Electric Fan After Heatsoak

I’m trying what Kirby has in the book on page 228 to have the electric fan come on after shutdown and after the car has been sitting a while. I’ve pulled the LG wire from Terminal 4 (middle) of the blue diode box, and connected it to a constant 12V supply.

When I short the thermostat sensor wires (to simulate sensor telling fan to come on), I hear what sounds like a diode click, but the fan does not come on.

Is there something I’m missing? The book is confusing, it says
Pull the LG wire off of terminal 4 and connect it to a 12V power supply.
I assume I connect the WIRE to a 12V power supply. Or do I connect the TERMINAL???

BTW, my 1988 has the built in feature that keeps the electric fan going after shutdown IF the fan was going right before shutdown, until the thermostat tells it to stop.

Ha, I’m a dummy. I pulled the 20amp fuse to use the live socket as a 12V power supply to test my circuit. Not knowing that the 20amp fuse supplied power to the fan!!!


And it’s connect the LG WIRE to 12V, not the terminal.

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I know this has been talked about to death over the decades, so feel free to ignore :wink:

Now that local temps are up in the 60Fs, I’m getting my rough idle after heat soak again. If car sits for at least 20 minutes after shutdown with hood closed, if i try to start engine 20-40 minutes after shutdown, it starts up instantly (some people have trouble starting) but then idles rough and at a low rpm for about 20 seconds. Giving it some gas helps speed up process. After half minute, idle is great.

I’ve tried turning ignition on/off a few times before starting to pump cooler fuel thru the fuel rail, but no help.

This winter i wrapped my fuel rail with heat reflective tape, but no help.

Opening the hood at shutdown does help, but it’s not something you can do at a grocery store parking lot :wink:

I’m currently testing the aux fan coming on after shutdown using a temp sensor mounted above fuel rail. It came on 20 minutes after shutdown. It stays on for about 40 secs and shuts off. It then comes on about 4 minutes later. It did this three times and then at 30 minutes after shutdown i tried to start. Same problem, but it WAS slightly better than before. I am testing lower temps for it to come on, but i don’t want to kill the battery.

A lot of theories in the archives are that the injectors soak up a lot of heat, and simply need some time for cool fuel to cool them. And/or that some vaporized gas gets stuck in them and flushing the fuel rail doesn’t flush the injectors. They need to be cycled.

One last test I’m going to try is when i cycle the fuel pump a couple times before a start, I’ll depress the accelerator to click open the injectors.

One sure fire solution I’ve heard of is louvers in the hood. I’m not there yet!!!

Greg, my post is not gonna help much other than to say I regularly left my bonnet cracked open when parked. Used a small piece of 2 x 4 placed at rear center of engine bay to rest the bonnet. Provides about a 4 inch air gap. Did this whether fan was running or not. And yes, even at grocery store. Got some crazy looks and occasional inquiry.

And please don’t go louvres

Does the electric fan run on after shutdown?

If yes, but not for long enough, maybe fan could be made to run longer after shutdown, if that would help?

Another thing mentioned in archives which I should try…

Two summers ago I don’t remember having this issue as bad. I was regularly using Chevron gas.

Last summer I switched to Safeway gas, when prices went thru the roof.

I should try switching back to Chevron. Different gas have different properties. Also, someone told me some gas stations may be using 15% ethanol? I need to stay away from that.

Wrote a lengthy piece here on injector flows/hard starting maybe a year ago. Also posted an article written by what I perceived as several rather intelligent guys doing testing on reducing vapor lock on injectors. As I recall, it got little attention 'cause nobody wanted to really dig into the meat of the article. After 20 years of watching postings here and working on injectors, I put vapor in the injectors as a real thing. I think I remember the engine crankover time to purge the vapor from the injectors at around 40 seconds. Maybe as a given…it may take 2-3 seconds under normal heat for the engine to catch and run, then 40 seconds would require 15 to 20 engine cranks. Enough to get any driver grinding their teeth and cussing at the engine. Which to me means, if you keep cranking and stopping, them right about the time your start cussing and give up, is right about the time the injectors are going to completely purge.

