Yeah, i wonder too, people who don’t have as much of an issue with heatsoak idle could have a tired old air temp sensor or a brand that is less prone to heatsoak? Mine is a $20 FAE sensor. The Ohm readings seem to be in spec. But it could absorb more heat than other brands? I’ve read online some other cars with similar problem go with a plastic air temp sender. Ours are brass, I think.
I also read, if you give it some gas, the air flow is enough for the meter to pick up the correct air temperature. But when you go back to idle, there just isn’t enough air flow and the sensor picks up the air stagnant around the heat soaked metal part of the sensor. (which might explain that if I rev it a bit, my rough idle goes away faster than if I just let it idle, because I’m speeding up the cooling of the sensor?)
Perhaps too, the original air trumpets on the V12 were so narrow, that the air sensor was guaranteed to have some air flow even at idle. With my bigger opening, and perhaps moving sensor away from intake box, there just isn’t enough air flow at idle. ?
On a side note, my alternative vacuum boost switch works! It is a Standard Motor Products PVS19 with sensor at 130F. I bought an adapter to go from common 3/8" npt to our M16.
I bought the three way because our Jaguar vacuum switch is backwards. Almost all 2 way vacuum switches out there OPEN the vacuum line at temperature spec, not CLOSE it.
Using the three way, manifold vacuum goes to D. Hookup at 1 allows vacuum to FPR below temp spec. Once the switch is above 130F, the switch goes to 2. I put a cap there. So the FPR will get no vacuum and the cap will prevent vacuum leak.
About the only issue, the new fuel cools the rail too fast! The switch only stayed on for about 5 seconds. LOL I guess it will help one day on a true hot start when it’s hot outside and the car has been sitting in the sun.
I realize I’ve done two things that lower air flow at idle around the air temp sender which hinder cooling its heat soaked state…
I no longer have the skinny air trumpets which have the air sender there
I now have my pcv flow coming from A air intake instead of B intake where sender is.
(The intakes for PCV are i would guess a good 200 rpm…so about 25% of airflow)
FYI, I’ve gone back to open loop at idle in P and N. Even with my o2 sensors having electric warm up, it seems to take a good half minute or so. So the ecu isn’t doing that much of an improvement right after startup.
For now im just gonna live with it …a little revving for 15 seconds is usually enough.
Today I found in a Jaguar publication I recently bought for the 6.0 engine that the fuel pressure regulator “operating pressure is raised from 2.5 to 3.0 Bar. This increased pressure helps suppress vapour formation within the injector body during hot soak conditions. Engine start and idle following a hot soak will be improved when using high volatility fuels”.
Pictures from the manual below.
Hey Jon…does that manual have a publication number? I might search for one. I assume preliminary means it was a final “draft” pending any refinement. But our number should stay same
It was just an interim manual until the regular shop manual could be updated to include the 6 liter engine and the 4L80E transmission, and other changes that were mostly made as a consequence of those major changes. There is no publication number on it anywhere. I ran across it on the Coventry Foundation ebay store. I’d never seen nor heard of it before, so I bought it. Most, if not all of the info is in the post facelift service manual set.
Holey Toledo Batman…finally some verification (from Jaguar) to what I have been preaching for a loooong time. The vaporization of fuel occurs in the injectors…(as opposed to the fuel rail…my addition) Now, where is that posting that went on for days ? Will someone better skilled in the software on this site post that portion of the manual on the old thread about vapor lock. Greg, where are you ? SD Faircloth
Of course that is entirely logical - the fuel (and vapour) in the fuel rail will recirculate and be pushed around back to the tank and that will start being cleared as soon as the fuel pump primes the rail with new fuel. The fuel and vapour in the injectors is in a dead space and can only be flushed out by using the injectors.
Exactly, which according to my past calculations would take +/- 40 seconds of cranking a vapor locked situation to purge the injectors. Enough time to frustrate even the most ardent owner, and therefor wait for the engine to cool. SD
This has probably been asked and answered, but I’m too lazy to go read the 93 posts in this thread, and however many in the other one.
Why not build a little circuit that would run the fuel pump and either hold the injectors open, or cause them to fire a few times once the key was turned on. The circuit could be activated by a push button, it only works when the button is held down.
In my mind, the two risks are some sort of damage to the injectors or resistor pack from holding them open, or plain old flooding.
It’s a terrible idea to turn the injectors on without cranking because you are guaranteed to flood the engine. How can you not? If you haven’t flooded the engine, then you are still vapour locked.
A more sensible idea is to have the fuel pump run intermittently whist the car is left standing. That way, you’d flush the fuel rail of hot fuel and replace it with cooler fuel from the tank.
The fuel injectors are unlikely to be damaged by running them wide open. Accelerating at high rpm and wide open throttle will have them open up to ~80% of the time (or maybe more) anyway.
That won’t help much, as it is the fuel injectors that are vapor locked. The injectors are deadheaded unless actuated, and separated by a couple inches of rubber fuel line from the rail, so no cool fuel gets to them.
If the vapor lock is caused by vapor, forming in the top of the injector, as described above, what’s the harm in open the injector and pushing out this mostly vapor and whatever fuel comes with it? Might be slightly flooded or rich on start up, but possibly more tolerable than a minute or more of surging idle, which is how long it takes my car to clear the vapor lock.
I think the duration of how long you hold the injectors open would have to be experimented with to find the best comprise between vapor lock and flooding.
Flushing fuel through the rail has already been tried and discussed, doesn’t work, because because vapor is still present at the top of each injector, as I understand it.
Your surging idle can’t caused by vapour lock, because it isn’t even fueling the engine if it is vapour locked. It’d be caused by a heatsoaked temperature sensor, which is leaning out the mixture more than it ought to because the sensor is returning a higher temperature than it ought to.
The 1992 onwards fuel rail had the injectors directly mounted to it, so flushing the rail would be beneficial. Even with the earlier fuel rail designs, flushing the fuel through BEFORE vapour lock has happened would be beneficial. This is no more than how the car normally runs when you have the engine on.
I think that idea has some merit. A button activated circuit to hold the injectors open…maybe 2-3 seconds at a time…manually activated. MarekH thinks it would flood the engine. I think not. Remember… during a vapor lock situation, the injectors are full of vapor and not liquid fuel. also, I doubt a 2-3 second burst would overheat the injector coils enough to burn them out. Stuck in the intake, they are already 180F ? SD SD
