewhere. Am I on the right track.
I now have access to a hoist so I will inspect the length of the pipe / hose from the tank to the canister.
Again is my thinking correct?

15

**
I’m not sure if it is the right track, Testie…

‘Boiling’ implies hot fuel - and the boiling point of fuel lowers with increasing vacuum. However, fuel may get very hot - heated with the engine is running for some time, particularly in hot weather…

Vacuum normally build up as fuel is consumed - but this may be accelerated by engine vacuum if the charcoal clogs the canister venting. Or indeed if the vent line is blocked…

Vacuum is noticed as a filler lids is opened - a "whoosh’; which increases with vacuum and with low fuel level (more air volume). So what happened when you opened a lid? And what happens to the ‘whoosh’ after some engine running? A small underpressure in the tanks is designed into the closed vent system - a little ‘whoosh’…:slight_smile:

To test the system; locate the charcoal canister and trace the vent hose from the canister down to the pressure relief valve - this valve sometimes fails. Disconnect vent hose from the valve at the tank side - this bypasses both the canister and the valve. Tank venting is now to open air - and sucking on the hose ‘should’ be free. Repeat the ‘whoosh’ test - there should now be no tank vacuum…

I’m simply not sure whether hot fuel or faulty venting is the problem…

Frank
xj6 85 Sov Europe (UK/NZ)
**

16

Thanks Frank, I suspect the boiling was more movement in the filler neck from the return line which was under the level of the fuel. The level of the fuel is unusually high as I syphoned some out yesterday and another 4 litres today. A couple of weeks ago there was a leak from the overflow, again fuel level too high. I will pull the hose off and see what happens.

Col.

17

**
This may imply a changover or return valve issue, Tetie…

It’s imperative that you check fuel return at the tanks. With the tank lids open and the pump running; listen to fuel return in either tank selection. Fuel must only(!) return to the selected tank. If it returns to both tanks; one return valve has failed - which will transfer fuel from one tank to the other…

If return is correct and fuel is still transferred; the changeover valve is the culprit…

As an aside; a flooding tank may cause liquid fuel into the charcoal canister. Which will ‘poison’ the charcoal and cause it to solidify - which may clog the tank venting…

Frank
xj6 85 Sov Europe (UK/NZ)
**

18

Thanks the return is only to the tank in use. Is there anyway of checking the charcoal canister?

19

It appeqars to me that if the vent line or cannister is blocked that the vent can be restored by merely opening the tank kid and lewt the tank vent to atmosphere. If that restores the engine to running correctly, then on to Franks suggestion as to further isolation of the blockage in the vent system.

Carl

20

It can also be that the suction side valve is stuck halfway. Fuel is taken from both tanks or only the wrong one and is returned to the other, selected tank only. I overfilled my left tank recently and it came out the top (european model, drain that exists at the filler and just goes down to the wheel well) and bottom - I thought the tank blew, but it didn‘t. However I did lose way too much fuel, otherwise lucky. Did I have problems starting and running? Not at all.
However the venting system appears to have cleaned out as it gave some degree of whoosh and resulting roughness after a long time of driving - and I wasn’t yet prepared to fix that. Now no whoosh and it runs fine.
I can imagine that a clogged return and overfull tank cause richness and the engine dies that way. But cooling the fuel line shouldn’t help there, just waiting. Maybe look at the sparking plugs right away, switch tanks when it happens, and so on. Follow Carls suggestion or, safer, put a washer or toothpick across the filler so it’s not airtight anymore when you close them.
David