So rather than do the extended cranking, you focus on cooling the engine with a fan.

So…Greg…we are long time posters here and I know you have done a wheel barrel full of work on your engine. What is the thermostat rating on the switch regulating your fan ? As is…what temperature does your fan come on and cut off ? The thermal vacuum valve operates at 158F…a temp the Jag engineers calculated years ago to increase the fuel pressure in the rail. Maybe you should shoot for something similar ? And I’m gonna guess your fan needs to also run longer. And maybe, a battery with more CCA ?

I would personally aim for a continuous fan run time after shutdown, until the valley temp gets to 158-160F. How long the fan runs to do that I’m sure some mechanical/thermal engineer could calculate the heat transfer time…but absent that…trial and error in the garage. Buy a cheap temp gauge and a sensor. Mount the sensor on the fuel rail and the gauge outside the engine bay, where you can read it. Then start you fan run time and your experiment.

SD Faircloth


I do know that the fuel rail temp remains over that 160 deg for hours, 2-3 even in the 40-45 ambient we are experiencing in Michigan.
I believe that the fuels today have a RVP that is too low for the injector temperature and fuel pressures that our Jaguars operate at. Solution is to raise the fuel pressure (which would require the ECU to be remapped) and lower the injector temperature.
Modern injectors, 3 bar fuel pressure, and a different engine management system….sounds like where Jaguar was headed at the end of V12 production.
Gives a different perspective on plastic intake manifolds and plastic body injectors on newer engine designs…not necessarily just to save money.
Starting in 1989, the US EPA has lowered the RVP of gasoline from 10.5 psi to less than 7.5. RVP is the pressure that stops the fuel from evaporating at 100 deg F. If I understand correctly. And fuel temperature doesn’t necessarily have a linear relationship to RVP, it is just one datapoint.

I have an XJ6 S2 & use this setup for twin electric fans, very accurate in selecting temperature and can be setup to run when engine off… very cheap on ebay

Run electric cooling fan for up to 15 minutes after run-on shuts fan OFF as shown here with one SPST relay and one time delay off relay (Hella relay is adjustable from 0-900 seconds).

(New drawing.)

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Right on target … plastic swapped for steel, higher fuel pressure, and lowering the RVP on gasoline. All three measures to eliminate vaporization of the fuel. Most of the newer vehicles as you say,have plastic injectors, fuel rails and intakes, no fuel line between the fuel rail and the injector, and fuel pressures at the 50-55 psi range.

Problem is, a bit difficult to do on the V12s. So, insulate the rail, keep the cooling system up to par, and keep blowing air on that big hunk of metal, while you go shopping.



the 6.0 litre has the electrical going after shut off when the temp stays high, and it will restart if needed

the setup is pretty simple : temp switch on the lower part of the coolant system, switching the ground of the fan
the thermo fan switch suggested by Tom wold do the same on your earlier engine

I have installed a switch measuring the air at the fuel rail which will turn on aux fan anytime, which is why I’m posting all this info.

My latest test on 30 minutes after shutdown:

The fan came on 4 minutes after shutdown, and then cycled on/off 9 times. According to this data, the heat soak seems to peak at around 11 minutes in? (BTW, my switch turns off the fan once it’s 10F below the trigger temp.). I also have a Revotek electric fan, much improved over original.
OFF 4 mins
ON 1 min
OFF 1 min
ON 2 mins
OFF 1 min
ON 2 mins
OFF 1 min
ON 1 min 30 sec
OFF 1 min 30 sec
ON 1 min 30 sec
OFF 2 mins
ON 1 min 30 sec
OFF 2 mins
ON 1 min 30 sec
OFF 2 mins
ON 1 min 30 sec
OFF 2 mins
ON 1 min 30 sec

Unfortunately, started engine at 30 minutes and the car still idled a bit low and rough, only slightly improved. It still took about 30+ seconds to get a nice normal idle.
Also, I have no idea what temps my fan was switching on/off at. I’ve ordered a little external thermometer, so I can test more like SD said. Stay tuned! :slight_smile:

Greg, What is the CFM on your electric fan ? Have you swapped out the low flow aux OEM fan for an aftermarket that moves more air? You might be a candidate for an aftermarket single or dual electric fan setup ? SD

The Revotek kit with Comex fan I installed is 1160cfm.

