Fitting an IMDU
(Inlet Manifold De-icing Unit)
What's an IMDU and why would I want one?
One of the most common problems for beetles which retain the stock carb and inlet manifold set-up is manifold icing.
Has your bug run rough when cold, or cut out at idle until the engine has warmed up?
If it has, the problem could well be Manifold icing. The problem with the stock manifold is that it is a long way from the carb to the cylinder head (dual-carbed engines do not have this problem).
When the air/fuel mix from the carb reaches the base of the manifold, just before it (the manifold) splits into two pipes (one to each cylinder head)
it is at its coldest and this is where icing takes place. You can see this in the form of condensation on the outside of the manifold)
VW themselves were dogged with this problem. The two smaller bore tubes that run from either side of the exhaust to the base of the manifold are heat-riser tubes designed to combat this problem. These are known as 'single hotspots'.
The factory tried again by giving later models 'Dual hotspots' - that is - four small bore pipes (2 each side) from the manifold to the exhaust - with little success.
One solution to this problem is an IMDU
This is a basically a small electrically heated blanket which wraps around your manifold to warm it up.
Price is about twenty pounds (sterling).
Fitting an IMDU
Fitting is relatively straight-forward, if a little awkward (although you do not need to remove the manifold or distributor).
An IMDU looks a bit like this:
What you have is a leatherette blanket with two eyes cut out and two wires (both usually white in colour but red and green for display purposes here).
This needs to be wrapped around the neck of the manifold, just below where the carb is bolted to. You might need to remove the petrol pump for access and you will need to remove the breather pipe from the neck of the carb.
When the unit is wrapped, it can be held in place with a cable tie if needed. Connect the wire containing the fuse to the 2 way switch on / under (one of the type that light up when on are
best) the dash. The best way to run the wire to the front is through the firewall (protected by a
grommet) or through the foam insulation in the top corners of the engine bay (which will come out under the seat) Connect the lead without the fuse to a good earth such as where the coil bolts to the fan housing.
Alternatively, if you're just using one through winter, connect one to earth and
the positive to the positive on the coil. If you want to disconnect it in the
summer simply leave it in place and take the fuse out.