Quick warm-up using hot air from exhaust manifold

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#1 Quick warm-up using hot air from exhaust manifold

Post by jagwit » Sat Mar 14, 2020 3:22 pm

I find it rather pleasing when one can attend to the little things that has actually needed attention years ago, but because they “never” reach the top of the priority list, they “never” get attention.

So it was, in my case, with those flaps in the air filter housings that should switch (upon receiving vacuum) air intake from the trumpets to the hot air duct that comes from the exhaust manifold cowling.

PO had the air filter housings powder coated and in the process the diaphragms (pic below) in the vacuum servos were fried to dust. Thankfully the failure mode is for them to remain switched to the cold air position (kinda important in my climate).

So, I decided to have a go at repairing the vacuum servos. First order of business was to remove them. The bracket that hold them is spot welded to the filter housing, so drilled out the spot welds with a 4.5mm drill, knowing that I would refit them using 4mm pop rivets. This did not break the welding completely but it was then possible to remove the bracket by wedging a screw driver between the servo bracket and the housing and twisting the screw driver. This tore the bracket away from the housing and I could remove the actuator.

Now to open the servo. This required bending open the edge that has been folded over. An old jeweler’s screwdriver was perfect for this job.This only lifted the edge a little bit but then I could use progressively wider screw drivers until the edge stood upright which allowed the one half of the servo to be removed. (Careful, its spring loaded). Here are all the bits once cleaned up and primer coated:
tn_20200314_135937.jpg (171.49 KiB) Viewed 943 times
Next was to make a diaphragm. My wife raged when she discovered someone had stolen just one of her dish washing gloves. Wasn’t me of course. :roll_eyes:

I used the cover with the vacuum nipple to trace the correct size onto this diaphragm material I found, cut the diaphragm out, folded it in half and then into a quarter to find the centre of the circle where a snipped a tiny hole through which the L-shaped hook will go. I then used grey gasket maker to “glue” the diaphragm onto the base.

Now is assembly time. Note in which direction the L-shaped hook must go for each of the housings. In my case one went down and the other up. You now also have an opportunity to position the vacuum nipple where you want it - but keep the mounting bracket in mind so the nipple won’t interfere with the bracket. (Now why would I mention that…:roll_eyes: )

Install the diaphragm into the base (L-hook in the correct direction), make sure the diaphragm sits nicely on the ledge of the base, no folds, install the spring and then the top cover (with the vacuum nipple" pointing where it should. Now press the whole lot together. I first used pliers to bend the edges back down and then I used the bench vice for that final firm press. Voila! Now re-install them to the air filter housings.

All this work is of no value if the thermostatic “vacuum switch” (C33025, Temperature sensor), situated between the two carb ports inside the air cleaner, does not function correctly. Again I removed mine and first investigated how they work. I opened one of them completely as I could not figure out how they work… Here it is in pieces:
tn_20200314_115656.jpg (169.94 KiB) Viewed 943 times
The two vacuum nipples that point downwards towards the crank BOTH feed into a common chamber. This is why it does not matter to which nipple you connect what. One of them is connected to permanent manifold vacuum feed and the other goes to the trumpet/hot air flap servo.

Vacuum will thus come from the intake manifold into this common chamber and will be fed through to the servo UNLESS the “vacuum switch” (C33025, Temperature sensor) becomes hot and then opens a little valve (small round brass piece on left part in pic above using a bimetallic lever). When this valve opens, this common chamber is now exposed to ambient air pressure. This results in 1) the vacuum to the servo disappearing and 2) a “vacuum leak” to the intake manifold. The orifice size in the two nipples on the vacuum switch is very small and thus this vacuum leak is negligible. One of the orifices in one of the two vacuum switches was blocked with residue (no doubt from the fried membranes).

There is a small pozidrive screw on the vacuum switch that determines the temperature at which the ambient air valve will open. To access this screw, it is necessary to remove the plastic housing. Cut off the two “rivet” heads, open the plastic cover and remove the metal part inside. I used a tiny zip tie to close it up again.
tn_20200314_115708.jpg (158.69 KiB) Viewed 943 times
Turning the screw clockwise, will increase the temperature where the ambient air valve will open ( and thus remove vacuum to the servo, thus making it switch to cold air from the trumpet). Today was roughly 25ºC here and I turned the screw out such that if I sucked really hard on a nipple (having closed the other one with a finger), I could only just break the seal of the ambient air valve.

Then reinstall everything, connect up the vacuum lines and start the car. Having removed the ducts, I could see both flaps had switched to the hot air port. I then re-installed the ducts and I could feel them becoming hot quite rapidly. It surprised me to experience find how quickly the air in that duct became hot!

To check if the thermostatic switch opened the flaps to the trumpets when it was warm enough, I removed the trumpets so I could look down the front and it had already switched to cold air by the time I looked down the front.
Best Regards
Jag: 72 E-type V12, 80 XJS (Megasquirt + 5sp manual O/D)
Jensen: 74 Interceptor (EFI by Megasquirt + O/D 4sp auto)
Chev: 59 Apache std, 70 C10 (350V8, 700R4)

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