Compared to my original one, this one takes half the time to cycle on/off when coolant temps get high enough. I really like it. Also has a nice protective screen over it, so no finger amputating. Also came with an aluminum shroud. My original one was rusting out, as they do.

Just did a test. No electric fan after shutdown so i could get a good heat soak.

Sure enough, start up 30 minutes later, car idled low and rough. But then i disconnected air sensor which automatically enriches fuel 10% because ecu thinks air is cold. Idle went back up to normal rpm and although idle wasn’t perfect, it was acceptable. So it really needs richer fuel during heat soak starts. Which is probably why Jaguar has the vacuum switch on fuel rail. But I’ve tested it, and during my bad heat soak idle, it doesn’t even switch on blocking the FPR vacuum, thus higher pressure thus richer mixture. The fuel rail temp isn’t high enough! My opinion is they set the temp (158F?) based on non ethanol fuels. Now we are running ethanol fuels with a lower vaporizing point. Too bad we can’t lower that switch sensor.

Another angle, i have replaced my 45 second timer (extra air at startup) with a programmable relay. I’ve currently got it set to 15 seconds. I may try increasing that to 1 minute. It won’t enrichen the fuel, but at least it will help rpm.

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Tried increasing extra air at startup to 1 minute. Fixed the low rpm, but still quite a rough idle.

Another angle that Roger Bywater suggested to me a couple years ago, is disconnecting the two prong plug on ecu idle mixture feedback that forces open loop to closed loop. He said since i had o2 sensors that had electrical warm up, i could run like this.

Sure enough, idle is much improved now at hot start up, since ecu is enriching the fuel for me. I’ll have to see how this works all the time, and cold start up. I know i tried this two years ago, and got a loopy idle in P. But ive fixed a lot of faults since then. So we’ll see.

Still need to try switching gasoline next fill up too!

From a question I received:

Yes, the “GN from 133” (cooling fan relay) is the same GN from the cooling fan thermostat, so that can be tapped at any convenient point.

And, yes, the “BG from 133” is the same BG to the cooling fan motor, so that can be tapped at any convenient point.

I referenced 133, the cooling fan relay, to differentiate the two 12V sources. The “GN from 133” is using the 12V from the source to operate a relay, while the “BG from 133” is being fed 12V to the cooling fan motor. To avoid back-feeding an unwanted 12V into the electric circuit, the “BG from 133” goes to a ‘now open’ switch within the relay.

All of my electrical schematics show the relay the same way, however, being as the schematic (even in the Jaguar printed book) are printed from old reproductions, the schematics are not 100% crystal clear.

If the cooling fan relay terminal to which the BG is connected is not a “open switch” when the relay is not activated, that “open switch” effect can easily be addressed with a relay which simply disconnects the BG wire between the cooling fan motor and the cooling fan relay. And, considering that this possibility exists, I am including a drawing with a relay which makes such schematic reproduction clarity (lack of) a moot point.

Now that I am not driving the XJS as much, I use non-ethanol gas in mine. There are several gas stations near here with it, and one has it at each pump.

Sounds like it may be issues other than just additional cooling air.

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Kirby’s book outlines - just pull and connect LG wire from terminal 4 on blue diode pack to a constant 12V supply. Voila, fan will come on anytime the relay is triggered, even when car is sitting off. This is what i did.

I may make a switch for this so i can disconnect when working on car, JUST in case. My electric fan has a screen, but i could easily get startled if it came on!

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