Hot Wires
 

As a new owner of an old A4 motor I recently went through my first spring start up which has gone relatively well. One thing I did notice was the spark plug wires, specifically where they plug on top of the spark plug were of different temps. One was very hot to the touch and another not as much. Does this makes sense or does it indicate some misfiring or other issue? Thx!

If you carefully pull a wire off each plug one at a time with the engine running, does the sound of the engine change? I found that I had a defective plug once, which wasn't firing.

On the one which seems the coolest the sound of motor doesn't change, on the one which seems the hottest most definitely so I get it that that would indicate a defective plug. On another which was only moderately warm the sound doesn't change which is what drove my original question. Why would one spark plug be hotter than another if both are firing?

If you can pull off a spark plug wire and the engine continues to run the same as before, that means that the cylinder is not firing. I would focus there first.

Once you get the engine running properly on all cylinders, then I would go hunting for new challenges. ;^)

So...you have two plugs that aren’t making a difference once the boot is removed?

Are you sure the boots are on the plugs correctly, and those two boots aren’t switched? The correct order is 1-2-4-3...it says so right on the side of the motor but is easy to forget, and a lot of engines run surprisingly well on fewer than 4 cylinders.

A frequent cause of an unhelpful spark plug: stuck valves. Measure the compression, treat fuel with a double dose of MMO, and try to bump the valves down with an Allen wrench through the plug hole before becoming a head surgeon.

Maybe no power is getting to one plug and the moderately warm plug is getting power intermittently. Maybe the wiring is shot or there's corrosion in the distributor cap towers.
If you want to test the plugs swap them around and see if the problem follows the plug.

ex TRUE GRIT

Edit: Have a look inside the distributor cap for excessive burning of the rotor and distributor cap contacts.

Plugs & BOOTS
 

Sparkplugs have an ideal temp at which they like to run, thus the various heat ranges of plugs. The heat range is governed by what the engine is designed to do so the plug temp easily starts the "burn". If the plug is to cold it is hard to start the burn and if it is to hot it can start the burn before the spark gets there:eek: preignition. Also the "boots" on the plugs are part of the design to regulate the cooling rate of the plug and keeping it warm. Boots are important in most applications!!

Now a cold or cooler plug when the engine is running can be 2 things, either the cylinder has a malfunction, like fuel mix, sticky valves or vacuum leaks. If the cylinder is compromised it will not make the power which is the heat thus a cooler plug. The other is the ignition side. If there is a weak spark or a compromised wire that spark can be weak, it will not lite the burn very well and not create much heat (power) and thus a cooler feeling plug.

Dave Neptune :cool:

Coolant flow?
 

In general, the coolant or waterflow is more active toward the thermostat or water-outlet end of the head. Because of this, it may just be a case of heat transfer to a hotter part of the head. Which plug(s) are running hotter?
Tom

Proper Torque on plugs
 

A spark plug that is not properly seated will have less heat transfer to or from the head. A poorly seated plug will run hotter than a properly seated plug. An easy check.

Tom

Grow thoughts and ideas, thank you everyone and really appreciate it. Hope it's not a sticky valve and maybe just a faulty plug but will check into all of these next weekend when I'm back at the boat and will report back!

Certainly rule out simpler causes, but a sticky valve is not necessarily a disaster. Direct lubrication, a bit of manipulation, fuel treatment, and some exercise can clear up a lot of situations. Valves WANT to work, they sometimes just need a little help.

So got down to the boat, pull all the plugs and checked compression and that the valves were moving, all good. Cleaned the plugs and still had the issue with one not firing so put a new plug into that one and now hitting on all 4 cylinders. Thanks for everyone's help and advice. I still need to look inside the distributor cap, is there annual maintenance that is recommended here?