My Mom's '95 940 engine detonates badly (pinging and rattling). There is also a pretty strong gas smell sometimes. We notice it when sitting still with the engine running. Her trusty Volvo mechanic can't figure out what the problem is. He told her the timing is set properly and can't be adjusted because it's self-adjusting. Beside the timing, he has no idea what the problem could be caused by. It's got 177k miles. It's been doing this since 140k miles and has been getting worse. It's also guzzling gas, 23mpg on the highway and it's a non-turbo 2.3L.
Thanks for the tip. I'll have to look into this. I already did a full tune up and it didn't effect the pinging/rattling problem at all.

Oops, just reread your comment and no, I don't believe Volvo's 2.3L 4cyl. is throttle body injected.