My '94 1.9L - charcoal cannister vacuum hose dry-rotted at the charcoal cannister. (length was excessive causing it to sway & jiggle and eventually break) Simply cut the dry-rotted end + excessive length off. Cold engine spark knock under load, that was a problem through all last Fall & Winter is now gone in recent cool weather.
Idle problem that was an issue during the hottest weather, has seemed to disappear in this cool weather though. My running theory is that the PCV valve might've been leaking. (diaphragm could be weak) PCV Valve can be manually opened with a hand-operated vacuum pump. But I never took a chance to test that theory out. But at idle, whenever I allowed the engine to run for a really long time and heat up enough to run that fan for a short time, the idle problem would disappear for the time that the fan was running. Reduced exhaust temperatures may have reduced backpressures against that PCV diaphram and prevented it seeping exhaust back into the intake. I've been told that the PCV is not supposed to open except for at highway speeds. If open or leaking at idle, that extra, unmetered intake may cause rough idle or kill the engine. Problem was kind of intermittent and only occurred when the engine was warmed up. Oh, and the only other change I did beside the vacuum hose, was cleanup the battery terminals. (not dirty before, but worth mentioning anyway)