I was told that my 94 940 could use a tune up. What all does a tune up consist of, beyond changing the plugs, air filter, and distributor cap?

Related questions: how do I tell if the air filter needs to be replaced? I'm not sure when the PO did it, and when I pulled it wasn't filthy or anything.

For the the distributor cap--is it something I can tackle without any special tools? Do I need to re-adjust the timing after? I had a new timing belt installed when I bought the car three months ago, so I figure it was done then.

will a tune up make the engine knock less?

thanks

Engine knock can be carbon build up in the combustion chamber from cheap gas or not enough driving. I brought a little old lady car that knocked like crazy on the hiway. A couple of cans of treatment and some 91 octane for a while cleared it up.

Waterboarding and an Italian tune up are quicker and cheaper but can be hard on older head gaskets

The knock sensor is pretty robust, but a known good one used is cheap and easy to swap. The only one I saw bad was due to weeping HG directly over it.

A knock should be address as pre-ignition left untreated can cause severe engine damage.

Yes, replace the spark plugs, air filter, and rotor and cap. Go to www.stepbystepvolvo.com and click on "Noises, Poor Performance, Easy Fixes, and Tune Up." That will help you out some. I will be updating that info in the near future, such as using an 8mm (or is it 10mm, I can't remember) ratcheting box end wrench for removing the distributor cap. Also, use two long pieces of wood underneath the rotor to pry it off. Works like a charm!

You cannot really tell if your air filter is good by looking at it (at least I couldn't). My 1987 740 turbo was blowing out blue-black smoke going up small hills. I replaced the air filter and it solved the problem. Old air filter didn't really look bad at all. So, if in doubt, replace it. It's a cheap fix.

Also, check the condition of your spark plug wires. An easy test is to put a timing light over each wire. You should see light. If not, that wire is bad.

As for engine knock, perhaps you need a new knock sensor.